I came to the same conclusion and fitted a 50l volumiser when i installed my system. The 50l was plumbed in on the return after the diverter valve. On a hot water cycle in cheap rate the volumiser heats to 60 deg and then opens up around the heating circuit giving it a boost. Works well.
I have the same arrangement. Works great in winter, but in summer it means we are heating an extra 50L of water to 55-60C in order to complete the DHW cycle, and then that heat is largely lost to an already hot house over the next 24h. An online calculator suggests heating 50L of water from 25C to 60C will use around 2kWh of energy. It must be at least doubling the energy required for DHW cycle, although as you say this is still useful energy in the heating season.
Thanks for this info. I’m finally getting a heat pump installed by Octopus (assuming planning approval). First time round they said they couldn’t do it because UFH would cause a pressure drop. Now, calcs are passing, the only design change is they’re leaving most of the radiators alone. 25ltr volumiser, 11kW Daikin heat pump. No one ever explained to me what the actual issue was regardless of how many times I asked. I’m guessing though that the total water volume was more than the unit maximum once the radiators were upgraded to the original design, so the opposite of what you described in this video
Ah, interesting. Volume and pressure/resistance can't be used interchangeably. More volume is always good but more resistance is always bad because circulation pumps have to work harder. Sometimes with more system volume comes more pressure but not always. The 11kW Daikin likes a lot of system volume and definitely doesn't have a maximum amount. Good luck 👍
That's very interesting, adding up my existing radiators, excluding the two towel rails comes to just 85 ltrs, not much really, pipe work will add more, not much though. I've started the ball rolling for our ASHP, just need to book some surveys, one with a Heat Geek, other with an approved Vaillant installer. In other news I put a deposit on a 3 year old E-Niro today, so I'll soon be joining the EV club.
Thanks for this, I'm currently doing research before deciding on a Heat pump. I was unaware of the importance of volume so that's gone into my crib sheet. Had solar and battery installed just over a year ago, and with my latest Octopus bill have a years worth of data so I can see exactly how much gas and electricity I used. Being on Agile and Gas tracker my gas averaged out to 5.2p per kWh and my electricity costs just about 9p per unit, so it looks like that with a decent COP a heat pump is definitely the way to go.
Great video. I had a 7Kw Arotherm Plus fitted in November and it has had constant defrost cycling in every cold snap currently resulting in a very cold house! The installers are swapping it out to a 10Kw unit shortly but I will look at system volume and do some rough calcs, we have a large downstairs UFH slab with 15mm poly pipe so hopefully in the recommended range.
@@HappyDays2602 ouch. I wonder if you had a couple of weeks before a cold snap, you could have built up thermal mass in the property that could have helped?🤔
We had warm(ish) weather afterwards, more than enough to build up the thermal mass, and it’s happened every cold snap so far. The unit works perfectly when at +5C, COP up to 5.5 so looking forward to it being sorted.
Really interesting, thank you, another lesson in the road to binning our LPG boiler. I'll have to look into underfloor for volume calcs. We've 100 m2 of modern underfloor, I'd imagine there's good volume there. I always wondered what buffer tanks are for, so thanks again, very educational. Why do any of us ordinary folk need to understand this? to keep an eye upon a perspective installer, there's such a small installer skill base out there. Buyer beware!
Yep, a shame that this is all very common knowledge in many other european countries but we've been left so far behind. There are lots of good heating designers out there but we need many thousands more of them to train and upskill. The primary purpose of a buffer tank isn't to add system volume. It's to allow the flow rate of the heat pump and the heating circuit to be different. The volumiser effect of a buffer tank is secondary really.
I had an Octopus 4kw Daikin system installed 2 weeks ago. With 3 small radiator upgrades. Interesting challenge with the weather! It has performed fantastically well in terms of heat delivery. Cosy the whole time. My only issue is noise, about 66db at 1m. I need go check octopus removed the transportation bolt and/or get them to have a looksee. Maybe that's normal at freezing temps, even outside defrost cycles...
Update here what you find because we had a giant (16Kw !) Daikin installed by Octopus in November and it's had a a proper cold weather workout since. House is delightfully warm but the noise is very high. Same 60+db regularly during the day and night and probably higher during defrost cycles. I've been thinking of contacting them to see if this is right because that level is loud enough to annoy.
I can see why many installations have been suboptimal. It seems that the idea that I have so often heard of "we increase the size a bit just in case" really could have a huge detrimental effect on the system when it comes to defrosting.
Yes, but MCS heat loss calcs are a joke, yet followed slavishly by many installers. Octopus insisted on a 9kW Daikin, which can only modulate down to 900w (it's the same unit as the 14kw (16 possibly?)). They refused to consider the 8kw Daikin for my property. My defrosts don't seem too bad anecdotally this first winter - there doesn't seem to be any useful data out of the box).
Interesting video. I have a Samsung 12kW heat pump, and have just calculated my system volume to be ~190L (94L in 12 rads. 46L in pipework and 50L volumiser). The Samsung recommended minimum volume is 50L. During a defrost cycle, the flow temp drops from 35C to around 20C, but recovers again fairly quickly presumably to the large system volume. When the system was installed, I didn't want the volumiser, but after the first winter and observing defrost cycles first hand, I now recognise it's value. I can see how a heat pump that is not oversized (i.e, just able to meet the heat loss requirements at -3C) and low system volume may cause the system to really struggle on the coldest of days. I thought the 8kW Samsung unit and no volumiser would have been more suitable for us, but having now lived through some colder weather, I can see the larger unit is able to handle this with ease.
For info re suppliers , Eon have a tarrif next drive 5 which is 6.7p for 7 hours midnight till 07:00 , then it 24.67 and a 64p standing charge . Helpful if you have big battery system to charge .
We have solar panels and battery, but we don't have an electric vehicle ( or any vehicle come to that, unless you count the bicycles 😉 ). It's interesting to note that several energy companies have tariffs that are more advantageous if you own an electric car.
volumizer or buffer also helps as heat store with reducing hp cycling at lower load and higher outdoor temps, when input is higher then output and hence increases efficiency
Really interesting. I didn't know system volume was related to defrost. I'll have I'll have to look up minimums for my Aira 8kw but I'm starting to think the buffer i have is even more necessary than I thought
Why would oversizing matter for defrosts? Is it just that larger units have a greater surface area to defrost and that's why the extra volume in the system is needed?
Great short video again! And definitely interesting to hear about the 'true' system volume needed. Interestingly in the video and comments people are talking about buffer tanks, but from reading places like the Renewable Heating Hub... Buffers are the devil (efficiency losses etc). I'm not a Heat Pump expert though should people be pushed more towards volumisers to counter the topics in the videos?
Tomato have interesting tariffs. But I saw the energy mix for last year shows they actively buy fossil generated electricity. Almost no renewables. Their carbon intensity is 466gco2 /kwh compared to 222 UK average! Showing finance and environment are still very poorly coupled.
@@joewentworth7856 They just don't purchase REGOs, a way of 'greenwashing' the energy mix; it's all explained on their website. The actual energy mix feeding your home will be the same as all other suppliers.
All very interesting. No idea what my system volume is, as I have ufh on the ground floor, but I think I'll compare what my heatpump did to the others on there to see if it's 'normal'.
Thanks for this. I've also got a 7kW Vaillant aroTHERM plus running on a single-circuit, open-loop radiator system, with approximately 135 litres of volume (no TRV's), so well above the minimum 55 litres in the spec sheet. Defrost has worked well over the last 3 winters, so I'm curious where you got the recommended 180-200 litres figure from?
I came to this figure by too many hours of reading forums, Facebook groups and studying heatpumpmonitor. Some more experienced heat pump engineers seem to use 25L / kW as a starting basis to allow heat pumps to work very comfortably. This doesn't mean that you'll immediately run into problems below this recommended system volume. I have no doubt that at 135L, your system will work just fine. Especially if your heat loss is not near the max capacity of the unit. Here's some maths behind it. Credit to Paul Spence for this : "Defrosts and the benefits of system volume. I’ve always worked on the basis of 25L/kw to both limit starts per hour and limit the effects of defrosts. As 1L of water is more or less 1kg and requires 4186J to raise its temperature 1°c, to raise 1L x 5° requires 4186x5=20,930j Similarly to lose 5°c 20930j are expended. Assume your flow temp is 45° and return is 40°c , ie dt5 during normal heating mode. The following is a general description, not exact as there are many variables. Assume a defrost is 1 per hour and has a 5min duration. The system volume is 100L and the heat pump is 7kw with nominal flow rate 20L/min. When the heat pump enters defrost mode, the circulation pump continues to run at 20L/min. All the energy required for deftost is provided by the return water. As the return water enters the condenser it is 40°c, at 20L/m x 5 mins the whole 100L has passed through the condenser losing 5°c, flowing out to the heating circuit.. but now at 35°c… some 10° cooler than normal operation. Whilst flowing round the heating circuit the whole 100L loses another 5° via the emitters, so returns to the heat pump at 30°c. The heat pump has to raise the circulating water 15°c to design flow temp.. that’s 3x the dt required.. to reach 45°c again. So the 100L has to recirculate 3x which is 15 minutes at 20L/min just to recover flow temp. 100L x 4186 x 5=2093000J have been expended.. just on that 5 minute defrost. Given 1kwh =3600000j then it follows 2093000/3600000=0.581kwh of energy have been extracted from your system. Additionally, the mwt of the circulating water has dropped, from 42.5° to 32.5°, a full 10° lower.. which in turn affects the output from the emitters, so room temps start to drop during the defrost and then less do during the following 15min recovery time. 100/7=14.286L/kw is in my view too low. Now assume 7x25=175L volume.. still the 20L/min, same 100L circulates during the defrost.. but this time there’s a surplus 75L, consequently the average temperature of the circulating water is ((100x35)+(75x40))/(175)=37.14° rather than 32.5° or thereabouts, that’s 5° warmer and less work for the compressor, recovery time is 5mins quicker. (This is a fairly good approximation but not exact) Now assume defrost happens more than once per hour, which is quite possible, the effect can be significant, lower room temps, high bills etc. Defrosts mainly happen when we most need the heating, colder and wet climate, but it can also indicate a loss of refrigerant. Some heat pumps have booster heaters for defrost, some have dedicated defrost buffers.. both will still use energy, you’ll still pay. But with an open loop, adequate volume to satisfy the defrost and cause negligible difference in room temps is my preference. Appliance minimum flow rate typically and 3 starts per hour. Look at the arotherm 7kw, 1205L/hr at max but minimum flow at 540L/hr. 540/3600=0.15l/s 0.15x5x4.18=3.135kw min output of the compressor, hence the min flow rate for fault. That’s more to do with the min compressor rate 20hz, the refrigerant enthalpy, the suction temp when it’s 15°c externally, the compressor discharge temp would add 3.135kw. If the min flow rate is 540L/hr and I wanted 3 starts per hour, I’d divide 540/3=180L volume passes per rotation of water. 180/7=25.714L/kw. Yes the emitters are also offloading at the same time.. so if at base temp your load is 7x.25=1.75kw but the min is 3.135, then the surplus is the difference but it also means system volume would be lower if I took the surplus to be 3.135-1.75=1.385kw rather than 3.135kw just for starts per hour. I’m aware others hold a different view but smooth compressor run times and less defrost disruption works better at 25L/kw it seems. If on a fixed speed circulator the dt narrows from what ever your initial design dt is to where ever it ends. In principle if dt5 at 7kw with a fixed speed 1205/3600=0.335L/s at min load 3.135/(0.335x4.18)=2.239°dt That’s nothing to do with a volumiser, it’ll always narrow to that.. to add a bit more.. if as with many heat pumps the compressor stop time is 3 mins, and two stops per hour occur, the compressor actual output is 54/60=0.9x3.135=2.822kw Each compressor stop deducts 3 mins of output.. so it doesn’t seem to me the volumiser is an issue. My best performing systems have either added volume, 300L buffer on closed loop system, 100L volumiser on open loop systems etc. what’s really needed is longer stop times."
