MVHR is Largely Bogus

[MJNewton]

25 minutes ago, Jeremy Harris said:

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. 

 

Again Jeremy I think you are viewing this from a perspective that doesn't align with the motivations of why many of us are fitting MVHR systems. We are doing it to provide a high quality comfortable living environment. If the selected mechanism can do this more efficiently than others (e.g. thanks to any amount of heat recovery from MVHR compared to a PIV system) then that's a bonus, particularly if that mechanism adds further to the main goal (e.g. the added comfort from MVHR giving a supply of relatively warm fresh air rather than the cold that PIV or trickle vents would).

 

I do agree with you that, if measured financially or in terms of efficiency, older and less-airtight homes reap fewer benefits but again it is not these benefits we are seeking, particularly in these types of homes. JFDIY's experience is a great example of the true benefit of MVHR for many. In my case we didn't have a problem per se to fix (e.g. condensation) but still have benefitted from it and thus consider it most worthwhile.

Edited December 19, 2019 by MJNewton

[ToughButterCup]

10 hours ago, MJNewton said:

...

the chosen unit of measurement for assessing the value/benefit of MVHR is '£'. In my view this is wrong, 

...

Well maybe not wrong. But it is certainly useful. Made me sit up and think

[SteamyTea]

38 minutes ago, JFDIY said:

We've always suffered with condensation on windows,

This can be an indicator to a fairly airtight building.  I get condensation on my windows after I have used the shower, never happened before I improved the place.

 

Using heating degree days can useful to gauging how effective the MVHR is.  Just a case of measuring temperature differences and energy usage with the MVHR on and off.  Kind of thing that is worth doing when you go away for a week.

[Jeremy Harris]

13 minutes ago, MJNewton said:

 

Again Jeremy I think you are viewing this from a perspective that doesn't align with the motivations of why many of us are fitting MVHR systems. We are doing it to provide a high quality comfortable living environment. If the selected mechanism can do this more efficiently than others (e.g. thanks to any amount of heat recovery from MVHR compared to a PIV system) then that's a bonus, particularly if that mechanism adds further to the main goal (e.g. the added comfort from MVHR giving a supply of relatively warm fresh air rather than the cold that PIV or trickle vents would).

 

I do agree with you that, if measured financially or in terms of efficiency, older and less-airtight homes reap fewer benefits but again it is not these benefits we are seeking. JFDIY's experience is a great example of the true benefit of MVHR for many. In my case we didn't have a problem per se to fix (e.g. condensation) but still have benefitted from it and thus consider it most worthwhile.

 

 

I'm just viewing it from the title of the bit of kit, Mechanical Ventilation with Heat Recovery.  It's primary function is heat recovery, that's why it was developed and it's why it has been fitted to passive houses, in fact it has become a key element in making a passive house possible.  The air quality improvements are a side effect.  A nice side effect, for sure, but still just a side effect.

 

It so happens that to stop the heat exchanger getting clogged up with dirt and dust a MVHR unit needs a filter on the intake, and that has the side effect of delivering dust and pollen/spore free air to the house, which is very definitely nice to have.  However, that filtration wouldn't be there if it wasn't needed to stop the heat exchanger getting clogged up.  You could have exactly the same clean air ventilation benefit from just fitting a whole house positive input ventilation (PIV) unit.  Cheaper, just as good at delivering clean air and good ventilation, but with no heat recovery.

 

 

[MikeGrahamT21]

Out of curiosity, to the bungalow owners, and previous bungalow owners, did you ever have an official measured air test performed? And if so, what did you achieve?

 

I plan on having an air test done once all the improvements have been completed, and then take it from there, the dream is to have MVHR installed, and I suspect I will do even if the air test isn't so good, as I believe in the technology and what its going to do in terms of fresh air, and since I probably spend more than 90% of my life in here as I work from home, it may as well be as good as it can be.

