MEV & MVHR Retrofits and Why They Should Not Be the GoTo Strategies
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- Опубликовано: 24 авг 2024
- There are around 28 million existing properties in the UK that must have insulation retrofits if we are to get anywhere close to achieving our Net Zero ambitions. We've known this ever since the 2050 carbon reduction deadlines were first committed to well over a decade ago.
Whilst these insulation improvements are mandatory for each and every property so too is simultaneous ventilation performance improvement without which we will further compound the increasingly well recognised and understood problem of health and wellbeing issues caused by poor indoor air quality.
So what is the best way to improve ventilation in an existing building ? Opening windows is the usual suggestion but it wastes energy and costs money and is often not effective. Install trickle vents or air bricks is another common one but this too wastes energy, is usually not effective and will make a condensation and mould problem worse. In fact any strategy that relies on natural background ventilation via opening windows, trickles or air bricks should be avoided in this context because it wastes energy without being very effective.
So that leaves mechnical retrofits. As mentioned above we can forget System 1 Decentralised or System 3 Centralised Extraction (MEV) because the supply component of these is natural and wastes lots of energy without being very effective at all. How many times have you seen a property with extraction in the bathroom and kitchen but with condensation and/or mould in the bedrooms ? Now you know why ..
The two other retrofit approaches are MVHR System 4 and PIV. Lets talk about MVHR first. Since the pandemic MVHR has been cited as the go to retrofit by certain membership bodies without any reference to the practical issues affecting its performance. MVHR was originally envisaged for whole building new builds as the ductwork infrastructure can be designed with the building taking into account run lengths, resistances etc. In a retrofit it may not be possible to route ductwork to be relevant spaces not to mention other key factors such as air permeability. Even with today's auto compensating MVHR units and semi rigid duct systems retrofits are usually not feasible and certainly offer less energy efficiency than PIV even with 95%+ heat recovery. In our experience, unless you have a loft space or sunken ceiling void with no obstructions and covering the entire building footprint then MVHR is never feasible.
PIV is the only whole building strategy designed for retrofit. It is cheaper and easier to install than MVHR (yes even ducted flat units) and if done correctly it is equally or more energy efficient than MVHR. Many PIV installs are not done correctly due to lack of proper knowledge in the industry. Most unqualified specifers or installers think you just put it in a loft or on a wall and turn it on and thats it. There's a lot more to it than that. Perhaps the biggest issue with PIV understanding is the PAS2035 suggestion that trickle vents or air bricks should be used. Anyone with serious PIV installation experience will tell you that is nonsense and in fact reduces performance and (you guessed it) wastes energy. Another key aspect of a PIV solution is the importance of effective extraction which means energy loss compared to MVHR but this can be minimised with humidity tracking extraction or brought into similar line with MVHR with the use or SRHR units. In our experience PIV is always feasible in a retrofit and almost always preferable to MVHR for energy efficiency. PIV (and properly installed MVHR to be fair) means the entire building gets all the ventilation it needs regardless of occupancy size and habits without the need to open windows at all during the colder months of the year. No wasted energy, reduced heating bills and better health and well being.
To sum up we must stop making the mistake of saving energy by improving insulation only to throw that energy away by opening a window, trickle vent or airbrick to ventilate. If we are to have any hope of hitting Net Zero without creating a health and wellbeing crisis we need to retrofit 28 million buildings primarily with PIV but without the PAS2035 trickle/airbrick recommendations.
I'm really not sure I understand this advice.
We live in a cavity-insulated 1980s house 2 floor house which is not airtight but certainly does not breathe. We have the upvc windows cracked all year round wasting lots of money on heating but even then, with the internal doors shut, the house becomes stuffy and the smaller south-facing rooms are an absolute nightmare to live in for the kids, especially in the summer.
In the summer we argue over shutting the windows entirely because some of us have hay fever but others can't sleep well without fresh air.
Trickle vents were not included in the windows by the previous owner but they tend to be thoroughly useless anyway imo.
A PIV system seems a crazy idea heating-wise as it will just push heat out and also, probably blow warm humid air through to the cold structure of the house creating condensation and damp in areas we can't see.
Houses which aren't properly ventilated are usually stacked full of pollutants - CO2, plastics and paint vapours plus various air 'freshener' sprays etc etc.
So, faced with the above, we either leave our house full of pollution or pollen and waste lots of money on heating and live a miserable life, or, we try and retrofit an MVHR system which will recover some (not the 85% as advertised) of the heat and at least deliver filtered fresh air to a minimum of the bedrooms.