@@UpsideDownFork Thanks. Very detailed. Yes, my heat loss is around 6.1kWh at -2°C, so there's enough headroom for defrost recovery with my 7kW unit. At the recommendation of my installer, I also make sure that my hot water "Max. cyl. charging time" and" Cyl. charg. anti-cycl. time" settings are both set to 60 mins, so that long overnight hot water runs are split up into blocks of 1hr, with enough time for the heating circuit to recover in the event of frequent defrost cycles.
thanks for the vid, do you have a reference for where the recommended volume figure came from? Just had a quote and they recommend a 25L buffer piped as a volumiser as I think they are assuming any rad with a TRV is not included in the system volume even if I plan to run it all open loop.. My Current volume without volumiser is 165L in theory
I came to this figure by too many hours of reading forums, Facebook groups and studying heatpumpmonitor. Some more experienced heat pump engineers seem to use 25L / kW as a starting basis to allow heat pumps to work comfortably. Some heat pumps have slightly different defrost strategies that don't require quite as much volume to be comfortable. Viessman and Ebac are examples of companies who do things differently.
Great insightful video on the water volume thank you. You keep plugging Octopus on your videos and asked why would you not be on Octopus and what are you doing if not? This irks me because my experience of them has been awful. I asked them a year ago for a smart meter, and they had to fit 3 that didn't work before admitting they could not get one to work at our address due to signal issue. Then as a compromise we asked for an Economy 7 which can work offline to get a time of day tariff. This was 3 months ago, and they forgot to commission it so think we still have the old meter and have been estimating our usage wrongly because we cannot submit a reading, and billing us at the fixed standard variable rate tariff while we have been trying to time shift our 2 x EV charging and home battery. They have been an absolute joke for us, and if I didn't know better, I would actually wonder what they do for a living...
I'm really sorry to hear this about octopus energy. I hope through your persistence you will get there in the end. The smart meter roll out is a joke for many technical reasons.
We've had some issues with Octopus but I still recommend them to friends. I think some are problems out of their control and others are part of what happens when a company grows exponentially fast. Despite any problems I believe we still have to thanks them for being positive market disruptors.
Interresting! But what do you do with your bedrooms? Any in use in my house get turned down to 14 degrees at night, i could not sleep in a room heated to 20 like the rest of the house? I had a Hive system with my old boiler so just kept that to control any bedroom temperatures as required, all the rooms are normally set to a temperature greater than they will reach so the TRVs are always open except occupied bedrooms, seems to work.
It's definitely a challenge. I like a cool bedroom but my wife likes it hot so we've both learned to compromise. Our bedrooms typically sit at 18 degrees during the day and cool to about 16 at night. If we know our system volume and know that we can safely isolate a couple of bedroom rads without affecting performance then it's no problem. The danger comes when people start turning rads off and the heat pump doesn't have enough volume to defrost itself.
I have mine set to 18 degs from 10pm to 2 am and 21 degs all other times. That allows the temp ramp up overnight using cheap tariff ready for morning...
This is great timing, as I have been doing some calculations for my heating circuit volumes for an aroTHERM plus 12kW. (As I will be having UFH, and have the potential to the increase pipe bore to therefore increase the circuit volume) I had looked at the technical information sheet for the 12 kW and assumed that as I was over the minimum 45l (I was roughly 140l) everything was OK, but you mentioned a 'recommended' circuit volume which seems much larger. However, I couldn't find this recommended volume on the technical sheet, does anyone know where to find it or how to calculate it? Thank you.
@@UpsideDownForkThat's great thank you. Gives me a figure to aim for when I go over the calculations again and look to see where I can potentially increase the pipe bore to give me more volume.
Thank you, that was really useful. I asked the octopus surveyor about the implications of using smart trvs (my office tends to get warmer than other rooms) and he did not have an answer I guess the opposite applies, too much volume will be detrimental too
Smart TRV in one or two rooms will be ok until it gets really cold outside then you really don't want to be isolating some of your system volume. Yes, there will be a point at which too much system volume will not be helpful but most residential properties won't need to worry about that. My heat pump for example can easily accommodate at least double my system volume without any detrimental effects.
Column rads are pretty good for volume. 1x triple column 1800 high x 450 wide has 27 litres! Big thermal mass in radiators will smooth out cycles and defrosting.
@@UpsideDownFork they take up the same space as the equivalent K2, maybe 1cm deeper. They have a slightly lower output Vs same K2 but benefits in other ways. Horses for courses 🙂
My system (Samsung ASHP) will default to using which ever is in operation at the time the defrost is needed. If heating is running, it takes from that, but if it's in the middle of a DHW reheat cycle, then it will take from the DHW tank. Other makes may vary, but there's no way to set or change that behaviour on Samsung systems.
Hi Mate. Thank you for your great job. Did you try or do you know If I can set up set up pure weather compensation for the day time 5.30 - 0.30 and fixed value for overnight 0.30 - 5.30 to use cheaper tariff charging my 800l buffer tank (20°C-52.5°C= approx 30kW ). I have the same HP. Many thanks
Set the room temp mod. to inactive and you'll be utilising pure weather compensation on a Vaillant. Then you will need to set different internal temps on your schedule for those different time periods. You'll need plenty of experimentation to find the sweet spot.
@@UpsideDownFork Thank you for the advice. Currently I am on heat curve 0.50 which is fine as a sweet spot. But when I want to charge the 800 l buffer ( second pump controlled by the app and thermostat is responsible for heat distribution) I have to change all the settings and I am not keen to get up at 5.30 to change the settings back for the weather comp. I do not have the kit for app control (probably there is no option in the app either) It might have been challenging for Vaillant to set up controller to run Weather Comp control and later use fixed value settings. But luck of settings to set up different heat curves for different time periods that is not really justifiable. I would like to use night tariff to charge my system but without pumping heat into the property when is not needed. I would set up the right curve to get me the required temperature during cheap tariff. This looks like Vaillant could have done more to have that option in the Senso Comfort. Imagine you can't set up your PV system to charge your batteries using cheap electricity unless you do it everyday manually.
I don't suppose you have any idea how to calculate the system volume of UFH per square meter rather than run length do you (as I don't know the pipe run lengths). It's a 128sqm property with an estimated 120sqm of UFH. All the calcs seem to be by length of pipe (which makes sense).
@UpsideDownFork thanks. Pipe spacing is 100mm and I will be able to get the internal diameter from manufacturer. Would it be as simple as 10m per sqm in that case? Not allowing for turns etc.
I've been eagerly waiting for your December north facing panels generation stats. Have I missed it in one of the heat pump / tariff videos released this month?
Apologies. I woke up with a headache on 26th December and have been very unwell since then. This video and likely the next couple will be videos I recorded back in November. I think i'm now on the mend so hope to get back on my feet soon, but the regular schedule will be severely delayed i'm afraid.
@UpsideDownFork I'm sorry to hear you're not well, that's quite a long time to be feeling ill. Rather concerning 🤔 I thought it odd that you missed two Saturdays release days for your previous monthly stats, either that or I had somehow missed them. At least your last few videos will have a few more repeat viewings from me looking for your Dec stats... Get well soon, very soon... I want them there stats 🤪
@@roblinc2u Thanks. I don't think it's anything serious. Covid/Flu style symptoms. Very severe coughing, headaches and fatigue like i've never had in my life before. Less severe symptoms intermittent fever, loss of taste/smell/appetite, streaming head cold type stuff. I promise, when I can, I will update you!
quick (maybe dumb) question is your 45 litre tank a buffer tank or a volumiser tank? Does it hydraulically seperate the heat pump from the radiators or just add 45 litres to the system volume?
Mine is plumbed as a buffer tank. If I can ever get written confirmation that I can replumb it as a volumiser without voiding my warranty then I will do so. I was actually hoping to do this back in November but verbal agreement is not good enough for me.
@@jooie444 a volumiser has just 2 pipes. One in, one out. It can be fitted on the flow or return. A buffer is a 3 or 4 pipe setup. Typically 4 pipe in the UK. It allows different flow rates to be used by the heat pump and the heating circuit.
How many customers give this any thought before pressing ahead with ASHP installation? I guess it’s only when your system develops a problem it comes to light.
Yep. Unfortunately that might be the case. Hopefully as we increase awareness and all get a better understanding of heat pumps, things like this will be normal to consider.
If you have a professional installer the customer shouldn't need to consider anything. The installer will design a system that takes account of all manufacturer design rules. The problem is some installers don't adhere to the installer guidance.
Great technical video and very timely for myself. I have Octopus coming out on Wednesday to revisit/resurvey my proposed installation to see if they will do the replacement of existing trunk Steel Pipework in my property so I can discuss with them the overall system volume as this was not a figure presented in the survey document. At the moment they are adamant that they will not increase the size of any of the radiators calculated from the survey last month even though I expressed reservations over the designs ability to maintain 21 degrees internal temperature in my 80 year old property. I'm interested also in their process for confirming/certifying the stated BTU outputs of the radiators from their chosen supplier, there seems to be a lot of talk about stated BTU not being accurate and actual being considerably (20%) lower. If this component of the design is not accurate and they will not upsize radiators it throws the whole design out and the system will not achieve the planned efficiency and required comfort.