[Oz07]

Will update on air score in a few months. Building regs measurement different to the ach as used by better houses. I think I read score has to be below 3 on building regs scale for mvhr to stack up? Less than 3 should be comfortably achievable on self builds where more attention to detail is the norm. Less so on developer houses

[MikeGrahamT21]

9 minutes ago, Oz07 said:

Will update on air score in a few months. Building regs measurement different to the ach as used by better houses. I think I read score has to be below 3 on building regs scale for mvhr to stack up? Less than 3 should be comfortably achievable on self builds where more attention to detail is the norm. Less so on developer houses

 

Would you drop me the result on a PM at the time, as I will have totally forgotten about this in a few days, let alone a few months lol!

 

My bungalow is a 1968, wet plastered with a couple of modern extensions, one of which is airtight, and the other which is dot and dab (which i'd sprayed copious amounts of airtight expanding foam behind the boards all round the perimeter). Floors are the main problem in this house, which i'm slowly working my way round.

[MJNewton]

34 minutes ago, Jeremy Harris said:

I'm just viewing it from the title of the bit of kit, Mechanical Ventilation with Heat Recovery.  It's primary function is heat recovery

 

Its primary function is mechanical ventilation. Heat recovery is secondary - a non-functional requirement if you like (and given your working background where functional vs non-functional requirements will have been bread and butter!).

 

If it was called Heat Recovery with Mechanical Ventilation that'd be different. (Hmm.. gap in the market?! ;-))

Edited December 19, 2019 by MJNewton

[MJNewton]

28 minutes ago, AnonymousBosch said:

"the chosen unit of measurement for assessing the value/benefit of MVHR is '£'. In my view this is wrong, "

 

Well maybe not wrong.

 

Agreed - I wasn't entirely comfortable with the word but couldn't put my finger on anything better!

[Onoff]

I think the only downside is if you're unlucky enough to drag a nasty smell like bonfires or sillage through your single intake. 

[Jeremy Harris]

9 minutes ago, MJNewton said:

 

Its primary function is mechanical ventilation. Heat recovery is secondary - a non-functional requirement if you like (and given your working background where functional vs non-functional requirements will have been bread and butter!).

 

If it was called Heat Recovery with Mechanical Ventilation that'd be different. (Hmm.. gap in the market?! ;-))

 

 

No, if all you want is mechanical ventilation, then just fit PIV.  Simpler, cheaper, provides just the mechanical ventilation function and can filter the incoming air to get good air quality.

 

If you also want to improve thermal efficiency then fit mechanical ventilation with heat recovery.

 

[MJNewton]

I don't just want mechanical ventilation, I want the added benefits of some heat recovery too. Whilst both MVHR and PIV satisfy my primary requirement of mechanical ventilation, only MVHR also satisfies the secondary requirement of providing it with some heat recovery hence it being selected as the preferred solution.

 

(Edit: Reading these two posts I think we're in violent agreement on this particular point.)

Edited December 19, 2019 by MJNewton

[MJNewton]

7 minutes ago, Onoff said:

I think the only downside is if you're unlucky enough to drag a nasty smell like bonfires or sillage through your single intake. 

 

I suppose the flip side to that is that the 'attack surface', to coin a security phrase, is reduced with only having that single intake.

[Adrian Walker]

13 hours ago, DavidHughes said:

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.

 

 

 

 

 

 

 

 

What are your air tightness figures?  I'm guessing well over 1 (ACH50).  Just re-read your post and it say "...professionally designed and supplied MVHR system...".  It couldn't have been professionally designed without any air tightness figures IMHO

Edited December 19, 2019 by Adrian Walker

[Ed Davies]

30 minutes ago, MJNewton said:

If it was called Heat Recovery with Mechanical Ventilation that'd be different. (Hmm.. gap in the market?! ;-))

 

That's why I write MHRV, not MVHR which sounds awkward to me. It's heat-recovery ventilation which happens to be mechanical. You can also have passive (non-mechanical) heat-recovery ventilation.

