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sorry for the delay, i overlooked your ,message. There should not be a problem linking an existing UH. So your dt is about 2.5 degrees and flowrate 14 lit/min? I think that equates to about 2.5kW of heat. is it likely that your heat pump is on low-speed (throttled down). Is your UH 157sqm? To increase the DT, either the heat quantity needs increasing (turn up the set-point/curve) , or the flow rate needs reducing. I would be happy enough with a dt of 2.5 if your temperature is being achieved. I assume no mixing valves on manifold? anyhow, underfloor tends to prefer a closer dt than radiators.

You can experiment by limiting the radiator flow temperature and running more constantly, rather than the on-off cycling. For much of the year, the actual reqired radiator temperature can be consideranly lower, especially in reasonably well insulated houses. The response time may not be as fast as a boiler, so the way its used may take a bit of adjusting to.

If you could limit it to 5kW... It should help. Most heat pumps go to a high output (around 8kW in your case). This is to warm the system up. However, you may only need say 2kW at times. By limiting the output, the system should spend more time at a lower water temp, so COP should be better. Very few heat pumps allow power limiting, but 'quiet' mode might help for some. Quiet mode seems to simply reduce maximum setting for both compressor and fan. However, some manufactureres may slow the fan down more than the compressor. This could reduce efficiency a little, but the efficiency gains of running the unit lower should outweigh this for cases of oversized ASHP

@@johncantor4056 Tnx

@@johncantor4056 I just found some info for the silent mode , 30% less on both the fan and the comp , its good enough since i have a problem , the pump is next to my bedroom and its very loud :S It feels like the fan is on max all the time .

@@toniilievski3934 seems sensible to enable quite mode. You can go back to normal if needed in the depth of winter. I expect a lot of people with oversized heat pumps might benefit from quiet mode

@@johncantor4056 It went to 2 cycles /h , its working better , cheers .

The old-school way of plumbing rads is like I show, in at the top, and as you say hot water will stay above cold water, so the temperature and flow is even as the water flows (pumped) down the radiator. The return should always be at the bottom (the coldest part), but if you find a thermal image of a normal radiator (both connections at bottom, you will see the hot water rising to the top in just one of the vertical channels, then the hot flows along the top, and down the area of the radiator. The old method of top entry can be 'same side' or 'opposite' side. ususlly depending on if its height-length ratio. Re the pump position, it makes very little difference where this is in the circuit. The pressure gauge and expansion vessel would be in the 'suction' side of the pump.

Yes, exactly, inverter systems should do that, and will reduce to about 40% of full power in milder weather. (they go lower in colder weather, but that is not when you want them low). However, they cannot modulate down as far as we would like, so some sort of cycling is bound to happen in milder weather. In mid winter, i would hope many would run constantly, modulating between say full and 1/2 power as required. Before inverter motors, compressors would start with quite a kick, causeng wear & tear and if frequent, overheating of windings. But now they start nice and slowly, so I am not sure if stop-start is quite as detrimental as people think. I think very few heat pumps actually 'die' of too many stop-starts. Its mostly other component failure that writes them off. That said, its definately good to reduce the number of starts if possible.

Sorry for the delay. Yes, you are right, often no room for buffers. A low loss header should take little room. Anyhow, i do like the simplicity of a direct connection as you describe. The flowrate for heat pumps should be higher, but heat pumps are usually considerably smaller than the boiler they replace, and run for far more hours (longer and lower). So the head pressure mau be similar. Persionally, I sometime look at existing radiators, and ask myself, what size heat pump would be happly with existing rads. Increasing house insulation, so as to reduce heat need, can e a good plan. I think I answered for 1) & 2) there. If you mean CO2 refrigerant, then the flow rate will probably be lower

Well... buffer tanks have been used in the way you describe for a long time. If space is available, yes, they can be a good idea. There are however a few down sides. Often, the average water temperature that the heat pump 'sees' is a few degrees higher than the emitters actually get. This rise tends to reduces to COP. Also, if the buffer is going to store any amount, it may need to rise several degrees above what is needed... e.g. a 200litres (typical size) tank , with 10kW heat input, would rise 10c (18F) in only 14 minutes, (using heatpumps.co.uk/calculators/ ) So buffer tanks need to be quite big if they are going to store a wothwhile abount. They also add a considerable expense and complexity. It may be worth it in some cases. Again, the majority of houses would probably prefer not to loose the space required for the tank.