Ah, this is interesting stuff. Octopus are definitely hesitant to deviate from their cookie cutter style at the moment so i'll be interested to hear how you get on with them.
What I don't understand with my Vaillant system: Why doesn't it use the energy stored in my 150l warm water cylinder for defrosting ? Or am I just missing this option somewhere ?
The hot water circuit is too small to use for defrosts....it's not the volume of the hot water cylinder that counts.... it's the volume of the heating circuit that matters......so only the larger heating circuit can be used for a ashp defrost cycle
Well it is the total heat capacity that counts i.e. volume of water times temperature. As the volume of 150l is comparable to the volume of the complete heating circuit that should be no problem. What makes the difference however is the higher temperature of 45+ C in the hot water cylinder. This should be sufficient for multiple defrost cycles. Of course you have to reheat your hot water after doing so. But there is no difference to heating up your warm water for the heating cycle. Only downside I could see is an efficiency argument: to heat up the water in your warm water cylinder you need 50C hot water whereas the heating runs with 38C - depending on the outside temperatures of course.
Interesting. Wonder how this affects efficiency because it take more power to heat up more water. I guess the additional power required is offset by the lower on off cycles.
Sounds like this adds to the argument for having a buffer tank? It seems like the heat pump world is quite divided over whether buffer tanks are good/necessary or not. I reckon everyone has had a lot of cycling going on this week!
@@Burtis89 Yes, the volumiser is there specifically to increase the volume of water in the system, whereas a buffer tank is to separate flows. (I know you know this, just pointing it out to others who may not).
Thanks for this insightful video! I am afraid, however, I am missing a point here. I understand that more volume results in more thermal mass in the system. But what is the effect of more thermal mass on a defrost cycle? (Or: how does having "more warm water" help reduce the number of defrosts?) I guess the heatpump is doing its defrost cycle, no matter the temperature of the heating water. I understand that more volume reduces the lowering of water temperature during a defrost cycle. But how does it impact the defrosting itself?
Credit to Paul Spence for this : "Defrosts and the benefits of system volume. I’ve always worked on the basis of 25L/kw to both limit starts per hour and limit the effects of defrosts. As 1L of water is more or less 1kg and requires 4186J to raise its temperature 1°c, to raise 1L x 5° requires 4186x5=20,930j Similarly to lose 5°c 20930j are expended. Assume your flow temp is 45° and return is 40°c , ie dt5 during normal heating mode. The following is a general description, not exact as there are many variables. Assume a defrost is 1 per hour and has a 5min duration. The system volume is 100L and the heat pump is 7kw with nominal flow rate 20L/min. When the heat pump enters defrost mode, the circulation pump continues to run at 20L/min. All the energy required for deftost is provided by the return water. As the return water enters the condenser it is 40°c, at 20L/m x 5 mins the whole 100L has passed through the condenser losing 5°c, flowing out to the heating circuit.. but now at 35°c… some 10° cooler than normal operation. Whilst flowing round the heating circuit the whole 100L loses another 5° via the emitters, so returns to the heat pump at 30°c. The heat pump has to raise the circulating water 15°c to design flow temp.. that’s 3x the dt required.. to reach 45°c again. So the 100L has to recirculate 3x which is 15 minutes at 20L/min just to recover flow temp. 100L x 4186 x 5=2093000J have been expended.. just on that 5 minute defrost. Given 1kwh =3600000j then it follows 2093000/3600000=0.581kwh of energy have been extracted from your system. Additionally, the mwt of the circulating water has dropped, from 42.5° to 32.5°, a full 10° lower.. which in turn affects the output from the emitters, so room temps start to drop during the defrost and then less do during the following 15min recovery time. 100/7=14.286L/kw is in my view too low. Now assume 7x25=175L volume.. still the 20L/min, same 100L circulates during the defrost.. but this time there’s a surplus 75L, consequently the average temperature of the circulating water is ((100x35)+(75x40))/(175)=37.14° rather than 32.5° or thereabouts, that’s 5° warmer and less work for the compressor, recovery time is 5mins quicker. (This is a fairly good approximation but not exact) Now assume defrost happens more than once per hour, which is quite possible, the effect can be significant, lower room temps, high bills etc. Defrosts mainly happen when we most need the heating, colder and wet climate, but it can also indicate a loss of refrigerant. Some heat pumps have booster heaters for defrost, some have dedicated defrost buffers.. both will still use energy, you’ll still pay. But with an open loop, adequate volume to satisfy the defrost and cause negligible difference in room temps is my preference. Appliance minimum flow rate typically and 3 starts per hour. Look at the arotherm 7kw, 1205L/hr at max but minimum flow at 540L/hr. 540/3600=0.15l/s 0.15x5x4.18=3.135kw min output of the compressor, hence the min flow rate for fault. That’s more to do with the min compressor rate 20hz, the refrigerant enthalpy, the suction temp when it’s 15°c externally, the compressor discharge temp would add 3.135kw. If the min flow rate is 540L/hr and I wanted 3 starts per hour, I’d divide 540/3=180L volume passes per rotation of water. 180/7=25.714L/kw. Yes the emitters are also offloading at the same time.. so if at base temp your load is 7x.25=1.75kw but the min is 3.135, then the surplus is the difference but it also means system volume would be lower if I took the surplus to be 3.135-1.75=1.385kw rather than 3.135kw just for starts per hour. I’m aware others hold a different view but smooth compressor run times and less defrost disruption works better at 25L/kw it seems. If on a fixed speed circulator the dt narrows from what ever your initial design dt is to where ever it ends. In principle if dt5 at 7kw with a fixed speed 1205/3600=0.335L/s at min load 3.135/(0.335x4.18)=2.239°dt That’s nothing to do with a volumiser, it’ll always narrow to that.. to add a bit more.. if as with many heat pumps the compressor stop time is 3 mins, and two stops per hour occur, the compressor actual output is 54/60=0.9x3.135=2.822kw Each compressor stop deducts 3 mins of output.. so it doesn’t seem to me the volumiser is an issue. My best performing systems have either added volume, 300L buffer on closed loop system, 100L volumiser on open loop systems etc. what’s really needed is longer stop times."
@@UpsideDownFork Wow! Thanks for the longest post I read ever! My gut feeling now is: more volume means less impact on heating water (and room) temperature, means less re-heating needed, means less icing on the heatpump means more time until the next defrost...
The mechanics of a defrost cycle requires a 4-way valve inside the outdoor unit to reverse the flow of hot refrigerant gas from the compressor, from the indoor refrigerant/water heat exchanger, to the frosted over outdoor air coil. The hot refrigerant gas from the compressor defrosts the outdoor air coil quite quickly. When complete (time & temp sensor controlled) the hot refrigerant flow is re-directed back to to the indoor refrigerant / water heat exchanger to resume heating the water loop again. While in the defrost mode the indoor refrigerant/water HEX (condenser) becomes an evaporator which takes heat from the water loop to effect the defrost process. If there is insufficient water volume in the water loop there is not enough heat available in the loop (which cools down quickly) to complete the defrost cycle in a timely fashion & this extends the time it takes to complete the defrost cycle or the frequency of them. In some cases the defrost cycle may not actually clear all the frost on the outdoor coil before a timer stops the process which seriously effects the system performance. While the HP is in defrost mode (cooling) it cannot produce any heat for the house so it is important that the defrost mode is completed as soon as possible so the system can resume heating the house. A combination of an oversized HP (which requires a larger water volume) &/or, a low volume water loop pipework system, will promote poor system performance & frequent defrosts sequences to the point where very little heat is delivered to the house in colder weather which in turn causes the so called "defrost cycle of death" & short cycling issues. Proper system design is therefore a must. Defrost cycles for air source HP's occur most frequently between outdoor air temps of +5 to -10c when outside air is damp & humid which then freezes on the outdoor air coil. The outdoor coil (evaporator in heating mode, condenser in defrost mode) operates at around 12c below the ambient air temp (heating mode). Below an outside air temp of -10c the outside air is less humid so not so much moisture freezes on the outdoor coils of HP's & the need for defrost cycles reduces but is not eliminated altogether. Hopefully this helps a bit.
@@MT-zn6tc Nope. Volume is advantageous in milder temperatures too. It helps to prolong cycles. More capacity to put the thermal energy into before the dt narrows too far on the heat pump when it's down on minimum modulation.
What do you mean by pressure level? Pressure usually increases with water temperature. A larger system volume could potentially stabilise the pressure because the heating up and cooling down cycles can be minimised. I can't see it making much of a difference in any healthy system.
@ I have a gauge on the system which tells what the water pressure is ie 1.5 bar the recommended pressure. Surly if I increase the Volume of water the pressure gauge goes up past the recommended level?
@chrisholland4952 if you add water without anywhere for that water to go, then yes that's true. The real way to add system volume is to fit a volumiser or larger radiators. 👍
I have a 50 or 60 litre buffer tank in my system and based on the rule of thumb a total capacity of ~ 277 litres. This seems a bit small given I have two 9kW heat pumps, but the LG Therma V manual doesn’t specify anything about the system water volume, so who knows. But not that I could do much about it, can’t really add another buffer tank and I would only change radiators for bigger (which I have done 3 post-install) if it was required to achieve room temperatures. BTW, be careful about the advice you gave on draining down and fully refilling the system to calculate actual system capacity. My system was at least part filled with ethylene glycol mix in the water to prevent freezing with the outside pipes when the system was turned off. I have subsequently had a leak so that’s all leaked away and been replaced by water, but if like me your system has anti freezing liquid in it, then draining down isn’t the best thing to do
Very good point about draining down. I doubt anyone would go to that length to measure their system volume. Some heat pump 'experts' seem to use 25L /kW as their rule of thumb but I'm not sure how this would be applied to twin pumps and could certainly differ if running in cascade.
Octopus will have fun sizing the buffer tank for my cosy 10 with 10mm microbore then 😅 Maybe they'll just repurpose my old 210ltr HWC instead of swapping it over 😉
All interesting stuff👍, but not talked about much, when comparing the costs compared with a modern gas boiler.Because the system has a lot more water volume with the Buffer/Volumisers and larger rads, it will take more energy to heat that water. Also in the cold/humid weather, the heat pump will need to de- ice more often, using the heating water with or without the buffer/Volumiser, this water will then need to be reheated by the heat pump over and over again. Is there a rule of thumb of how much Kwh is being wasted by extra volume and the de- icing process? Also No one seems to factor the heat losses of the extended exterior pipe runs, I see on these heat pump installalations that arent required with a gas boiler coz its normally within the house envelope.
