MVHR is Largely Bogus

[DavidHughes]

Well, having found this forum a few months ago and benefited greatly from the well-informed posts concerning MVHR I felt I'd like to share my own experiences and give a condensed list of useful practical tips. Unfortunately on reflection I feel compelled to write a different kind of post but still in the spirit of helpful advice.

 

Having just DIY installed a professionally designed and supplied MVHR system in my bungalow, and looking closely at all the time and numbers I conclude that it is largely not worth it in terms of either cost or performance. I have come to the conclusion that a carefully designed system of adjustable wall, window and ceiling vents will fulfil the same needs at a fraction of the price and effort.

 

The primary reason for this conclusion is that building regulations section F requires a continuous ventilation flow rate for an MVHR system of 0.3x the floor area in L/s. For a 100m^2 bungalow = 30 L/s. At this ventilation rate, assuming an average 21C inside and annual average 10C outside, with gas central heating, you will be saving about £200/year on heating with a 90% efficient MVHR.  My experience with both this house and a previous one with an MVHR system is that you actually need only a small fraction of 0.3 ACH for any house to ventilate it adequately. Previous other posts on this forum also conclude that the required flow rate which they actually run their houses on is much lower than 0.3 ACH. The 0.3 ACH value is an outlier worst case, most owners of modern houses will find 0.1 ACH completely comfortable, even with cooker hoods and showers taken into consideration. I have personally found that about 0.05 ACH continuous is absolutely fine for a few people in a small house. If you now look at 0.05-0.1 ACH continuous you find that the heat saving per year is £30-£60/year. Out of interest I also do scuba diving occasionally and can confirm that about 0.5 L/s for one person doing moderate exercise adjusted from 10-20m pressure to 1 bar at the surface is normal.

 

Now lets look at cost and effort.

 

For my 100m^2 bungalow I paid BPC ventilation (who, despite what I have said are excellent in every way) about £2000 to design and supply an MVHR system for my 100 m^2 bungalow.

I have very conservatively spent 100 hours planning, installing and testing the system.

I have had to take paid time off work to allow building control in to inspect.

I have had to hire a calibrated flow meter at a cost of about £140 including postage.

I have messed about with it for many hours (about 20) to balance the system.

I have spent at least 10 hours preparing the relevant documentation for Building Control.

I have had to shave the bottom off all of my doors to make a 10mm gap for through house air flow and in some cases re-paint, I don't even want to think about those hours!

I could go on but shall we call it £5000 if I paid someone to do all this? That's not unreasonable.

 

So I pay £2000 and do it DIY for a £50/year payback = 40 years

Or I pay someone £5000 for a £50/year payback = 100 years

The MVHR unit will probably last no more than 25 years but the ducting will hopefully last a lot longer.

According to the manufacturers of the MVHR I need to inspect the filters every 3 months and replace them at 2x£20 every 6-12 months.

And if you are CO2 conscious lets not forget the cost of manufacturing the kit in the first place - who knows?

 

So why did I do it?

 

Well I got suckered into it. The last house in which I DIY installed a system like this I swapped an old heat exchanger for a crate of beer, put simple ducting and fans in about 20 years ago. I didn't need to consult building control, just did it and ran it for 10 years. It worked really well and cost me about £400 in bits and about 50 hours time. I started this new house recently and suddenly building control and the 0.3 ACH rule comes in. Still determined I pressed on to make sure my house was compliant but I should really have stopped and thought about it before I spent my £2000 on kit and began work.

 

Are there any benefits to MVHR?

 

Not many. I'm confident that in new build in 20 years time it will all be gone and that carefully planned vents incorporated into high quality glazing units and ceilings will be the order of the day. The only things which I can think of are filtration of pollen and pollution and maybe noise reduction (vents let in noise).

 

If I were to do it again?

 

No MVHR.

High quality adjustable wall vents low down in the corners of all bedrooms (the rooms which you want to be coolest).

Similar ceiling vents in living areas to let the air out.

Bathrooms with good, possibly motorised, flap valves on the extractors.

A recirculating cooker hood fan or total extract with similar good flap valve.

Chase and fix drafts religiously including floors with insulating and low air permeability underlay.

Hire a cone flow meter to see what's really happening. https://www.bsria.com/uk provide this service  for £80-140 depending on whether you collect or have it delivered.