Please could you explain why a retrofitted and slightly poorly ducted MVHR system is in any way worse than a PIV system which does absolutely zero heat recovery? Would it not be possible to put a flow and return vent into each of the bedrooms and do one floor only?
I understand that not running extract ducts to the kitchen etc will reduce the effectiveness of the system but surely having some system is better than no system.
Sometimes I just think we're missing the point on retrofit... we're not looking for passive house certification or even meeting any form of building regs... we just want to improve our lives.
Happy for any pointers as I'm really struggling to find a route forward in this. I should note that we don't have any issues with mould in any part of the house (except a few poor-quality blown upvc windows) - just a lack of fresh, pollen-free air.
Another issue we face (I forgot to mention) is that with the cracked windows, I get woken up by the pigeons and birds at 4am - it's like a tropical rainforest in our bedroom!!! Being able to close the windows all year round will make all of us much happier!
A lot to unpack there. Lets go point by point:
1. You have decent insulation characteristics. Cavity, windows and probably loft. This will lead to significant heat accumulation in summer.
2. Your supply ventilation strategy is currently natural (via openable windows probably) which creates the risk of pollen ingress in summer. I expect your internal air transfer is also quite inefficient.
3. The problem you are experiencing is mainly heat accumulation in summer. This will not be fixed by PIV or MVHR both of whose performance is limited during the warmer months of the year by summer cut off which turns off the distribution of supply air above preset temperature thresolds. Some units have a an override mode to create continuous supply airflow but this will more often than not result in supplying warm air into an already warm house during warm weather so the stuffiness will remain a problem
4. The solution is to use vapor compression AC to remove the heat from the air (via recirculation in each room on the top floor) whilst at the same time improving the performance of the supply ventilation which will remove the pollutants.
5. Your conceptual understanding of what PIV is doing is wrong. PIV pushes fresh filtered air into the building which mixes with the stale indoor air, diluting and displacing pollutants which are ultimately removed via extraction points. PIV does not create interstitial condensation. There is no evidence of that phenomena in millions of successful PIV installations across the UK. That idea is simply a myth.
6. Your suggestion of installing MVHR on the first floor only and fitting each bedroom with supply and extraction is not sound. MVHR systems are designed to extract from wet rooms only and supply into living spaces.
7. We can achieve the same whole house reach and arguably better (more controllable) ventilation performance and better energy efficiency performance with our whole house PIV solutions as compared to MVHR systems for far less time and cost:
a) Our PIV systems take advantage of the solar gain (worth at least 500 kW of energy per Energy Saving Trust) when MVHR systems do not.
b) MVHR systems run continuously and are usually around 95% efficient in recovering heat when heat recovery is not being bypassed. This means around 5% of the heat energy from the extracted air is lost to atmosphere continuously. Our PIV systems do not have heat recovery but our extraction components only run when they have so if you add up all the minutes you spend having showers and baths each day and compare the cost of this against the 5% energy loss over every 24 hour period I would argue the energy loss is less with our systems than with MVHR. Furthermore the running cost of the PIV and Extractor units we install is usually less than a centralised MVHR system.
We are both F-Gas certified for AC and a highly experienced ventilation retrofitter and we have installed this type of solution (Whole House PIV and AC in the main Living and Bed spaces) in numerous properties including our own. We know it works very well and is the best value retrofit for cost and time. Put simply, ventilation works for most of a typical UK year to help create cooling comfort for the occupants because it creates air movement and we humans lose heat from our bodies by relying on air movement over the skin to help evaporation. However, as our homes tend to be very well insulated this process is not reliable once the temperature gets very warm when the only way to cool is literally to move the heat from inside to outside via refrigerants inside lines.
@@andrewhobbs4265 Many thanks for taking the time to reply. I have been distracted by my son's 18th birthday but I really appreciate your input. I'm currently having a rethink on the subject after your response.
I bought a nuaire PIV unit and it does not work for my detached, 5 bed bungalow. W have had it on for the last 5 years. We have damp and mould in patches around the house regardless of the PIV and moisture levels are above 60% in most rooms. I am thinking of MVHR
PIV is the best strategy for whole house retrofits but you must know what you are doing. Most people in the trade dont have a clue. They just think all you have to do is buy a unit online or from a wholesaler and install it and youre done. Theres a lot more to it than that. Let me ask you the following questions and I will help you identify the issues. The outcome will be that your solution has not been properly specified and/or installed and/or the performance relevant dependencies have not been properly observed. It always is.
1. Who specified PIV for your house ? Exactly what PIV unit from Nuaire did you install ?
2. What extractors do you have in the kitchen and bathrooms ? Are they intermittent in operation and have you checked their performance ?