No energy is being wasted by having extra volume. It's in fact the opposite. Less compressor stops and starts and less defrosts means more volume is always more economical. It may take longer to heat the extra volume from a complete standstill but it will have better thermal inertia, smoothing out any peaks. The external pipework heat loss is very minimal as long as they are insulated properly but can be calculated if you wish to. Poor performing pipe insulation will require a higher flow temp and result in a lower COP. COP is the relevant metric to use when comparing gas to HP consumption.
Good information and analysis. I find comparing your own system against others on OEM is a bit of a double-edge sword. Unless you know the system you are looking at in detail comparing against your own is tricky. Knowing how the household runs their system, things like DHW run frequency, temperature, legionnaire cycles, setback, operation mode (normal, eco etc), rad sizes and so on. So many variables to consider. Our system defrosts once every 60 minutes when it drops below 5ºC and when it falls below 0ºC perhaps twice every 60 - 90 minutes which is quite normal. We do have a large system volume (which I will calculate now having watched your video) we also have very long primaries much of which are outside. I'm sure they take a hit on heat loss because of that. Humidity has a massive impact on defrost frequency as well as flow temperature. The more humidity has more moisture in the air which means more freezing onto the fins at the back of heat pump. A higher the flow temperature will also result on more frequent the defrosts as the heat pump has to work harder to get up the temperatures.
I agree about OEM. Without knowing how people run their systems, it can be tricky to compare against your own. As for system volume, I imagine that there's a minimum needed - enough water at a sufficient temp to defrost the system - but beyond that I can't see that more volume is helpful. I suppose a few minutes of defrosting and a few degrees lower in flow/return temp is normal. My system has been defrosting every 1/2 hour at times, but even at 1C daily average, the COP is 3 - SCOP is still over 4 (including DHW). The best performing Arotherm on OEM's heatpumpmonitor site appears to have similar performance (although a higher COP).
They're not a real problem for most people, just at certain temperatures (mostly 0ºC-4ºC) together at the same time as high humidities the efficiency is lowered. A bad cycling problem indicates an installation problem, as if the volume is too low a volumiser should have been fitted. A gas boiler by contrast loses 20% of its energy all the time in normal operation, even the 95% efficient ones usually aren't much better than 80% installed, we just know about heat pump efficiencies much more because of the monitoring.
Defrosting is no different to what happens in a domestic frost free fridge/freezer. The only difference is that the fridge freezer uses an electric heater (imbedded in the coil) to defrost the evaporator inside the freezer section instead of reversing the flow of refrigerant from the compressor, as per the domestic heat pump. Nothing new in the world of refrigeration & air conditioning which has been using heat pump technology since 1856 (Peter von Rittinger) & used domestically since the 1940's. New refrigerants have come & gone but the basic principle remain the same.
Apologies that I just jumped straight into the deep end with this video. Here's another video I made showing what to expect with a defrost cycle ruclips.net/video/aY8UKI6qHCE/видео.html Perhaps I can do something a little bit more beginner friendly in the future.
Gas boilers were also very commonly hugely oversized. Seems to be programmed into the mindset of the heating industry the idea that bigger is better and now even more of a problem when applied to heat pumps. We are not alone. Domestic heating in the USA is on a totally different scale of oversizing with massive basement furnaces that would be suitable for a factory or a school building in the UK.
Thanks for sharing. If you go outside and observe your heat pump you will see that it takes much longer than 20 seconds for the whole process. A quick defrost on a Vaillant pump will be something like 2 minutes. Humidity pays a critical role in defrost frequency too.
@UpsideDownFork probably, I just timed from when I see the electric consumption change followed by the start of visible steam at the rear and front then the finale of steam ejection with the fan being activated.
If you are getting good performance with your 7Kw Vaillant, I would leave well alone and keep your buffer! Don't try and eek out a little bit more performance, for something that might help when the weather gets below the design temps. In our UK weather the defrost cycles can cause havoc and the published output data on the 7Kw unit, doesn't seem to live up to the figs that is purported. The heat geek guru Urban Plumbers ( they don't fit buffers!) has had so many problems with the Vaillants, noisy, under performing by 20% etc, he has fitted resistive heaters into the systems for extra heat, and is now NOT specifying Vaillants. It also seems the COP figs arent correct either, coz they dont take in consideration of the kwh cooling of the heating system on defrosts cycles. Which on some systems was 0.4Kwh per defrost. The system only counts the energy in and the heat produced.This seems for all manufacturers though🙄
I have Vaillant 5kW, 200l+ volume, lots of high intertia UFH, running very low flow temps (25-35). I'm still seeing worst case defrosts of every 30 mins or so, but the defrosts themselves are short, less than 5 mins.
I hear a lot of comments saying don’t have TRVs to increase volume and thermal mass but we like having different rooms at different temperatures at different times of the day. I guess having more emitters in the hottest rooms and a volumiser can help. Interested to know your thoughts?
Different rooms at different temps is no problem. Good heating design will address that. Larger rads in rooms you want at 22 degrees and relatively smaller rads at rooms you want at 18 degrees. Now changing the temp of those rooms throughout the day relative to each other is a different challenge and one that is not ideal for most domestic heat pump installations. You will sacrifice some efficiency to meet your goal from what I understand you are trying to achieve.
I didn't look into water volume before I got mine installed. I just calculate mine and it's 170L for the Vaillant 5kw. So about right I guess. Its been running 529 hours and 216 on/off cycles? Anyone else on/off's similar?
170L for a 5kW pump is great! Your run time vs cycles also looks excellent, but it probably won't be able to maintain that as we get into the shoulder months and you hit minimum modulation.
And you didn't mention the pitfalls of system volume when in DHW mode........ The correct way of calculating system volume is considering the number of stop/starts permissable and the kW output of the unit.
@@71brp84 If you're talking about the rule of thumb from 2:50 - 3:50, that's not 7L/m for a K2. It's 7L per panel. If you're talking about the calculation method from 4:00 - 7:00 then it is 6.6l/m for a 600mm tall K2. My calculations at 4:50 look fine.
@71brp84 I agree it's flawed. It's not a rule of thumb that I concocted. It was shared with me by an experienced plumber. He seemed to think that anyone of a certain generation who built domestic heating systems uses this exactly as I described in the video.
@@GrahamNewman-mq7gr I've just had a heatpump installed. It's made a huge difference to the overall warmth of my home and been great over the freezing weather we've just had. It's embarrassing shouting about things you have no understanding of.
@AlanTov No UNDERSTANDING realy. I am fully converstant how these things work. Marine career Chief Engineer 50,000 ton LPG carriers .Large Container Vesels up to 200 Refridgerated Containers. These systems are a "Complicated answer to a non existant problem". A Combi Boiler turn it on "Hot Water " as you want it , as much as you want , Bath / Shower ect. Heating turn it on Radiators hot 62C after 20 minute All house up to temp under 1 hour. Your "Heat Pump will never do this". Latest Combi Boiler £2000 Installed, 10 year parts / labour warranty. You probably have "A 4 WHEEL ELECTRIC LEMON" DEPRECIATING £500 A MONTH ON THE DRIVE" to complement the HEAT PUMP.
Love this channel. Even without heat pump, this is very enjoyable content. Although I’ve been sold on heat pumps
Thank you!
I came to the same conclusion and fitted a 50l volumiser when i installed my system. The 50l was plumbed in on the return after the diverter valve. On a hot water cycle in cheap rate the volumiser heats to 60 deg and then opens up around the heating circuit giving it a boost. Works well.
Nice! 50L can make a huge difference during the very cold periods and the very mild periods.
I have the same arrangement. Works great in winter, but in summer it means we are heating an extra 50L of water to 55-60C in order to complete the DHW cycle, and then that heat is largely lost to an already hot house over the next 24h. An online calculator suggests heating 50L of water from 25C to 60C will use around 2kWh of energy. It must be at least doubling the energy required for DHW cycle, although as you say this is still useful energy in the heating season.
Thanks for this info. I’m finally getting a heat pump installed by Octopus (assuming planning approval). First time round they said they couldn’t do it because UFH would cause a pressure drop. Now, calcs are passing, the only design change is they’re leaving most of the radiators alone. 25ltr volumiser, 11kW Daikin heat pump. No one ever explained to me what the actual issue was regardless of how many times I asked. I’m guessing though that the total water volume was more than the unit maximum once the radiators were upgraded to the original design, so the opposite of what you described in this video
Ah, interesting.
Volume and pressure/resistance can't be used interchangeably.
More volume is always good but more resistance is always bad because circulation pumps have to work harder.
Sometimes with more system volume comes more pressure but not always.
The 11kW Daikin likes a lot of system volume and definitely doesn't have a maximum amount.
Good luck 👍
That's very interesting, adding up my existing radiators, excluding the two towel rails comes to just 85 ltrs, not much really, pipe work will add more, not much though. I've started the ball rolling for our ASHP, just need to book some surveys, one with a Heat Geek, other with an approved Vaillant installer. In other news I put a deposit on a 3 year old E-Niro today, so I'll soon be joining the EV club.
Big news day for you! Congratulations 🎉
Thanks for this, I'm currently doing research before deciding on a Heat pump. I was unaware of the importance of volume so that's gone into my crib sheet. Had solar and battery installed just over a year ago, and with my latest Octopus bill have a years worth of data so I can see exactly how much gas and electricity I used. Being on Agile and Gas tracker my gas averaged out to 5.2p per kWh and my electricity costs just about 9p per unit, so it looks like that with a decent COP a heat pump is definitely the way to go.
@@devjon123 nice! It sounds like you're well prepared and you'll end up with a good result 👍
Great video. I had a 7Kw Arotherm Plus fitted in November and it has had constant defrost cycling in every cold snap currently resulting in a very cold house! The installers are swapping it out to a 10Kw unit shortly but I will look at system volume and do some rough calcs, we have a large downstairs UFH slab with 15mm poly pipe so hopefully in the recommended range.
Ouch. How close was your heat loss?
Did you also consider the electrical backup heater option?
7.6 Kw / hr heat loss at -3C, unit went live and next day the snow arrived and the cycling started!
Considered the BUH but not enough detail on how COP could be monitored etc with it so elected to go for bigger unit.
@@HappyDays2602 ouch. I wonder if you had a couple of weeks before a cold snap, you could have built up thermal mass in the property that could have helped?🤔
We had warm(ish) weather afterwards, more than enough to build up the thermal mass, and it’s happened every cold snap so far. The unit works perfectly when at +5C, COP up to 5.5 so looking forward to it being sorted.