Set the system at about 0.05-0.1 ACH

On a two storey house a duct and low power fan to circulate the air top to bottom (my last house had this and it was excellent)

If the layout of the house were amenable ( I have solid floors so no chance) I would probably attempt to draw air in over the foundations of the house to either pick up heat in winter or let go of heat in the summer.

Follow the building regs guidelines section F which do have some good rules but don't bother telling them unless you absolutely feel the need to - it's just some wall vents in a house...

 

Regards,

 

David Hughes

Ex F35 Lift System Stress, Thermal and Dynamics Engineer and home energy efficiency enthusiast.

The Wirral.

 

 

 

 

 

 

 

[ProDave]

You are entitled to your opinion.

 

I paid a lot less than you for my kit, I got the mvhr unit from ebay and the ducting from BPC, self designed.

 

From measurements I know I am running mine at about half the BR recommended rates and it is fine. The air is always fresh inside the house.

 

The trouble with trickle ventilation, is a lot of the time it will be under ventilating, on a still day,  but on a windy day, will be grossly over ventilating.

 

MVHR really comes into it's own when you are building a really well insulated, and sealed, air tight house.  

 

But fitting mvhr might not always be optional. One self builder near me did a good job on sealing his house, so much so that when he had an air test done, the result was so good, building control insisted he fit mvhr.

 

[Onoff]

MVHR...less dust in the house, less pollutants like radon, off gassing from furnishings and carpets, people on here with allergies report benefits, mould issues reduced, constant fresh air. 

 

 

[NSS]

The MVHR in our home has, quite literally, transformed my wife's life, and probably added years to it. Frankly I don't give a damn whether it ever pays for itself in a financial sense.

[Ed Davies]

0.3 l/s/m², assuming typical 2.4m ceiling heights, is 0.45 AC/h, not 0.3 AC/h.

 

24 minutes ago, DavidHughes said:

Out of interest I also do scuba diving occasionally and can confirm that about 0.5 L/s for one person doing moderate exercise adjusted from 10-20m pressure to 1 bar at the surface is normal.

 

This is irrelevant. With normal scuba kit the air you breath out is dumped completely so water vapour and CO₂ content is got rid of completely. In an enclosed space like a house you finish up with the exhaled gases diluted in the contained air which you rebreathe so the purpose of ventilation is to keep the concentrations down as well as to get rid of humidity and contaminates (like VOCs) from other sources.

 

(In neither case is getting enough oxygen a significant consideration.)

[vivienz]

Speaking as someone whose MVHR was broken down for a few weeks in a highly insulated and air tight house (0.25 ac/h) I strongly disagree on the ventilation aspect.

 

You've also largely ignored the HR part of MVHR.

 

My MVHR is now fixed and I'm very pleased.

[Russdl]

I'm happy to go for it just so I don't have trickle vents. Why would you install reasonable/good/excellent glazing and then put a big hole over the top of it?

 

Edited December 18, 2019 by Russdl

[joe90]

I am like @ProDave, cheap unit from Ebay, self designed, self installed (yet to balance it, other  things tend to be more important) BCO simply looked at the vents and said “no trickle vents because you have MVHR.” and didn’t ask fir any paperwork ?. We live in a very windy position near the coast and any trickle vents and such would whistle loudly. I am glad I installed mine.

[Thedreamer]

I watched some videos on MVHR recently and wondered whether the ducting ever gets mouldy?

 

 

[Jeremy Harris]

I spent a lot of time modelling the design of our house, and then measuring its performance, both when setting systems up, and through life with embedded sensors that are logged every 6 minutes and the data stored for later analysis.

 

In terms of energy saving alone, our MVHR reduces our heating requirement by a significant amount, enough on its own to justify the expenditure.  First, a look at the house heat loss, versus the difference between the inside and outside temperature, when not fitted with MVHR, but with the level of trickle ventilation/extraction required by building regulations:

Note the blue line, which is the proportion of total heat loss attributable to the required level of ventilation to ensure that the house remains comfortable and damp free.

 

 

Next, plotted to the same scale, the house as built with the MVHR system we installed.  All other parameters are exactly as in the plot above, the only difference is the use of MVHR, rather than trickle vents, extraction fans, etc:

 

Note that the ventilation heat loss is very significantly reduced, as is the total heat loss.