3. Which rooms have the mould and have you check internal air transfer capability to/from those rooms ?
4. Do you have air bricks and/or trickle vents ? Are they open or closed ? Do you leave your windows open anywhere in the house ?
5. Who installed your unit ? Where is the ceiling diffuser located ? Have you checked the loft for moisture sources ?
You will be far better off identifying and rectifying the issues (because this is where the problem is) than installing MVHR which not good for retrofits and especially not in homes with 2 floors or more.
Happy to help if you want my help.
We went from air bricks in a solid wall 1930's 3 bed house to positive pressure ventilation (nuaire drinaster). It's been brilliant and we are mold free, 50-60% humidity. However looking at next steps for ventilation after we get external wall insulation, be double glazing and additional loft insulation. Given that we have to ventilate one way or another are saying what we have is optimal or is there a in between solution that has some heat recovery. I was tempted by a DIY mvhr retro fit but only if it will be of benefit.
Hi. Thanks for your email. It is highly unlikely you will be able to retrofit MVHR into your property especially if you have more than one floor. This is because MVHR is a ducted strategy and every room must be served. Duct routing should be optimised and length minimised and this is usually not even feasible in a building that has already been erected. It often not even feasible in bungalows with open loft spaces above. Part F stipulates you should only invest in the more complex strategies if the building will give you all the benefits of that strategy and considering MVHR is designed for the most airtight energy efficient buildings you wont get all those benefits in a leakier older building even if it could be installed properly. PIV is the best option and you can achieve heat recovery on a decentralised basis by using room only heat recovery units in your wet rooms. Note if you consider MVHR only recovers heat from extracted air from the wet rooms on a centralised basis anyway there is virtually no difference between MVHR and PIV with Room Only Heat Recovery.
@@andrewhobbs4265 thanks for the detailed response Andrew. MVHR really doesn't sound like it's worth looking at then for us. Maybe when we do the external insulation, windows and loft insulation it will feel like a big step change anyway (it would certainly cost enough in the just place!).
@@tuscan440r PIV is a scalable supply strategy just like MVHR so you will find you can add as much insulation as you like without creating any humidity damp issues. If you add insulation to any naturally ventilated property it makes humidity damp issues worse because its not scalable. This is one of the more common causal issues we come across where surveyors, contractors and agents recommend such treatments for a mould problem actually make it worse because they dont understand how relative humidity behaves indoors in naturally ventilated properties. You'd be surprised at the extent of ignorance that pervades the property industry in this area.
@@andrewhobbs4265 I can appreciate that. I had an external wall quote and the guy clearly didn't understand even the basics about thermal bridging suggesting I could insulate the external walls but leave the loft gable end out to save money...! We want a wood fibre/Lime render EWI and he suggested he was willing to 'give it a go'. Needless to say I want interested. I can't help but feel a national insulation scheme is sorely needed but is at big risk of causing big problems when designed/installed poorly.
@@tuscan440r Its true. Nobody in the Trade understands these issues. I dont mean to be rude to anyone because the ignorance is hardly ever malicious but Ive never met anyone in the Trade who understands all my years of doing this. Regarding a national insulation scheme governments of the day have been giving out free insulation for years but the benefits are nullified if you then advise people to open windows in winter to stop condensation and mould and all that saved energy goes out the window. And dont forget all the properties where airbricks are fitted that make condensation and mould WORSE not better. Our solutions stop all these problems AND save energy as clients never have to open windows in winter saving loads of energy and £££. Its the right retrofit and if the UK is to have any chance of hitting Net Zero every property with a natural background ventilation strategy (and there are 28 million of them) must be retrofitted. I could go on but you get the picture.
Obviously, there are many ductless MVHR units available - either single room or multi room/cascading. Reversing or (better) simultaneous balanced input/extract. Blauberg, Lunos, Prana, Fresh-R, BluMartin. (PIV seems the most ridiculous choice to me - all it is is a gigantic draft, pushing heated air out of your house.)
Your view is nonsense. PIV does no such thing. In recent years there are increasing numbers of people gravitating toward centralised MVHR/decentralised SRHR on the flawed understanding it is easy and simple to specify and install and all the benefits just happen once you turn the unit/s on. As a highly experienced ventilation professional I can assure you MVHR is usually not suitable for a retrofit and SRHRs are usually very ineffective. Happy to have a constructive discussion but you've kind of burnt your bridges using words like "ridiculous" when speaking to someone who does this for a living with thousands of successful PIV and MVHR installs under their belt.
So PIV causes heated air to exit the building through leakage points, correct? With none of that heat energy being retained?