Really interesting, thank you, another lesson in the road to binning our LPG boiler. I'll have to look into underfloor for volume calcs. We've 100 m2 of modern underfloor, I'd imagine there's good volume there. I always wondered what buffer tanks are for, so thanks again, very educational.
Why do any of us ordinary folk need to understand this? to keep an eye upon a perspective installer, there's such a small installer skill base out there. Buyer beware!
Yep, a shame that this is all very common knowledge in many other european countries but we've been left so far behind.
There are lots of good heating designers out there but we need many thousands more of them to train and upskill.
The primary purpose of a buffer tank isn't to add system volume. It's to allow the flow rate of the heat pump and the heating circuit to be different.
The volumiser effect of a buffer tank is secondary really.
I had an Octopus 4kw Daikin system installed 2 weeks ago. With 3 small radiator upgrades. Interesting challenge with the weather!
It has performed fantastically well in terms of heat delivery. Cosy the whole time.
My only issue is noise, about 66db at 1m. I need go check octopus removed the transportation bolt and/or get them to have a looksee.
Maybe that's normal at freezing temps, even outside defrost cycles...
Congratulations. Good to hear that it's mostly working well for you.
Hope the noise issues gets ironed out.
Update here what you find because we had a giant (16Kw !) Daikin installed by Octopus in November and it's had a a proper cold weather workout since. House is delightfully warm but the noise is very high. Same 60+db regularly during the day and night and probably higher during defrost cycles. I've been thinking of contacting them to see if this is right because that level is loud enough to annoy.
I can see why many installations have been suboptimal. It seems that the idea that I have so often heard of "we increase the size a bit just in case" really could have a huge detrimental effect on the system when it comes to defrosting.
Indeed. Heat pumps really need calculations instead of rule of thumbs that plumbers have lived by before for gas boilers.
Yes, but MCS heat loss calcs are a joke, yet followed slavishly by many installers. Octopus insisted on a 9kW Daikin, which can only modulate down to 900w (it's the same unit as the 14kw (16 possibly?)).
They refused to consider the 8kw Daikin for my property.
My defrosts don't seem too bad anecdotally this first winter - there doesn't seem to be any useful data out of the box).
@@DomSta03 They insisted on fitting a 16Kw at ours despite me not managing to calculate anything above 12Kw using Heatpunk.
Interesting video. I have a Samsung 12kW heat pump, and have just calculated my system volume to be ~190L (94L in 12 rads. 46L in pipework and 50L volumiser). The Samsung recommended minimum volume is 50L. During a defrost cycle, the flow temp drops from 35C to around 20C, but recovers again fairly quickly presumably to the large system volume. When the system was installed, I didn't want the volumiser, but after the first winter and observing defrost cycles first hand, I now recognise it's value. I can see how a heat pump that is not oversized (i.e, just able to meet the heat loss requirements at -3C) and low system volume may cause the system to really struggle on the coldest of days. I thought the 8kW Samsung unit and no volumiser would have been more suitable for us, but having now lived through some colder weather, I can see the larger unit is able to handle this with ease.
Good to hear that your system is working well for you.
I find humidity a major factor contributing to the number of defrosts. At 95+% i get one every 40min. At 80% once every 2hrs. All at about 0°c
Yes! Humidity is critical! Good numbers you've got there. I need to pay better attention and record mine.
For info re suppliers , Eon have a tarrif next drive 5 which is 6.7p for 7 hours midnight till 07:00 , then it 24.67 and a 64p standing charge . Helpful if you have big battery system to charge .
That looks like a really attractive deal. Do you know what the export rate is on that?
@@UpsideDownForkJust checked and it looks pretty decent. 21p/kWh (eon installed panels) or 16.5p/kWh for everyone else. Up to 15kW solar array.
We have solar panels and battery, but we don't have an electric vehicle ( or any vehicle come to that, unless you count the bicycles 😉 ). It's interesting to note that several energy companies have tariffs that are more advantageous if you own an electric car.
volumizer or buffer also helps as heat store with reducing hp cycling at lower load and higher outdoor temps, when input is higher then output and hence increases efficiency
Agreed 👍
Really interesting. I didn't know system volume was related to defrost. I'll have I'll have to look up minimums for my Aira 8kw but I'm starting to think the buffer i have is even more necessary than I thought
Your recommended system volume will be very roughly similar to mine.
25L / kW is a very rough rule of thumb to work with.
@UpsideDownFork when you measured pipe runs. Did you times by 2 to count flow and return?
Why would oversizing matter for defrosts? Is it just that larger units have a greater surface area to defrost and that's why the extra volume in the system is needed?
Yep.
Great short video again! And definitely interesting to hear about the 'true' system volume needed.
Interestingly in the video and comments people are talking about buffer tanks, but from reading places like the Renewable Heating Hub... Buffers are the devil (efficiency losses etc).
I'm not a Heat Pump expert though should people be pushed more towards volumisers to counter the topics in the videos?
Yes, volumisers are preferable!
Have been with octopus now with Tomato nuch cheeper just commenting as you asked for. Interesting video as most are
Tomato have interesting tariffs. But I saw the energy mix for last year shows they actively buy fossil generated electricity. Almost no renewables. Their carbon intensity is 466gco2 /kwh compared to 222 UK average! Showing finance and environment are still very poorly coupled.
Thanks. When I last looked, they also offered no export tariff.
@@joewentworth7856 They just don't purchase REGOs, a way of 'greenwashing' the energy mix; it's all explained on their website. The actual energy mix feeding your home will be the same as all other suppliers.
All very interesting. No idea what my system volume is, as I have ufh on the ground floor, but I think I'll compare what my heatpump did to the others on there to see if it's 'normal'.
UFH typically has good system volume as long as it is kept all open.
Thanks for this. I've also got a 7kW Vaillant aroTHERM plus running on a single-circuit, open-loop radiator system, with approximately 135 litres of volume (no TRV's), so well above the minimum 55 litres in the spec sheet. Defrost has worked well over the last 3 winters, so I'm curious where you got the recommended 180-200 litres figure from?
I came to this figure by too many hours of reading forums, Facebook groups and studying heatpumpmonitor.
Some more experienced heat pump engineers seem to use 25L / kW as a starting basis to allow heat pumps to work very comfortably.
This doesn't mean that you'll immediately run into problems below this recommended system volume. I have no doubt that at 135L, your system will work just fine. Especially if your heat loss is not near the max capacity of the unit.
Here's some maths behind it.
Credit to Paul Spence for this :
"Defrosts and the benefits of system volume.
I’ve always worked on the basis of 25L/kw to both limit starts per hour and limit the effects of defrosts.
As 1L of water is more or less 1kg and requires 4186J to raise its temperature 1°c, to raise 1L x 5° requires 4186x5=20,930j
Similarly to lose 5°c 20930j are expended.
Assume your flow temp is 45° and return is 40°c , ie dt5 during normal heating mode.
The following is a general description, not exact as there are many variables.
Assume a defrost is 1 per hour and has a 5min duration. The system volume is 100L and the heat pump is 7kw with nominal flow rate 20L/min.
When the heat pump enters defrost mode, the circulation pump continues to run at 20L/min.
All the energy required for deftost is provided by the return water.
As the return water enters the condenser it is 40°c, at 20L/m x 5 mins the whole 100L has passed through the condenser losing 5°c, flowing out to the heating circuit.. but now at 35°c… some 10° cooler than normal operation.
Whilst flowing round the heating circuit the whole 100L loses another 5° via the emitters, so returns to the heat pump at 30°c. The heat pump has to raise the circulating water 15°c to design flow temp.. that’s 3x the dt required.. to reach 45°c again. So the 100L has to recirculate 3x which is 15 minutes at 20L/min just to recover flow temp.
100L x 4186 x 5=2093000J have been expended.. just on that 5 minute defrost.
Given 1kwh =3600000j then it follows
2093000/3600000=0.581kwh of energy have been extracted from your system.
Additionally, the mwt of the circulating water has dropped, from 42.5° to 32.5°, a full 10° lower.. which in turn affects the output from the emitters, so room temps start to drop during the defrost and then less do during the following 15min recovery time.
100/7=14.286L/kw is in my view too low.
Now assume 7x25=175L volume.. still the 20L/min, same 100L circulates during the defrost..
but this time there’s a surplus 75L, consequently the average temperature of the circulating water is ((100x35)+(75x40))/(175)=37.14° rather than 32.5° or thereabouts, that’s 5° warmer and less work for the compressor, recovery time is 5mins quicker. (This is a fairly good approximation but not exact)
Now assume defrost happens more than once per hour, which is quite possible, the effect can be significant, lower room temps, high bills etc.
Defrosts mainly happen when we most need the heating, colder and wet climate, but it can also indicate a loss of refrigerant.
Some heat pumps have booster heaters for defrost, some have dedicated defrost buffers.. both will still use energy, you’ll still pay.
But with an open loop, adequate volume to satisfy the defrost and cause negligible difference in room temps is my preference.
Appliance minimum flow rate typically and 3 starts per hour. Look at the arotherm 7kw, 1205L/hr at max but minimum flow at 540L/hr.
540/3600=0.15l/s
0.15x5x4.18=3.135kw min output of the compressor, hence the min flow rate for fault.
That’s more to do with the min compressor rate 20hz, the refrigerant enthalpy, the suction temp when it’s 15°c externally, the compressor discharge temp would add 3.135kw.
If the min flow rate is 540L/hr and I wanted 3 starts per hour, I’d divide 540/3=180L volume passes per rotation of water.
180/7=25.714L/kw.
Yes the emitters are also offloading at the same time.. so if at base temp your load is 7x.25=1.75kw but the min is 3.135, then the surplus is the difference but it also means system volume would be lower if I took the surplus to be 3.135-1.75=1.385kw rather than 3.135kw just for starts per hour.
I’m aware others hold a different view but smooth compressor run times and less defrost disruption works better at 25L/kw it seems.
If on a fixed speed circulator the dt narrows from what ever your initial design dt is to where ever it ends. In principle if dt5 at 7kw with a fixed speed 1205/3600=0.335L/s at min load
3.135/(0.335x4.18)=2.239°dt
That’s nothing to do with a volumiser, it’ll always narrow to that.. to add a bit more.. if as with many heat pumps the compressor stop time is 3 mins, and two stops per hour occur, the compressor actual output is 54/60=0.9x3.135=2.822kw
Each compressor stop deducts 3 mins of output.. so it doesn’t seem to me the volumiser is an issue. My best performing systems have either added volume, 300L buffer on closed loop system, 100L volumiser on open loop systems etc.
what’s really needed is longer stop times."