 

In simple terms, at a differential temperature of 15 deg C between indoors and outdoors, the house needs about 45% less heating than if it did not have MVHR.  That is, in my view, a massively significant benefit, nearly halving the heating requirement, just from fitting MVHR, and is, alone, enough to justify fitting it.

 

Perhaps the most important benefit, though, is the one that everyone who visits our house notices almost immediately they walk in the door, and one that @NSS has made a very powerful argument for, the improvement in air quality.  Having a house that is always fresh, has no residual odours from cooking etc, is free from pollen (that alone is a godsend for anyone who suffers from hay fever) and which results in bathrooms staying condensation free, with damp towels etc drying very quickly is probably as great a benefit as the saving in heating cost, and in my view probably worth fitting MVHR for on its own.

 

However, there are many, many, examples of poor MVHR systems, either by poor design, poor installation, or a combination of both.  There's also the fact that some people are persuaded to fit MVHR to houses that simply will not benefit from it, because they have an inadequate level of airtightness to allow MVHR to work effectively.  Very few houses in the UK are built to an airtightness standard that will allow MVHR to work well, as even the current building regulations level of airtightness is inadequate for MVHR, and mass housebuilders struggle to even get houses to meet that requirement. 

 

There's little hope that a house built ten or twenty years ago could be made adequately airtight to allow MVHR to work efficiently, without a great deal of major improvement work to the core fabric of the house.  I tried to improve the airtightness of our old 1980's built bungalow, and spent weeks air testing and going around sealing up every gap I found.  Despite my best endeavours it still ended up at least 20 times more leaky than our current house.  The main problem was that houses need to be designed to be airtight, it is pretty damned hard to try and bodge them to some level of airtightness when their basic structure was never intended to be free from many thousands of small air leaks.

 

 

 

 

 

[ToughButterCup]

. Welcome David.

Interesting post. Tell us how you went about achieving an airtight design...

[DavidHughes]

Thanks for pitching in Jeremy. It was your posts which I found most useful during planning and installation.

 

I'll digest your numbers over Christmas.

 

I too have my house fitted with albeit simpler measuring gear. My house is very well draft proofed, insulated and glazed. I'm currently getting 90% efficiency +/-5% on air heat recovery but will be fitting some more accurate and more sensors from old work kit to take a closer look.

 

I'm still struggling to see how I'm making more than about £100/year at the most optimistic estimate in energy savings. Might be a house size/type thing.

 

David.

 

[DavidHughes]

>Interesting post. Tell us how you went about achieving an airtight design..

Start with a relatively modern single storey solid floor house with few leaks to start with then...

With great difficulty, persistence, pressure sensors, flow meters, aluminium tape, silicone, insulating foam, some new windows, new door seals, smoke....

Took two days.

Although I'm not attempting passivhaus standards I might just get someone in to do the pressure hold test if it's not too expensive.

.

 

[SteamyTea]

10 minutes ago, DavidHughes said:

I'm still struggling to see how I'm making more than about £100/year at the most optimistic estimate in energy savings

Have you look at the times when you are heating? (and maybe cooling)

Or are you assuming that anything below a fixed external temperature is a heating time?

(just about to turn my heating off as it has gone above 10°C)

[Jeremy Harris]

19 minutes ago, DavidHughes said:

Thanks for pitching in Jeremy. It was your posts which I found most useful during planning and installation.

 

I'll digest your numbers over Christmas.

 

I too have my house fitted with albeit simpler measuring gear. My house is very well draft proofed, insulated and glazed. I'm currently getting 90% efficiency +/-5% on air heat recovery but will be fitting some more accurate and more sensors from old work kit to take a closer look.

 

I'm still struggling to see how I'm making more than about £100/year at the most optimistic estimate in energy savings. Might be a house size/type thing.

 

David.

 

 

 

The critical thing is really the level of airtightness.  What was your air test result?  Unless it was significantly lower than current building regulations requirements (something mass house builders still really struggle to achieve) then I doubt it would be really worth bothering to fit MVHR.  The ventilation level is modest, as the MVHR fans don't provide much pressure (as anyone who has tried to set up and balance an MVHR with any sort of a breeze outside will testify) so unless the house is pretty airtight it is very easy for natural ventilation from the leakage through the fabric of the house to dominate, and make the MVHR pretty ineffective in practice.