Hi John, thanks for taking the time to reply! My 8kW unit's pump is plenty powerful, it pushes over 1400 litres per hour when it does DHW. That part of the circuit is 32mm multilayer pipework, it allows for such flow and so the pump delivers it. The low flow rate problem only appears when pumping towards the radiator circuit, it's probably caused by the 16mm multilayer pipework (specifically, the multilayer pipe connectors, which cause significant internal constriction at every junction), combined with the fact that my radiators are connected in series. Why do I think that this is the problem and not the pump? Because my brother owns the 6kW version of the same unit, which has a slightly weaker pump... yet he gets 1000+ liters per hour when pumping towards the radiator circuit. Granted, my installation has 14 additional radiator elements (equivalent to an extra medium-sized radiator), but this isn't why his flow rate is so much better. He gets higher flow rates because his radiators are all connected in parallel, and also because the circuit is made with 16mm copper pipework (free of internal constrictions). Brute forcing my unsuitable radiator circuit and pipework with a second pump doesn't seem like a good idea to me... the increased stress on the connectors, the noise, the potential pump coordination problems, etc... too many problems are likely to arise. I might try to replace the pipettes inside the radiators (the tubes attached to the radiator valves, which extend into the radiators, to ensure that hot water reaches the other end). When I added the 29 radiator elements, I extended the pipettes of several radiators using narrow pipe extensions...perhaps too narrow. If replacing those pipettes doesn't improve flow rate, I'll try replacing the radiator valves... if that doesn't work either, I'll have to accept the low flow rate and live with it. Thanks again John!

@@Goreuncle I didnt think there has ever been a heat pump connnected to an in-series radiator connection. normally there is around 5 degrees temperature drop from radiator inlet to outlet. If you had 10 radiators in series you would need 1/2 degree drop across each, and 5 degrees over the total. internet search for radiator connections and look into copying your brothers setup. Actually, the restrictions due to fittings can be less that they might appear.

@@johncantor4056 My setup is as follows (excluding the DHW part): 8kW unit > (32mm multilayer pipe) > 1 1/2" 3-way valve > (32mm multilayer pipe) > 1 1/4" heating manifold (with only 2 ports) > (2x 16mm multilayer pipes, one for each radiator ring/series) > first radiator of each ring/series > second radiator of each ring/series > etc. I have 2 series of radiators (5S and 4S, 9 radiators total), these 2 series are connected in parallel, the pipework runs under the floor (ceramic tiles + cement) and inside some walls (brick + cement + plaster). I'd have to wreck my home in order to redo my radiator circuit completely in parallel, so copying my brother's setup is off the table 😭 As for delta T, my unit can't keep delta T 5ºC when it gets cold outside, the unit works at delta T 8-9ºC (~1ºC drop across each radiator, as a simplification). As for the 16mm multilayer pipe fittings, they look crazy small 😅 Thanks again for your insight, John!

I think installers are worried about what happens on the coldest day, and no doubt one or two people will control their system badly (not keep the 'plates spinning'), and expect to heat up a house from cold on the coldest days. So, fitting a big one can be a safe bet for an installer. I'm sure that like you, many people would be better off with a smaller unit, on the other hand, a few would be miserable in the depth of winter. Not sure I share your trust of social metdia.. a lot of good advice, but just as much bad advice, and impossible for the uninformed judge between the two. I often get depressed when browsing FB. In the old days, the only cost effective heat pumps were small ones (like yours), and geberally these won't need buffers or complexity. I have a lot of experience of tiny heat pumps feeding big existing radiators, and all supported with woodstoves for the coldest days. If you browse, you will see refernce to small heat pumps 'straining'... not the case. heat pumps love heating tepid radiators. People need to make sure there isnt an electric boost in there somewhere. As you experience.. a lot better running, with less cycling, on average winters days... of which there are many.