@@UpsideDownFork Thanks. Very detailed. Yes, my heat loss is around 6.1kWh at -2°C, so there's enough headroom for defrost recovery with my 7kW unit. At the recommendation of my installer, I also make sure that my hot water "Max. cyl. charging time" and" Cyl. charg. anti-cycl. time" settings are both set to 60 mins, so that long overnight hot water runs are split up into blocks of 1hr, with enough time for the heating circuit to recover in the event of frequent defrost cycles.
thanks for the vid, do you have a reference for where the recommended volume figure came from? Just had a quote and they recommend a 25L buffer piped as a volumiser as I think they are assuming any rad with a TRV is not included in the system volume even if I plan to run it all open loop.. My Current volume without volumiser is 165L in theory
I came to this figure by too many hours of reading forums, Facebook groups and studying heatpumpmonitor.
Some more experienced heat pump engineers seem to use 25L / kW as a starting basis to allow heat pumps to work comfortably.
Some heat pumps have slightly different defrost strategies that don't require quite as much volume to be comfortable. Viessman and Ebac are examples of companies who do things differently.
Great insightful video on the water volume thank you. You keep plugging Octopus on your videos and asked why would you not be on Octopus and what are you doing if not? This irks me because my experience of them has been awful. I asked them a year ago for a smart meter, and they had to fit 3 that didn't work before admitting they could not get one to work at our address due to signal issue. Then as a compromise we asked for an Economy 7 which can work offline to get a time of day tariff. This was 3 months ago, and they forgot to commission it so think we still have the old meter and have been estimating our usage wrongly because we cannot submit a reading, and billing us at the fixed standard variable rate tariff while we have been trying to time shift our 2 x EV charging and home battery. They have been an absolute joke for us, and if I didn't know better, I would actually wonder what they do for a living...
Sounds like you have had a bad experience. I can say nothing but good things about Octopus energy in my own experience 😊
Awful experience with Octopus? Don’t go anywhere near British Gas!
The smart meter rollout should have been done by the DNOs, this would have helped mitigate many of the problems and certainly been much cheaper.
I'm really sorry to hear this about octopus energy. I hope through your persistence you will get there in the end.
The smart meter roll out is a joke for many technical reasons.
We've had some issues with Octopus but I still recommend them to friends. I think some are problems out of their control and others are part of what happens when a company grows exponentially fast. Despite any problems I believe we still have to thanks them for being positive market disruptors.
Interresting! But what do you do with your bedrooms? Any in use in my house get turned down to 14 degrees at night, i could not sleep in a room heated to 20 like the rest of the house? I had a Hive system with my old boiler so just kept that to control any bedroom temperatures as required, all the rooms are normally set to a temperature greater than they will reach so the TRVs are always open except occupied bedrooms, seems to work.
It's definitely a challenge. I like a cool bedroom but my wife likes it hot so we've both learned to compromise. Our bedrooms typically sit at 18 degrees during the day and cool to about 16 at night.
If we know our system volume and know that we can safely isolate a couple of bedroom rads without affecting performance then it's no problem. The danger comes when people start turning rads off and the heat pump doesn't have enough volume to defrost itself.
I have mine set to 18 degs from 10pm to 2 am and 21 degs all other times.
That allows the temp ramp up overnight using cheap tariff ready for morning...
This is great timing, as I have been doing some calculations for my heating circuit volumes for an aroTHERM plus 12kW. (As I will be having UFH, and have the potential to the increase pipe bore to therefore increase the circuit volume) I had looked at the technical information sheet for the 12 kW and assumed that as I was over the minimum 45l (I was roughly 140l) everything was OK, but you mentioned a 'recommended' circuit volume which seems much larger. However, I couldn't find this recommended volume on the technical sheet, does anyone know where to find it or how to calculate it?
Thank you.
The recommended for the 12kW is 300L.
25L / kW is a rough rule of thumb for comfortable operation.
@@UpsideDownForkThat's great thank you.
Gives me a figure to aim for when I go over the calculations again and look to see where I can potentially increase the pipe bore to give me more volume.
Thank you, that was really useful. I asked the octopus surveyor about the implications of using smart trvs (my office tends to get warmer than other rooms) and he did not have an answer
I guess the opposite applies, too much volume will be detrimental too
Smart TRV in one or two rooms will be ok until it gets really cold outside then you really don't want to be isolating some of your system volume.
Yes, there will be a point at which too much system volume will not be helpful but most residential properties won't need to worry about that.
My heat pump for example can easily accommodate at least double my system volume without any detrimental effects.
Column rads are pretty good for volume. 1x triple column 1800 high x 450 wide has 27 litres! Big thermal mass in radiators will smooth out cycles and defrosting.
That's a great tip for anyone who has the house to accommodate them.
@@UpsideDownFork they take up the same space as the equivalent K2, maybe 1cm deeper. They have a slightly lower output Vs same K2 but benefits in other ways. Horses for courses 🙂
Good overview. Another reason why oversizing the heat pump (a common installer error) is bad for efficiency.
Thanks!
MCS pretty much guarantees an oversized heat pump, sadly. It's a giant stack of worst case assumptions.
that 99% percent slide actually does get me to like, I'm already subbed but I am forgetful, So I guess it works.
Thanks for the feedback!
Very helpful thanks!
You're welcome!
Very interesting. Can the hot water tank be used for defrost rather than the radiator circuit?
That is not advisable. Because topping up again requires higher flow temps.
My system (Samsung ASHP) will default to using which ever is in operation at the time the defrost is needed. If heating is running, it takes from that, but if it's in the middle of a DHW reheat cycle, then it will take from the DHW tank. Other makes may vary, but there's no way to set or change that behaviour on Samsung systems.
Hi Mate.
Thank you for your great job.
Did you try or do you know If I can set up set up pure weather compensation for the day time 5.30 - 0.30 and fixed value for overnight 0.30 - 5.30 to use cheaper tariff charging my 800l buffer tank (20°C-52.5°C= approx 30kW ).
I have the same HP.
Many thanks
Set the room temp mod. to inactive and you'll be utilising pure weather compensation on a Vaillant.
Then you will need to set different internal temps on your schedule for those different time periods.
You'll need plenty of experimentation to find the sweet spot.
@@UpsideDownFork
Thank you for the advice.
Currently I am on heat curve 0.50 which is fine as a sweet spot. But when I want to charge the 800 l buffer ( second pump controlled by the app and thermostat is responsible for heat distribution) I have to change all the settings and I am not keen to get up at 5.30 to change the settings back for the weather comp. I do not have the kit for app control (probably there is no option in the app either) It might have been challenging for Vaillant to set up controller to run Weather Comp control and later use fixed value settings. But luck of settings to set up different heat curves for different time periods that is not really justifiable. I would like to use night tariff to charge my system but without pumping heat into the property when is not needed. I would set up the right curve to get me the required temperature during cheap tariff. This looks like Vaillant could have done more to have that option in the Senso Comfort. Imagine you can't set up your PV system to charge your batteries using cheap electricity unless you do it everyday manually.
I don't suppose you have any idea how to calculate the system volume of UFH per square meter rather than run length do you (as I don't know the pipe run lengths). It's a 128sqm property with an estimated 120sqm of UFH.
All the calcs seem to be by length of pipe (which makes sense).
If you don't know your pipe spacing then I would suggest getting an infrared camera to expose the pipework under the floor to you.
@UpsideDownFork thanks. Pipe spacing is 100mm and I will be able to get the internal diameter from manufacturer. Would it be as simple as 10m per sqm in that case? Not allowing for turns etc.
I've been eagerly waiting for your December north facing panels generation stats. Have I missed it in one of the heat pump / tariff videos released this month?
Apologies. I woke up with a headache on 26th December and have been very unwell since then.
This video and likely the next couple will be videos I recorded back in November.
I think i'm now on the mend so hope to get back on my feet soon, but the regular schedule will be severely delayed i'm afraid.
@UpsideDownFork I'm sorry to hear you're not well, that's quite a long time to be feeling ill. Rather concerning 🤔
I thought it odd that you missed two Saturdays release days for your previous monthly stats, either that or I had somehow missed them. At least your last few videos will have a few more repeat viewings from me looking for your Dec stats...
Get well soon, very soon... I want them there stats 🤪
@@roblinc2u Thanks. I don't think it's anything serious. Covid/Flu style symptoms.
Very severe coughing, headaches and fatigue like i've never had in my life before.
Less severe symptoms intermittent fever, loss of taste/smell/appetite, streaming head cold type stuff.
I promise, when I can, I will update you!
quick (maybe dumb) question is your 45 litre tank a buffer tank or a volumiser tank? Does it hydraulically seperate the heat pump from the radiators or just add 45 litres to the system volume?
Mine is plumbed as a buffer tank. If I can ever get written confirmation that I can replumb it as a volumiser without voiding my warranty then I will do so.
I was actually hoping to do this back in November but verbal agreement is not good enough for me.
@@UpsideDownFork I thought a volumiser is piped on the return and a buffer on the flow side?
@@jooie444 a volumiser has just 2 pipes. One in, one out. It can be fitted on the flow or return.
A buffer is a 3 or 4 pipe setup. Typically 4 pipe in the UK.
It allows different flow rates to be used by the heat pump and the heating circuit.
How many customers give this any thought before pressing ahead with ASHP installation? I guess it’s only when your system develops a problem it comes to light.
Yep. Unfortunately that might be the case. Hopefully as we increase awareness and all get a better understanding of heat pumps, things like this will be normal to consider.
Ground source, no frost.
If you have a professional installer the customer shouldn't need to consider anything. The installer will design a system that takes account of all manufacturer design rules.
The problem is some installers don't adhere to the installer guidance.
Customers shouldn't have to
Great technical video and very timely for myself. I have Octopus coming out on Wednesday to revisit/resurvey my proposed installation to see if they will do the replacement of existing trunk Steel Pipework in my property so I can discuss with them the overall system volume as this was not a figure presented in the survey document. At the moment they are adamant that they will not increase the size of any of the radiators calculated from the survey last month even though I expressed reservations over the designs ability to maintain 21 degrees internal temperature in my 80 year old property. I'm interested also in their process for confirming/certifying the stated BTU outputs of the radiators from their chosen supplier, there seems to be a lot of talk about stated BTU not being accurate and actual being considerably (20%) lower. If this component of the design is not accurate and they will not upsize radiators it throws the whole design out and the system will not achieve the planned efficiency and required comfort.
Ah, this is interesting stuff. Octopus are definitely hesitant to deviate from their cookie cutter style at the moment so i'll be interested to hear how you get on with them.