 

After all the time I spent trying to improve the airtightness of our old 1980s brick and block bungalow, which had the really big airtightness advantage of having been wet plastered throughout, I came to the conclusion that it would be impossible to get the airtightness to the level needed to allow MVHR to work, without taking the roof and all ceilings off, removing all the skirting boards, removing all the kitchen units , ripping out all the wiring and essentially rebuilding the house from a bare shell.

 

I managed to get the easy stuff sealed, like the doors, windows and loft hatch, but had major air leaks through every electrical fitting, as all had fairly open channels running up the walls to the loft space.  I also had massive air leaks around the wall to ceiling junctions everywhere in the house.  The gaps weren't visible, but there was nothing sealing the plasterboard ceilings to the walls anywhere - the plasterboard was, at best, just resting on the walls.  The same went for all the light fittings, all had large holes directly to the loft space.

 

There were pipes going out though the cavity walls (which were ventilated)  in the kitchen, WC and bathroom, most of which were inaccessible, and none of which were properly sealed.  I tried sealing them outside, as that was really the only place I could get to them, but this made little difference, as they were still open to the cavity.  After tens of hours spent trying to get a decent level of airtightness, I couldn't get it anywhere near good enough to think about fitting MVHR, and concluded that retrofitting MVHR to an older house would probably not give a worthwhile reduction in the heating requirement.

[Onoff]

Welcome David. I'd urge you if you can to visit a passive build. The feeling is odd coming from a traditional house. Warm, totally draught free and quiet. Fitting MVHR to a passive place and yours, well there's really no comparison. Have an air tightness test done, I reckon it would shock you. Your place doesn't have an airtight, thermal envelope by design so you've multiple leakage points and thermal bridges. 

 

 

Snap btw! ?

Edited December 18, 2019 by Onoff

[DavidHughes]

My house is much better than yours was to start with but it was still a challenge. I think I spent about 6-8 hours sealing spot light fittings alone.

 

 

 

 

[NSS]

14 minutes ago, DavidHughes said:

My house is much better than yours was to start with but it was still a challenge. I think I spent about 6-8 hours sealing spot light fittings alone.

 

 

 

 

We have 48 spotlights in our ceilings. Not one of them has been sealed as there's no need to. If you have air leakage from outside to your ceiling voids then you're already fighting a losing battle, surely?

[MJNewton]

1 hour ago, Jeremy Harris said:

The critical thing is really the level of airtightness.  What was your air test result?  Unless it was significantly lower than current building regulations requirements (something mass house builders still really struggle to achieve) then I doubt it would be really worth bothering to fit MVHR.  The ventilation level is modest, as the MVHR fans don't provide much pressure (as anyone who has tried to set up and balance an MVHR with any sort of a breeze outside will testify) so unless the house is pretty airtight it is very easy for natural ventilation from the leakage through the fabric of the house to dominate, and make the MVHR pretty ineffective in practice.

 

Whilst I am not in a position to dispute the theory I would from personal experience dispute the conclusion.

 

I retrofitted MVHR to our 12yr old house (a Persimmon estate new build so whilst it wasn't 'drafty' I would not expect it to deserve any sort of air tightness label) and the results have been superb. All trickle vents have now been closed and extractors sealed up, and we now enjoy a noticeably fresher and more stable environment, no concerns about drying washing indoors, no cold drafts through the window vents, no noisy extractors etc. Heating costs are hard to compare year on year given the significant variation in winter climate however our annual gas costs look to remain at <£300 (for heating, hot water and hob) and this is a figure that I do not feel compelled to reduce or be concerned about.

 

The only arguable negative has been the cost and effort to install it but, given I don't have concerns about heating costs, it wasn't done for financial reasons (and it was <£1000 given I picked up some bargains along the way) and the effort has been (mostly!) enjoyable and rewarding in my view.

 

Believe me I wouldn't hesitate to label it not worthwhile if I had any doubts as I believe there's no shame in learning through first-hand experience and admitting if/when things haven't turned out quite as expected.

Edited December 19, 2019 by MJNewton

[Temp]

+1

 

Our house is far from airtight. We have two chimneys for starters. Despite this the incoming air from the MVHR vents is noticeably warmer than outside air so we are recovering some energy even if it not optimum.

 

The air quality is far better than any house we have lived in before. My feeling is that this is due to the constant high level of ventilation MVHR provides. Love it.