​@@johncantor4056 Hi John, thank you very much for making these videos, your calculators and taking the time to read comments and reply. Here in Spain many heat pump installers shove grossly oversized heat pumps down the customer's throat and, on top of it, blindly insist on buffer tank + secondary pump. If you tell them that you won't accept an oversized unit with an unnecessary buffer tank + additional pump, many refuse to do the installation, some even go as far as making you sign a waiver (they've watched too many lawyer movies 🤣). I understand that these companies are weary of clients who don't know how to run a heat pump system, so they err on the side of absurdity (and heftier bills... charging double of what they should and cashing in the EU subsidies that are meant for the customers). Anyway, one of the things that bothers me is that these companies don't bother running the numbers. For instance, I was repeatedly offered a 12kW unit despite the fact that my radiator installation could barely dissipate 6kW (assuming ΔT 30ºC). I don't know why I bothered making and sending those companies a detailed schematic of my home and radiator installation, they paid no mind to it. In the end I added 29 radiator elements (close to 2kW of extra dissipation capacity, just in case) and chose the 8kW unit, which turned out to be in the ballpark of what my home needed. A 12kW unit would've been beyond overkill, since my home needs around 6,6kW in the coldest hours of winter. Regarding social media, Facebook is the last place where I'd look for advice, I joined a Spanish Telegram channel dedicated to heat pump systems, over a thousand members, hundreds of heat pump owners sharing installation details and performance in a spreadsheet, technical manuals, schematics, tutorials, quotes, advice... pretty useful overall. But still, I didn't fully trust their advice either, they were telling me to get a 6kW unit and got an 8kW unit instead 😉 The only thing I'm worried about is the rather low flow rate I get when the 3-way valve directs the hot water towards the radiator circuit. I get ~670 liters per hour, when it should be closer to 1000 liters per hour, which I guess is caused by the narrow piping of the radiator circuit (16mm multilayer pipe, in series, only 2 rings, 60 radiator elements per ring). With this rather low flow rate, my unit is forced to increase its working ΔT to 7 or 8ºC (in order to output the same kW), instead of the ΔT 5ºC it's designed to work at. Do you think this is a problem I should be worried about? I can't change the piping of the radiator circuit, since that would involve destroying my home😩 I know it's not a problem with the Y filter, the dirt filter nor the unit, since the flow rate is nominal when the unit does DHW (3-way valve directs the hot water towards the DHW tank exchanger instead of the radiator circuit). Thanks again for your time and expertise!

When the heat pump is running, it will always be better with dt30 design (radiators at 50c). A problem I have seen with a hybrid was the radiators were quite small, and if TRV close rads off, it can cycle inefficiently. I will be fitting bigger radiators in the main living room simply to keep the heat pump running more efficiently

@@johncantor4056 thanks John 👍 have you an email I can contact you direct mate ?

Hello Anthony.. i look forward to watching your video through ( no time right now!!).. One initial question.. how accurate do you think your heat assessment is when dt is less than 2? A small sensor error will make a big difference. with no heat, bit pump running, what dt is showing on the heat meter? Is it zero? I will watch your video soon,

@@johncantor4056 Hello John I find the flow sensor minimum reading is 6.5l/min (which is also what the manual says) and the next lowest reading is 7.1l/min so that’s just over 10% error. Likewise when it comes to the primary current reading for the compressor the lowest readings are 0.5, 1.3, 1.8, 2.3 Amps etc… There’s absolutely no variation in between those discrete readings. What this means is very choppy current and flowrates and therefore very choppy instantaneous coefficients of performance. However when you average it out over the course of 10 minutes then you get some quite revealing trends happening, I tend to use 10 minute averages to determine what’s going on with my heat pump. DeltaT temperatures are accurate to within 0.1 degrees on both the flow and return temperatures. I am able to determine the accuracy of my calculated power consumption by sampling the readings against the power meter feeding the heat pump and the averages are very close but instantaneous readings can be off by quite a bit.

So, the pressure we are varying here is the difference in pressure across a system. Most circulation pumps will manage about 5m head pressure (yes some are more). this is about 0.5 bar. So, if your system is pressurised to 1.5bar, the circulation pump inlet should be 1.5 bar, and the outlet could be anything up to say 2bar, but you won't know this.. there is no outlet pressure gauge. If your system pressure were low, at say 0.5 bar, then the pump outlet might be 0.5 + 0.5 = 1 bar. The simulator relates to resistance of flow in a system, and in general how small-bore pipes can be a problem, needing excessive pump pressure and pump energy. does that make sense?

@@johncantor4056 Thank you for the clarification, then i can reference here as the difference in the pressure.

Its all done with JavaScript. Ellie did all the clever stuff.. its quite a learning curve I think... quite hard and time consuming

@@johncantor4056 I've already looked at the JavaScript and can see where the flow rate adjusts the animation speed of the pump and the dots in the pipes, but it was the graphical side that I was wondering about - Specifically the moving dots in the pipes which animate perfectly through the corners. The graphics are done using an SVG. I was wondering if she wrote the SVG himself by hand, or used a graphics tool to produce it? E.g. Adobe Illustrator or I'm hoping she used a free tool as that would make the graphical side of some of my own stuff much easier. I usually write my SVGs by hand but it's a pain to do so I'm hoping to find an easier way and this is a really nice example.