What I don't understand with my Vaillant system: Why doesn't it use the energy stored in my 150l warm water cylinder for defrosting ? Or am I just missing this option somewhere ?
Because you will run out of hot water when you need it.
The hot water circuit is too small to use for defrosts....it's not the volume of the hot water cylinder that counts.... it's the volume of the heating circuit that matters......so only the larger heating circuit can be used for a ashp defrost cycle
The system is configured as hot water priority. If you used the 150L hot water from the cylinder, it would be cold after a few defrosts.
Well it is the total heat capacity that counts i.e. volume of water times temperature. As the volume of 150l is comparable to the volume of the complete heating circuit that should be no problem. What makes the difference however is the higher temperature of 45+ C in the hot water cylinder. This should be sufficient for multiple defrost cycles. Of course you have to reheat your hot water after doing so. But there is no difference to heating up your warm water for the heating cycle. Only downside I could see is an efficiency argument: to heat up the water in your warm water cylinder you need 50C hot water whereas the heating runs with 38C - depending on the outside temperatures of course.
Interesting. Wonder how this affects efficiency because it take more power to heat up more water. I guess the additional power required is offset by the lower on off cycles.
Yeah, that's the basics of it. Volume just smooths things out. Decreases cycling. Allows the pump to work for longer at a lower power output.
@@UpsideDownFork Instead of a volumizer, what about those massive Victorian style high volume radiators?
@@ChidleyEngineering yes, that's a good solution if it works in the property.
Sounds like this adds to the argument for having a buffer tank? It seems like the heat pump world is quite divided over whether buffer tanks are good/necessary or not.
I reckon everyone has had a lot of cycling going on this week!
A volumiser to add volume on the return flow is probably more sensible to avoid separation and running a extra circulation pump
@@Burtis89 Yes, the volumiser is there specifically to increase the volume of water in the system, whereas a buffer tank is to separate flows. (I know you know this, just pointing it out to others who may not).
Almost every system would benefit from a volumiser, but a smaller amount may benefit from a buffer tank if things are calculated correctly.
Thanks for this insightful video! I am afraid, however, I am missing a point here. I understand that more volume results in more thermal mass in the system. But what is the effect of more thermal mass on a defrost cycle? (Or: how does having "more warm water" help reduce the number of defrosts?) I guess the heatpump is doing its defrost cycle, no matter the temperature of the heating water. I understand that more volume reduces the lowering of water temperature during a defrost cycle. But how does it impact the defrosting itself?
Credit to Paul Spence for this :
"Defrosts and the benefits of system volume.
I’ve always worked on the basis of 25L/kw to both limit starts per hour and limit the effects of defrosts.
As 1L of water is more or less 1kg and requires 4186J to raise its temperature 1°c, to raise 1L x 5° requires 4186x5=20,930j
Similarly to lose 5°c 20930j are expended.
Assume your flow temp is 45° and return is 40°c , ie dt5 during normal heating mode.
The following is a general description, not exact as there are many variables.
Assume a defrost is 1 per hour and has a 5min duration. The system volume is 100L and the heat pump is 7kw with nominal flow rate 20L/min.
When the heat pump enters defrost mode, the circulation pump continues to run at 20L/min.
All the energy required for deftost is provided by the return water.
As the return water enters the condenser it is 40°c, at 20L/m x 5 mins the whole 100L has passed through the condenser losing 5°c, flowing out to the heating circuit.. but now at 35°c… some 10° cooler than normal operation.
Whilst flowing round the heating circuit the whole 100L loses another 5° via the emitters, so returns to the heat pump at 30°c. The heat pump has to raise the circulating water 15°c to design flow temp.. that’s 3x the dt required.. to reach 45°c again. So the 100L has to recirculate 3x which is 15 minutes at 20L/min just to recover flow temp.
100L x 4186 x 5=2093000J have been expended.. just on that 5 minute defrost.
Given 1kwh =3600000j then it follows
2093000/3600000=0.581kwh of energy have been extracted from your system.
Additionally, the mwt of the circulating water has dropped, from 42.5° to 32.5°, a full 10° lower.. which in turn affects the output from the emitters, so room temps start to drop during the defrost and then less do during the following 15min recovery time.
100/7=14.286L/kw is in my view too low.
Now assume 7x25=175L volume.. still the 20L/min, same 100L circulates during the defrost..
but this time there’s a surplus 75L, consequently the average temperature of the circulating water is ((100x35)+(75x40))/(175)=37.14° rather than 32.5° or thereabouts, that’s 5° warmer and less work for the compressor, recovery time is 5mins quicker. (This is a fairly good approximation but not exact)
Now assume defrost happens more than once per hour, which is quite possible, the effect can be significant, lower room temps, high bills etc.
Defrosts mainly happen when we most need the heating, colder and wet climate, but it can also indicate a loss of refrigerant.
Some heat pumps have booster heaters for defrost, some have dedicated defrost buffers.. both will still use energy, you’ll still pay.
But with an open loop, adequate volume to satisfy the defrost and cause negligible difference in room temps is my preference.
Appliance minimum flow rate typically and 3 starts per hour. Look at the arotherm 7kw, 1205L/hr at max but minimum flow at 540L/hr.
540/3600=0.15l/s
0.15x5x4.18=3.135kw min output of the compressor, hence the min flow rate for fault.
That’s more to do with the min compressor rate 20hz, the refrigerant enthalpy, the suction temp when it’s 15°c externally, the compressor discharge temp would add 3.135kw.
If the min flow rate is 540L/hr and I wanted 3 starts per hour, I’d divide 540/3=180L volume passes per rotation of water.
180/7=25.714L/kw.
Yes the emitters are also offloading at the same time.. so if at base temp your load is 7x.25=1.75kw but the min is 3.135, then the surplus is the difference but it also means system volume would be lower if I took the surplus to be 3.135-1.75=1.385kw rather than 3.135kw just for starts per hour.
I’m aware others hold a different view but smooth compressor run times and less defrost disruption works better at 25L/kw it seems.
If on a fixed speed circulator the dt narrows from what ever your initial design dt is to where ever it ends. In principle if dt5 at 7kw with a fixed speed 1205/3600=0.335L/s at min load
3.135/(0.335x4.18)=2.239°dt
That’s nothing to do with a volumiser, it’ll always narrow to that.. to add a bit more.. if as with many heat pumps the compressor stop time is 3 mins, and two stops per hour occur, the compressor actual output is 54/60=0.9x3.135=2.822kw
Each compressor stop deducts 3 mins of output.. so it doesn’t seem to me the volumiser is an issue. My best performing systems have either added volume, 300L buffer on closed loop system, 100L volumiser on open loop systems etc.
what’s really needed is longer stop times."
@@UpsideDownFork Wow! Thanks for the longest post I read ever! My gut feeling now is: more volume means less impact on heating water (and room) temperature, means less re-heating needed, means less icing on the heatpump means more time until the next defrost...
The mechanics of a defrost cycle requires a 4-way valve inside the outdoor unit to reverse the flow of hot refrigerant gas from the compressor, from the indoor refrigerant/water heat exchanger, to the frosted over outdoor air coil. The hot refrigerant gas from the compressor defrosts the outdoor air coil quite quickly. When complete (time & temp sensor controlled) the hot refrigerant flow is re-directed back to to the indoor refrigerant / water heat exchanger to resume heating the water loop again. While in the defrost mode the indoor refrigerant/water HEX (condenser) becomes an evaporator which takes heat from the water loop to effect the defrost process. If there is insufficient water volume in the water loop there is not enough heat available in the loop (which cools down quickly) to complete the defrost cycle in a timely fashion & this extends the time it takes to complete the defrost cycle or the frequency of them.
In some cases the defrost cycle may not actually clear all the frost on the outdoor coil before a timer stops the process which seriously effects the system performance. While the HP is in defrost mode (cooling) it cannot produce any heat for the house so it is important that the defrost mode is completed as soon as possible so the system can resume heating the house.
A combination of an oversized HP (which requires a larger water volume) &/or, a low volume water loop pipework system, will promote poor system performance & frequent defrosts sequences to the point where very little heat is delivered to the house in colder weather which in turn causes the so called "defrost cycle of death" & short cycling issues. Proper system design is therefore a must.
Defrost cycles for air source HP's occur most frequently between outdoor air temps of +5 to -10c when outside air is damp & humid which then freezes on the outdoor air coil. The outdoor coil (evaporator in heating mode, condenser in defrost mode) operates at around 12c below the ambient air temp (heating mode). Below an outside air temp of -10c the outside air is less humid so not so much moisture freezes on the outdoor coils of HP's & the need for defrost cycles reduces but is not eliminated altogether.
Hopefully this helps a bit.
@@UpsideDownFork would you think there's a value in bypassing that extra 45l volume in temperatures above say 6°C? Get the best of both worlds?
@@MT-zn6tc Nope. Volume is advantageous in milder temperatures too.
It helps to prolong cycles. More capacity to put the thermal energy into before the dt narrows too far on the heat pump when it's down on minimum modulation.
Could some explain the relationship between system volume and the pressure level of the system. Yes, another newbie.
What do you mean by pressure level?
Pressure usually increases with water temperature.
A larger system volume could potentially stabilise the pressure because the heating up and cooling down cycles can be minimised.
I can't see it making much of a difference in any healthy system.
@ I have a gauge on the system which tells what the water pressure is ie 1.5 bar the recommended pressure. Surly if I increase the Volume of water the pressure gauge goes up past the recommended level?
@chrisholland4952 if you add water without anywhere for that water to go, then yes that's true.
The real way to add system volume is to fit a volumiser or larger radiators. 👍
I have a 50 or 60 litre buffer tank in my system and based on the rule of thumb a total capacity of ~ 277 litres. This seems a bit small given I have two 9kW heat pumps, but the LG Therma V manual doesn’t specify anything about the system water volume, so who knows. But not that I could do much about it, can’t really add another buffer tank and I would only change radiators for bigger (which I have done 3 post-install) if it was required to achieve room temperatures.
BTW, be careful about the advice you gave on draining down and fully refilling the system to calculate actual system capacity. My system was at least part filled with ethylene glycol mix in the water to prevent freezing with the outside pipes when the system was turned off. I have subsequently had a leak so that’s all leaked away and been replaced by water, but if like me your system has anti freezing liquid in it, then draining down isn’t the best thing to do
Very good point about draining down. I doubt anyone would go to that length to measure their system volume.
Some heat pump 'experts' seem to use 25L /kW as their rule of thumb but I'm not sure how this would be applied to twin pumps and could certainly differ if running in cascade.