 

24 minutes ago, Onoff said:

I think the only downside is if you're unlucky enough to drag a nasty smell like bonfires or sillage through your single intake. 

 

A minor advantage of a mechanical system (as opposed to most forms of passive ventilation which tend to be distributed around the house (in windows, etc)) is it's just one switch to turn it off until the neighbour's bonfire has gone out.

[mvincentd]

It might just be the underfloor heating that's my waste of money.  The mvhr is proving so efficient I never get to turn it on.

 

[Jeremy Harris]

49 minutes ago, Ed Davies said:

 

That's why I write MHRV, not MVHR which sounds awkward to me. It's heat-recovery ventilation which happens to be mechanical. You can also have passive (non-mechanical) heat-recovery ventilation.

 

 

A minor advantage of a mechanical system (as opposed to most forms of passive ventilation which tend to be distributed around the house (in windows, etc)) is it's just one switch to turn it off until the neighbour's bonfire has gone out.

 

 

I agree with your definition, it makes far more sense, however it seems that the abbreviation that has been almost universally chosen is MVHR.  May be we should start a thread to try and stem the use of MVHR, and get it changed to MHRV, a bit like my attempt to try and stop the use of "thermal mass" (although in that case it's because thermal mass appears to not have any defined units, so can't be measured).

 

5 minutes ago, mvincentd said:

It might just be the underfloor heating that's my waste of money.  The mvhr is proving so efficient I never get to turn it on.

 

 

Much the same here.  The UFH seems to only come on infrequently in the relatively mild weather we've been having over the past few days.  I have tweaked things a bit to allow the MVHR post-heating (from the integral air-to-air heat pump) to operate over a narrow temperature range now, as a way to improve temperature control if we get a sudden drop in temperature.  The UFH takes a fair time to warm up, and although we find the post-heated air from the MVHR a bit too dry, it does have the advantage of being able to respond relatively quickly, so seems to work well to just keep the temperature up by maybe half a degree or so until the UFH has had time to take over. 

 

TBH, it's really minor, though, as the house is massively more stable in temperature than any house we've ever lived in.  It's just that, having got use to the temperature being pretty constant, we now tend to notice a half a degree variation.  In our old house it wasn't uncommon to get 3 or 4 degree variation throughout the day in winter, sometimes more than this, so perhaps we're just being too fussy.

[vivienz]

I've had the UFH on for a while now as we are in an exposed and very windy spot. The MVHR helps but to maintain a steady temperature somewhere between 21 and 22 definitely needs some heat input in a twice daily basis. The common feature that I'm aware of with the houses of @jsharris and@mvincentd is that they both receive a lot of shelter from being in the pocket of a hill side. Ours is the opposite and behaves very differently as a result.

 

We still have no functioning UFH upstairs but it currently holds steady at about 19C. A little on the cool side of things for me if I'm working but an oil filled radiator sorts that out until the UFH gets connected up in January.

[joe90]

48 minutes ago, Jeremy Harris said:

That's why I write MHRV, not MVHR which sounds awkward to me. It's heat-recovery ventilation which happens to be mechanical. You can also have passive (non-mechanical) heat-recovery ventilation.

 

I tend to disagree, I have to have mechanical ventilation because I don’t have trickle vents and passive would not work very well in a windy location, it also has heat recovery so it’s prime function is the ventilation? ?

[Ed Davies]

12 minutes ago, joe90 said:

I tend to disagree, I have to have mechanical ventilation because I don’t have trickle vents and passive would not work very well in a windy location, it also has heat recovery so it’s prime function is the ventilation?

 

OK, so why don't you use trickle vents? Maybe ones like those in Fakro windows which automatically open and close to keep the airflow relatively constant as the wind and stack effect, etc, changes the relative pressures between inside and outside. Even at Fakro list prices they'd be cheaper than MHRV.

 

Because they don't do heat recovery.