 

In short I would install one again even if it actually cost me money to run.

 

[MJNewton]

47 minutes ago, Temp said:

In short I would install one again even if it actually cost me money to run.

 

...and I think that is good way of looking at it. Like a good mattress I am happy to pay for it for the non-financial benefits it provides.

 

All too often it seems that, like the original poster if I may be so bold, the chosen unit of measurement for assessing the value/benefit of MVHR is '£'. In my view this is wrong, and I can understand the potential for disappointment where it has been done. Sure, it might be the easiest to quantify but that doesn't make it right.

 

Edited December 19, 2019 by MJNewton

[Oz07]

Interesting thread and discussion. My system gets delivered today for similar size bungalow. Be interesting to see how I get on. 1700 excl designed and supplied

[Declan52]

You need to look at it in terms of what aspect of mhrv is the most important to you. In my case it's the clean fresh air. I have had sinus issues for a long time so moving from a stuffy old 1960s  house with its fair share of condensation and mould to my new build was a massive leap. 

I couldn't really care less if it recovered any heat to be honest. Once a year I spend about 30mins changing the filters and washing and drying the heat exchanger and the rest of the year it just works away doing it's job. 

[JFDIY]

Interesting topic, and if you look at the finances they probably don't add up. There doesn't appear to be many parallel comparisons either as most people fit it to new build houses or as part of a major refurb where lots of other work to improve the building fabric is being carried out. I'll be honest I've not done any work on the financial benefit in terms of reduced heating bills, this is my experience.

 

We have a converted barn, about 50-55m², two adults and one child, converted 10 years ago, built to building regs from around 2000 (when work started), so by no means air tight. Double glazed, UFH etc.

 

We've always suffered with condensation on windows, every morning I'd get up and wipe the bottom 3" of water from the windows and frames, the external door handles would be dripping wet with condensation. To try and combat this we'd keep several windows on vent and the bedroom skylight vent open all year round and even in the winter often have the skylight open several inches to help and reduce the stuffiness my wife complained of, and a chilly drafty bedroom on my side.... We never dried clothes in the house, have a tumble dryer in an outside shed.

 

Were building a large extension (125m²) and I've looked into mvhr in the past before the extension was a reality and shy'd away from the cost. However I decided fitted mvhr to do two things, get rid of the need for a downdraft extractor over the island hob and also try help with the above in our existing part of the build.

 

Over the summer  I've fitted the unit and ducting and turned it in in October, set it up by feel for now in just the original barn as we've not properly broken through to the extension yet. Cost was higher than many on here, but I bought new everything and DIY'd the install for about £3K, I think to get the quality of what I've done off trades people it would have been closer to £6K.

 

Since switching it on, doing a rough set up, I can honestly say the difference is night and day, the house is no longer damp, air in all rooms is fresh, as mentioned by @Jeremy Harris bathroom towels are always dry and we no longer need any window vents open, even dry a few clothes so reducing the tumble drier usage.

If it cost me £250 a year to run then I'd still be happy, in reality it won't - I bet we were using close to that having a skylight open all the time.

 

Like I said; same house, nothing else has changed, but the difference in feel is remarkable.

 

 

 

[Jeremy Harris]

The bottom line with regard to whether or not MVHR recovers as much waste heat as it could has to be related to airtightness, even if there is anecdotal evidence that sometimes this doesn't seem to be the case. 

 

If the house leaks air, such that the MVHR is effectively bypassed, then the house must lose heat from that bypass ventilation.  Whether that heat loss is significant depends entirely on how much air leaks into/out of the house without passing through the MVHR., relative to the proportion that does pass through the MVHR system.  Not only is airtightness a factor, though, location and environmental conditions are too.  Taking our location as an example, we get few days in the year when there's much of a breeze flowing past the house, because we're in a relatively sheltered location.  As such, our airtightness could probably be a bit worse than it is and the MVHR would still work OK.

 

The other factor relates to how much heat is lost through the fabric of the house relative to the heat lost through ventilation.  If the fabric heat loss is massively greater than the ventilation heat loss, then fitting MVHR may not make a big difference overall, as it can only ever reduce the ventilation part of the heat loss.  Again, in our case the house fabric heat loss is pretty low, so ventilation loss would dominate the total, if it were not for the MVHR.