Octopus will have fun sizing the buffer tank for my cosy 10 with 10mm microbore then 😅 Maybe they'll just repurpose my old 210ltr HWC instead of swapping it over 😉
@@steve_787 that's a great solution if you have space for it! 👍
All interesting stuff👍, but not talked about much, when comparing the costs compared with a modern gas boiler.Because the system has a lot more water volume with the Buffer/Volumisers and larger rads, it will take more energy to heat that water. Also in the cold/humid weather, the heat pump will need to de- ice more often, using the heating water with or without the buffer/Volumiser, this water will then need to be reheated by the heat pump over and over again.
Is there a rule of thumb of how much Kwh is being wasted by extra volume and the de- icing process?
Also No one seems to factor the heat losses of the extended exterior pipe runs, I see on these heat pump installalations that arent required with a gas boiler coz its normally within the house envelope.
No energy is being wasted by having extra volume. It's in fact the opposite.
Less compressor stops and starts and less defrosts means more volume is always more economical.
It may take longer to heat the extra volume from a complete standstill but it will have better thermal inertia, smoothing out any peaks.
The external pipework heat loss is very minimal as long as they are insulated properly but can be calculated if you wish to.
Poor performing pipe insulation will require a higher flow temp and result in a lower COP.
COP is the relevant metric to use when comparing gas to HP consumption.
@UpsideDownFork thanks for prompt reply👍
Good information and analysis.
I find comparing your own system against others on OEM is a bit of a double-edge sword. Unless you know the system you are looking at in detail comparing against your own is tricky. Knowing how the household runs their system, things like DHW run frequency, temperature, legionnaire cycles, setback, operation mode (normal, eco etc), rad sizes and so on. So many variables to consider.
Our system defrosts once every 60 minutes when it drops below 5ºC and when it falls below 0ºC perhaps twice every 60 - 90 minutes which is quite normal. We do have a large system volume (which I will calculate now having watched your video) we also have very long primaries much of which are outside. I'm sure they take a hit on heat loss because of that.
Humidity has a massive impact on defrost frequency as well as flow temperature. The more humidity has more moisture in the air which means more freezing onto the fins at the back of heat pump. A higher the flow temperature will also result on more frequent the defrosts as the heat pump has to work harder to get up the temperatures.
Great comment! Thank you.
I totally agree with all your points made.
I agree about OEM. Without knowing how people run their systems, it can be tricky to compare against your own.
As for system volume, I imagine that there's a minimum needed - enough water at a sufficient temp to defrost the system - but beyond that I can't see that more volume is helpful. I suppose a few minutes of defrosting and a few degrees lower in flow/return temp is normal.
My system has been defrosting every 1/2 hour at times, but even at 1C daily average, the COP is 3 - SCOP is still over 4 (including DHW). The best performing Arotherm on OEM's heatpumpmonitor site appears to have similar performance (although a higher COP).
For those of us completely new to heat pumps 'defrosts' are entirely alien
They're not a real problem for most people, just at certain temperatures (mostly 0ºC-4ºC) together at the same time as high humidities the efficiency is lowered. A bad cycling problem indicates an installation problem, as if the volume is too low a volumiser should have been fitted.
A gas boiler by contrast loses 20% of its energy all the time in normal operation, even the 95% efficient ones usually aren't much better than 80% installed, we just know about heat pump efficiencies much more because of the monitoring.
Defrosting is no different to what happens in a domestic frost free fridge/freezer. The only difference is that the fridge freezer uses an electric heater (imbedded in the coil) to defrost the evaporator inside the freezer section instead of reversing the flow of refrigerant from the compressor, as per the domestic heat pump. Nothing new in the world of refrigeration & air conditioning which has been using heat pump technology since 1856 (Peter von Rittinger) & used domestically since the 1940's. New refrigerants have come & gone but the basic principle remain the same.
Apologies that I just jumped straight into the deep end with this video.
Here's another video I made showing what to expect with a defrost cycle ruclips.net/video/aY8UKI6qHCE/видео.html
Perhaps I can do something a little bit more beginner friendly in the future.
Don't Heat Geek videos usually advise against buffer tanks?
Yes they do. Volumisers are preferred.
Gas boilers were also very commonly hugely oversized. Seems to be programmed into the mindset of the heating industry the idea that bigger is better and now even more of a problem when applied to heat pumps.
We are not alone. Domestic heating in the USA is on a totally different scale of oversizing with massive basement furnaces that would be suitable for a factory or a school building in the UK.
Everything is moving in the right direction.
At around 0c my 5kw heatpump defrosts around every 90 mins and it lasts about 20 seconds, impressive to see a huge cloud of steam being blown out
Thanks for sharing.
If you go outside and observe your heat pump you will see that it takes much longer than 20 seconds for the whole process. A quick defrost on a Vaillant pump will be something like 2 minutes.
Humidity pays a critical role in defrost frequency too.
@UpsideDownFork probably, I just timed from when I see the electric consumption change followed by the start of visible steam at the rear and front then the finale of steam ejection with the fan being activated.
If you are getting good performance with your 7Kw Vaillant, I would leave well alone and keep your buffer!
Don't try and eek out a little bit more performance, for something that might help when the weather gets below the design temps.
In our UK weather the defrost cycles can cause havoc and the published output data on the 7Kw unit, doesn't seem to live up to the figs that is purported. The heat geek guru Urban Plumbers ( they don't fit buffers!) has had so many problems with the Vaillants, noisy, under performing by 20% etc, he has fitted resistive heaters into the systems for extra heat, and is now NOT specifying Vaillants.
It also seems the COP figs arent correct either, coz they dont take in consideration of the kwh cooling of the heating system on defrosts cycles. Which on some systems was 0.4Kwh per defrost. The system only counts the energy in and the heat produced.This seems for all manufacturers though🙄
Thanks for commenting.
I’m getting half an inch of snow on the back of the unit and even when it defrosts the bottom third was solid ice. 500l thermal store but at only 23C…
A third staying frozen solid? Crazy!
@@UpsideDownForktried a hot water cycle still some left but don’t think that’s normal - maybe a faulty sensor or the low circulation temperature
I have Vaillant 5kW, 200l+ volume, lots of high intertia UFH, running very low flow temps (25-35). I'm still seeing worst case defrosts of every 30 mins or so, but the defrosts themselves are short, less than 5 mins.
Sounds good in this weather. High humidity and low air temps can be really punishing!
I hear a lot of comments saying don’t have TRVs to increase volume and thermal mass but we like having different rooms at different temperatures at different times of the day. I guess having more emitters in the hottest rooms and a volumiser can help. Interested to know your thoughts?
Different rooms at different temps is no problem.
Good heating design will address that. Larger rads in rooms you want at 22 degrees and relatively smaller rads at rooms you want at 18 degrees.
Now changing the temp of those rooms throughout the day relative to each other is a different challenge and one that is not ideal for most domestic heat pump installations. You will sacrifice some efficiency to meet your goal from what I understand you are trying to achieve.
Quand on a la place un ballon tampon de 1000 litres c'est l'idéal :) ça apporte aussi une réserve pour les heures ou l'électricité est cher .
Yes, that would be excellent if the space is available.
I didn't look into water volume before I got mine installed. I just calculate mine and it's 170L for the Vaillant 5kw. So about right I guess. Its been running 529 hours and 216 on/off cycles? Anyone else on/off's similar?
170L for a 5kW pump is great! Your run time vs cycles also looks excellent, but it probably won't be able to maintain that as we get into the shoulder months and you hit minimum modulation.
And you didn't mention the pitfalls of system volume when in DHW mode........
The correct way of calculating system volume is considering the number of stop/starts permissable and the kW output of the unit.
Thanks for commenting.
I was trying to keep this fairly basic for owner enthusiasts without getting into a serious maths/engineering class.
I don’t have octopus. Based in Ireland.
Thanks for commenting.
means nothing. outside humidty is the only thing that matters
Both matter. One we can control, one we can't.
Your rule of thumb estimate was off because you doubled the radiator capacity. Check your figures again.
7L is per panel, not per radiator.
All of my radiators apart from 1 are double panel rads so 14L each.
Does that make sense?
@UpsideDownFork 7l/m for a K2. You've then doubled that. Check again.
@@71brp84 If you're talking about the rule of thumb from 2:50 - 3:50, that's not 7L/m for a K2. It's 7L per panel.
If you're talking about the calculation method from 4:00 - 7:00 then it is 6.6l/m for a 600mm tall K2. My calculations at 4:50 look fine.
@@UpsideDownFork your rule of thumb also doubles all the pipework, not just the panels, which is why it's so far off.
@71brp84 I agree it's flawed.
It's not a rule of thumb that I concocted.
It was shared with me by an experienced plumber. He seemed to think that anyone of a certain generation who built domestic heating systems uses this exactly as I described in the video.
Just install a ground source heat pump No frost ever .
Fine if you have a load of land and/or a load of money.
Sound like a lot of messing about for a massively expensive system with no real savings compared to a efficient cheep to run gas combi boiler.
This system cost me less to install than the quotes I got for a combi boiler.
It saves me a couple hundred £££s in running costs every year.
Thanks
of course it did🤥
@@Cyberbobxxxx full details in my other videos including proof of quotes and energy bills etc 👍
Here we go again " The flow of B/S is never ending". My COMBI BOILER HAS NON OF THESE PROBEMS. I do not need UNDER FLOOR HEATING .
@@GrahamNewman-mq7gr who are you quoting? Also, why are you shouting?
@UpsideDownFork People want warning about all this HEAT PUMP B/S been thrown out by you EVANGELISTORS.
@@GrahamNewman-mq7gr I've just had a heatpump installed. It's made a huge difference to the overall warmth of my home and been great over the freezing weather we've just had.
It's embarrassing shouting about things you have no understanding of.
@AlanTov No UNDERSTANDING realy. I am fully converstant how these things work. Marine career Chief Engineer 50,000 ton LPG carriers .Large Container Vesels up to 200 Refridgerated Containers. These systems are a "Complicated answer to a non existant problem". A Combi Boiler turn it on "Hot Water " as you want it , as much as you want , Bath / Shower ect. Heating turn it on Radiators hot 62C after 20 minute All house up to temp under 1 hour. Your "Heat Pump will never do this". Latest Combi Boiler £2000 Installed, 10 year parts / labour warranty. You probably have "A 4 WHEEL ELECTRIC LEMON" DEPRECIATING £500 A MONTH ON THE DRIVE" to complement the HEAT PUMP.
@@GrahamNewman-mq7gr ok. We'll have to agree to disagree.
Thanks for stopping by.