 

I was, for a while, intrigued by the sort of passive heat-recovery ventilation used at BedZed (with the wind cowls on the roof) which actually needs a windy location to some extent. I'm still intrigued, to be honest, but it's too out there for a first pass on a self-build. Something to experiment with once the house is built and done, maybe.

 

This is a simple matter of emphasis: I think we're all pretty much agreed that both good ventilation and heat recovery are important, it's just a matter of outlook as to which you think is the reason for a mechanical ventilation system and which is a helpful by-product. Obviously, if you didn't need ventilation at all (e.g., something like Biosphere 2) then there'd not be much point in a ventilation system just to recover heat you wouldn't be needing to lose in the first place so heat-recovery is secondary ventilation in that simple sense. Assuming a ventilation system of some sort is required, though, the question is whether heat-recovery is the main reason for choosing a centralised mechanical one over something more distributed and passive. I think it is.

[SteamyTea]

42 minutes ago, vivienz said:

I've had the UFH on for a while now as we are in an exposed and very windy spot. The MVHR helps but to maintain a steady temperature somewhere between 21 and 22 definitely needs some heat input in a twice daily basis. The common feature that I'm aware of with the houses of @jsharris and@mvincentd is that they both receive a lot of shelter from being in the pocket of a hill side. Ours is the opposite and behaves very differently as a result.

I thought I would look at my particulate logger which I am playing about with.

Yesterday was very windy, even though my house is pretty airtight, I notice that the readings tracked the windspeed, even the lull as the storm moved East.

 

[Bitpipe]

Good discussion - and really appreciate the original alternative viewpoint / experience.

 

My view - if you're building an airtight, near passive standard house (with ACH under 2, ideally less than 0.6 - BR is 10) then MVHR is a necessity to have a BR compliant healthy environment. The Heat Recovery is a bonus and helps reduce heat energy waste through ventilation.

 

If you have an existing house and retrofitting, then it's more of a crapshoot for all of the reasons given above. I have a friend who refurbed and extended his 1980's developer detatched house and he fitted MVHR but was realistic about the energy efficiency savings. He feels he has much fresher air year round and no condensation in bathrooms or kitchens. 

[eandg]

14 hours 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.

 

I've been doing a little bit of reading on this lately and read that anything worse than 0.15 u-value for your walls and MVHR is a waste of time. Our plans are for 0.15 u-value walls/roof (kit only) and slightly better for the floor, with 3G windows and doors. Is this on the cusp of MVHR being fairly futile?

[SteamyTea]

7 minutes ago, eandg said:

I've been doing a little bit of reading on this lately and read that anything worse than 0.15 u-value for your walls and MVHR is a waste of time

I am not sure I understand the rational of that.

Maybe if you look at losses, as a percentage, then it may look like it is pointless.

But if you look at absolute losses from each component, then you get a different story, especially if you assign a cash price to them.

So say you loose £1000/year via air losses, but £8000/year via fabric losses, then it may seem that reducing air losses to £100 is pretty pointless.

But it is still £900/year.

 

Over, at the other place, there is a chap called PaulfromMontreal.  He has a relatively high fabric losses, but very good airtighness.

This has drastically reduced his energy usage.

Edited December 19, 2019 by SteamyTea

[Jeremy Harris]

6 minutes ago, eandg said:

 

I've been doing a little bit of reading on this lately and read that anything worse than 0.15 u-value for your walls and MVHR is a waste of time. Our plans are for 0.15 u-value walls/roof (kit only) and slightly better for the floor, with 3G windows and doors. Is this on the cusp of MVHR being fairly futile?

 

 

It's not hard to look at all the various elements that make up the total heat loss rate and then see what the impact of MVHR is.  The simple heat loss spreadsheet I wrote years ago does this (it's what I used to generate those two plots earlier in this thread) and will quickly show where the major heat loss elements are.  The spreadsheet is this one: Heat loss calculator - Master.xls