@@paddyeason9677 What makes you think PIV causes heated air to exit the building through leakage points ?
Unless the house is a balloon, then air brought in from outside will displace air that is already inside.
@@paddyeason9677 Air coming in from outside dilutes and displaces but the process doesnt create a one (input) for one (extraction) swap as you suggest and it never works like that in practice. I dont mean to be rude but your remark is total nonsense and reminiscent of those who simply dont understand the technical aspects of ventilation. PIV creates very slight net positive pressurisation throughout a property except during wet room usage (Im talking about properly specified and installed PIV by the way which involves a whole lot more than a PIV unit). MVHR is supposed to be balanced with the living spaces net positive and the wet rooms net negative. There is very little difference between the two in terms of whole house air movement (our PIV retrofits are actually more energy efficient than whole house MVHR retrofits for reasons I wont explain in this reply) but in practice it is very rare for PIV or MVHR retrofits to be properly specified and/or installed yet most people in the trade and property industry have no understanding of this matter of absolute fact and think all you have to do is get a contractor to install a unit/system and give it power.
I am having worsening condensation issues in my sons bedroom - worse since cavity wall insulation was installed last month - the room has 2 external walls and no ventilation was thinking PIV but there is nowhere for the ‘wet’ air to get out of - would a one off D-MEV be a better option?
Thanks for your message. Lots of people ask this question. The answer is misunderstood by almost everyone in ventilation. Firstly there is no need to create the "exhaust" process you describe with PIV just like there is no need to create an exhaust process for the other supply strategies - natural ventilation and MVHR. All ventilation strategies rely on extraction ventilation of sufficient performance in the wet rooms only. Secondly a D-MEV unit is not the right strategy for a living space and should only be used in wet rooms (ideally with automatic humidity tracking). Negative pressure as opposed to positive pressure is a far less process for managing relative humidity. Please note PIV must be properly specified and installed and I would recommend one of our surveys.
@@andrewhobbs4265 do you have an email address?
@@afaulkner6817 yes - andrew@betterindoors.com
Do you reckon MVHR will work if you are renovating the whole house ? We are buying a house which will be renovated completly , I am wondering if MVHR will be an option. I assume it will depend on the air tightness of the house (if that is a word). Is it possible to achieve it if you are renovating the house? Or would you still recommend PIV ? Can I also have the link to your company please?
Hi Vikram. Thanks for your note. MVHR probably wont be a possibility unless you are completely stripping out all floors and ceilings which is unlikely. MVHR designs contain duct routings which must not be deviated from as any change will affect overall system performance which will impact the central unit specification. Yes air tightness is also important. Part F states you should only invest in a ducted strategy if the building will give you full benefit but to be clear most modern retrofits give decent air tightness. Building Control is generally pretty good at policing insulation (not so good at ventilation). In our extensive experience PIV is far more suitable for any retrofit. It is cheaper and easier to install and far more effective than MVHR, it consumes less energy whilst leveraging solar gain (not possible with MVHR) and it turns leakages to your advantage (PAS2035 is WRONG - trickles or airbricks should not be used with PIV and the impact its performance - but you need experience to know this and those who wrote PAS2035 dont have that experience). We renovated our house in 2018 and the EPC rating is 89 (B) up from E. We have PIV and Active Air Purification PHI throughout installed by us. PIV is poorly understood especially by building associations/people who write the regs and many people in the trade. Dont assume any electrician, builder or damp proofer can do it. They all think its a case of buying any old unit from a wholesaler and putting it somewhere in the house. This is one of the big issues with the ventilation industry in Britain - manufacturer route to market via wholesalers who sell to anyone without caring if they have the right knowledge or not - and contractors without knowledge being able to get hold of products but not knowing how to install them properly and all the dependencies. So many people assume it simple but the reality is very different. Our company is www.betterindoors.com. You can email me at andrew@betterindoors.com. I will be happy to discuss your project. We can supply and fit for you if you wish. Our business covers the whole of the UK - from the Channel Islands to Scotland.
You can install a decentralised heat recovery ventilation system. Let me know if you need advice on this. Kirk (Cool Runnings)
@@kirkenicker3136 By decentralised I presume you mean SRHRs ? The same issues discussed in the video apply. We install these in our retrofits only in scenarios where the reach of the strategic background process is challenged. Other problems arise with these, especially with alternate flow devices.
@@andrewhobbs4265 We retrofit Prana recuperators in new build and older properties. We do not have any issues with these
@@kirkenicker3136 These are SRHRs. The noise levels are higher than the units we use and I cant see Part F performance certifications. Are they BBA approved ?