@@yngndrw. It was a lockdown project, and I know it took her a long time.

@@johncantor4056 Ah okay, it sounds like she did the artwork by hand then. Thank you for your responses, I was just hoping to find a tool to make it quicker than doing it by hand but it looks like I'll have to continue doing it that way. Thanks.

Yes, I am talking the average temperaure, so if radiator inlet were say 36c and outlet is 30c, then the mean is near enough 33c. The difference in temperure between water inside and the paint on the surface is very small. If you do a rough calc of heat transfer through the 2mm of steel (or aluminium), its small enough to ignore. No doubt a lot of laminar flow inside the radiator, but its still a small temperature difference, particularly at low heat pump temperatures.

Hi Craig, good question. Not that simple to answer. I would tend to go with R290 given that in the future, you should easily get it. R32 is expensive, and might get harder to get in time. Yes, you don't need the high temp, but either refrigerant should perform similarly at lower temperature. Re noise, I think some of the first R290 were (are) noisy, but now the big manufacturers are using it, noise should be the same. It is worth 'drilling into' the performance. Because R290 has refrigerant charge limit, some compromise may be sought, and this could reduce COP, but only in some cases. Sorry to be vague.

Thanks for your feed back. What else/ what topics do you think would be useful?

⁠The issue of maintaining a high flow rate for heat pump efficiency seems to come up a lot in discussion groups, whether that’s expressed as pros and cons of buffers, zoning, or TRVs.

Thanks for your comments.. I wish I could answer your question, but I dont actually know how the air-air control works. I would expect the system to register how many degrees the actual room temperature varies from the set-point , and respond accordingly... surely that is how it must work. i.e. as the room setpoint is approached, the compressor slows down, along with the fan speed. this gives the best comfoort. But does it also vary with respect to the outside temperature? I would expect it does. Given that a heat pump could give out twice the heat in mild weather compared to what it can give out in cold weather, then some sort of limitation for mild weather is surely part of the strategy. I would be interested to know.

Thanks, I think you’re right. Outside temp and temp differential (outside to set point) should be what the computer is most interested in. The heat pump must work harder if outside temp is cold and if your set point is a large jump up from current room temp; hence my theory of inching my thermostat up a couple degrees at a time as room gradually warms. Hopefully keeps compressor work light and air flow temp lower. My rough estimate is my heat pump electricity consumption goes up about 3% for every degree I turn up the thermostat and also goes up 3-4% for every degree the outside temperature goes down. Either way it makes sense as I’m increasing the differential from outside temp to thermostat set point. My electricity use was doubled with our recent week of 16 deg temp. House is old, could use better insulation, and Seattle homes not built for that. Thanks again for interesting discussion Steve

@@yt551217 In colder weather, the compressor will need to spin faster to get the same heat output, and in cold weather, more heat is needed, so surely it manages this intelligently in some way. Sounds like you are doing the right thing by increasing it slowly

Mmm.. its a little hard to properly monitor such a thing. I dont know of a way of knowing the heat ouput to air. Its useful to measure the power, and a CT clamp to measure current (assuming 1kw = 4 amps) should be good enough. What do you want to learn? It would be interesting to have room sensors (emonTH, with a probe sensing the warm air coming from the unit... I would find it interesting, but not sure what you would learn from it. cheaper options might be to buy some wifi thermometers and display it on an app. I guess you has fan-speed options?? if so, fastest air is probably most energy-efficienct, but noisy and possibly drafty. maybe you can help to compare input power with different air flows. possible my 'Carnot COP estimator might help you guess the COP

All I can think of is that you could try a compromise... lower the dt a little and see if you can record an improvement on running cost. There will usually be a region (maybe 4-7 dt) were the performance difference is not great. Circ pump power will increase if you speed the pump (unless you can open some flow restrictions to increse the flow) However, this exta may not be much with the latest pumps, and can often be outweighed by the compressor running cost saving.

​@@johncantor4056 Thanks for your answer. Everything started when I decided to install auto-balancing actuators, and I noticed I could not reach delta T of 7 degrees even if I opened all zones fully. I estimated a flow of 6 L/min through the manifold; that's why I think the pump runs at minimum speed, and I can run it faster. The difference between 1 and 2 speed levels is 55W, which makes 40 kWh for a month - not much. Also, with this low flow, the HP cycles a lot when there is low demand and weather outside is hot. Maybe with higher flow this would be better. But still I also thinks that I have to measure the difference.

@@nkitanov I have never tried auto-balencing valves.. I have never looked into them.. Have you adjusted them relating to the radiator size? I'm wondering if they could be an unecessary restriction?? Are some of them around the max setting? I should get one to try it.

@@johncantor4056 it's UFH, and the problem is that I do not know the lengths and specs, so tuning them blindly. That's why thinking of using auto balance valves, which have temp sensors of the flow and return flow, and by opening and closing the valves, they try to keep delta T of 7 degrees. Anyway, I checked the pump, and to my surprise, it's running at maximum speed. So, I won't be able to achieve better delta T without adding an extra pump and will run it like that. It's a bit strange that the pump even on max speed cannot reach the needed circulation.

@@nkitanov So, 7 degrees is quite a lot for UH. If say your flow temperature was 35C, then the return would be only 28C. this would be felt as quite a gradient in the room. (hot and cooler areas). Can you reduce the dt to 4 or 5 max? It sounds like the auto adjusters might be 'throttling' the flow in an attemp to get a dt of 7. In general, in a normal situation, at least one adjustable flow restrictor on the manifold would be fully open. Often almost all of them are fully open. It would only be the shorter loops that need restrictiong, (as you are trying to achieve).

I think I am getting at point that its generally better to keep the heat pump running in cold weather, so you dont really want the room thermosts turning it off... e.g. lets say it's running very happpily and steadily in cold weather (though might be interrupted by defrosts every say 40 mins). Anyhow, if the thermost turns it off, then there can be some catching up to do which may not be easy. Ideally the compressor will modulate down and keep running. The manufacturers thermosts is usually best, and this is where 'room target' or 'auto adapt' can work very well. In your case, I guess you are trying to vary/reduce the water temperure to see if say 40C is enough etc. Maybe your smart stats are trying to be too helpfull! can you simply turn the TRVs to max for your experiments?

Hello. I'm wondering if this 'cycling' is actually defrosting. This can happen about as you describe, and will a lot in cold damp conditions. Actually worse when say 0C to +5C. Nothing to worry about. If you catch it stopping, you should see the white frost on the fins melt over a few minutes. Otherwise, in this weather, the unit is likely to run steadily. They generally vary (modulate) between 33% and 100%. When the heat demand is lower, (milder weather) it will need to cycle, and cycling on/off mid range can sometimes be better than running steadily at 33%. difference between the two is often slight.

Unfortunately long runs are not good for ds18b20. A single sensor on a device might be ok, but I guess you want several. Problem seems to manifest as random occasional way-off readings that can be filtered out....to a point. Screened cable may not help. It might be a matter of experimenting. Good thing is.. being digital, if you get readings, they are accurate...

I'm in the exact same situation. My WORST scenario in actual heating usage (calculatede on 6 years bills) is 4,5Kw, they quote a 8Kw heat pump that give 7,28 Kw @ -7/35 ... and my external project temperature is -5 so .... WHY???

Can you explain where you mean? at 14:30 I'm seeing the 5kW inverter ASHP that starts to cycle 1/2 way through flow t says 36.5C. You are probably looking at a different graph. Where is the dt 0.8?

Hi John, thanks for your reply. I'm meaning at exactly 14:40 on your heat pump simulator video where the deltaT is showing as 0.8°C with the power input at 0.35kW. Hope that makes sense and thanks again!

@@brookejonny Ah.. I was looking at the wrong video! Yes, so from the point of view of the simulator, a flow-return dt of 0.8 is fine, but pumping power is high and circulation ight be noisy. For an old fixed-speed (pre-inverter) heat pump, such a high flow should not impair COP. However, we are observing oddites on the graphs when dt is fairly close, and this is likely to be the complicated algorythms that manufacturers use to comtrol the compressor speed and regulate the refrigerant flow valve. This control strategy is a compromise of doing a good enough job of control without excessive purchase expense and complexity. So, its probably best to stick to the manufacturers recommendations, and all seem to like a dt of around 5. However, some ask for a certain flow rate, and if your 5kW ASHP has a dt of 5 (@14litres/min), then if you stick to that flowrate, and the output modilated down to 2.5kW, then the dt is 2.5. This should be fine for many models. A bit complicated isnt it? However, often we change the dt and hardly see any differnce in performance (bear in mind that some heat meters lose accuracy dt less than 3!!!)

@@johncantor4056 Hi John, ahh very complicated indeed! So many variables which are all intrinsically linked. In my situation I have an 8.5kW heat pump that seems to be fixed speed around 24litres/min (Ecodan). I have been trying to reduce my flow temp as it was my understanding that getting this as low as possible would optimise COP.... BUT in doing so the deltaT reduces down to approx 1*C. I suppose my question is whether I what I am doing is actually a good thing for COP or if lowering the flow temp so much that it creates a small deltaT is counter productive for running costs? Very complicated indeed, but you seem to truly be an expert on all the variables! Thanks again John!

@@brookejonny 24 litres/min is what Mitsi ask for, and this should give a dt of 5 when on maximum. Do you ever get 5? At minimum speed, say 3.5kW? you dt might drop to 2 ish. If you rarely run at 8.5, than maybe a lower flowrate is a bit better for you?? Is your 1 degree measured accurately? Where are you reading it from? So, to drop your flow temperature, you can drop the set-point or curve settings, but now with cooler radiators, you will dissipate less heat, so you might cycle. Sometimes this is not too much of an energy reduction, but sometimes it is. Having more radiators in circuit (with trv valves on full) will help to dissipate the heat at low temperure. If all your rads are on, you will probably get a great COP, but arguably heating more of your house than you need is a watse.. it all depends on the size and type of house.... Dont just measure COP, record actaul energy input for comparisons. Yes, sorry, its complicated.

Good question. IF the flowrate is the same (fixed-speed circ pump), then the flow-return dt reduces in proportion to heat output. Not seen evidence of any units happily dropping output more than around 30% in meduim conditions (lower on coldest day though... when you dont want low output!). IF the pump can modulate, then it could 'track' a dt of say 5, but at minimum compressor speed, the flowrate might be too low, so I think there would be a minimum speed 'capping'. I dont see much of a problem with low dt when at low speed, but possibly, some algorythms assume a certain dt, so very low dt could mess up the compressor speed control... possibly... I dont know. Another thing to consider is that any heat meter can loose accuracy when dt gets less than say 2 degrees... some even stop recording!, so it can be hard to proberly record heat output at minimum output. dt of 5 is I think a compromise. not wasting circ power, not too noisy, and the flowrate when dt is 5 is about right. If dt more, then average radiator temperature is significantly lower than the flow temperature... not ideal.

It also for the glide of the refriger gas they use like R410, propane or R32, different gas different glide of condensation different delta T

@@Bushtuckerman71 can you please elaborate? possibly linking where to find the theory. thanks

Sinmple answer.. dt of 3 to 5 ? Its actually hard to be specific, and may depend on the pattern the floor pipes are laid, and also on the temperature its designed to run at. So, if the dt is too much, then the flow to the floor loop is significantly warmer than the return back, so the floor area at the start of the loop is notably warmer than the end of the loop.... the floor feels uneven, and may, or man not be warm where you want it. If the flow is 45 and return 40 (dt5) then uneven-ness may not be too much, But if the flow were only say 30, and return 25 (dt5), then the unevenness could be significant. That said, uneveness may be more acceptable when its fairly mild out. The other thing to consider is this... you would emit about the same heat (kW) from a system with flow 45C, return 35C (dt10), as you would with a floor with flow 42C and return 38C (dt4). (mean water temp is the same) The COP should be a little better with the lower flow temperature, even though the return temp is higher. Finally, a spiral pipe design wont have the one-side-of-floor cold problem. Personally, i like the simplicity of having no mixer, but some people would advocate having a mixer and pump so that you can have the floor dt closer than the HP dt. I dont think the dt at the heat pump and for the floor are actually so fussy

You are right that COP at start (after a long rest) is poor as it warms everything up. Compressor would be warm though if starting when cycling is frequent. There is however another thing at play... at start-up, the compressor pressures are equal, so the compressor has a very 'easy ride' at first, until the pressure difference across the compressor builds up, so at start, if it were not for various inefficienies, the COP would be much higher at start up

Hello Henry, sorry for the delay. This was never designed as a design tool, more a teaching aid to help people learn what happens without having to get a pump, some pipes, a bucket and a stopwatch. There is a good online pressure-drop calculator www.pressure-drop.com/Online-Calculator/ that is probably better for any size.. this also allows for temperures and other finer details