MVHR is Largely Bogus

[joe90]

26 minutes ago, Bitpipe said:

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.

 

+1 ?

[DundeeDancer]

I’m enjoying this thread as I have a plan to do a bare bones refurbishment on the apartment I live in.

Second floor apartment with solid concrete floor and ceiling. 

Going back to bare bones to hopeful get the apartment airtight enough to get a good return on the heat recovery element of the MVHR system and to significantly add to the insulation levels as well. Plan is to gut the flat, install at least 20mm of inner wall insulation on all surfaces and 50mm onto external facing walls, new triple glazing, new lightweight underfloor heating, re-wire, re-plaster new kitchen and two new bathrooms. I’ll be looking at a price tag of around £70k.  So I don’t won’t want to be investing in “Bogus” technologies.

 

Having stayed in a few places that have MVHR systems installed then I believe they are slightly over-hyped. 

One place I stayed I noticed the MVHR system wasn’t switched on. I had a decent sleep without the MVHR switch on but the CO2 levels did build up to around 1,500 ppm.  When the owner looked at the MVHR system he said that he must have forgot to switch the system on since he last change of the filters about 8 weeks ago!  None of his other guests has noticed! With the MVHR system on low the CO2 levels were around 900ppm which were decent.

 

To me it is the cumulative effect of all the following that justify the system installation: pollen free air, less dust mites, less humidity, reduction in heating bills.

Adding all that up makes the investment worthwhile I think but to be honest am not 100% sure yet so the plan stays on the drawing board for a little while longer.

 

Definitley need to get it done before I'm 60 and start to really feel the cold nights, so that just gives me a dacade or so to play with before fully committing. ?

[Ed Davies]

1 hour ago, Bitpipe said:

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.

 

My view - the reason Passivhaus requires airtight houses in the first place is to reduce heat energy waste through ventilation in two ways:

 

1) through not overventilating when it's windy or there's a large stack effect and

 

2) through recovering heat for the ventilation which is needed to keep the house comfortable and healthy.

[Jeremy Harris]

36 minutes ago, DundeeDancer said:

 With the MVHR system on low the CO2 levels were around 900ppm which were decent.

 

 

It's a nasty, wet, day here, and we've been shut indoors all day, not even gone outside once, so all doors and windows have been shut tight since yesterday.  Just taken this 'photo of the hall display from the house monitoring/data logging system:

 

 

That CO₂ level is around 100ppm higher than we normally see, most probably because the MVHR hasn't been on boost since around 08:00 this morning, and because neither of us has been out of the house.  If I started seeing levels as high as 900ppm then I'd be looking to find out why they had risen so high.  We find that the CO₂ level tends to fluctuate between about 450ppm up to about 800ppm, but never much higher than that.  The outside CO₂ level here seems to be about the global normal concentration ~420ppm.

 

 

By way of contrast, this plot was from a logger placed in the bedroom of our old house, with a small window left permanently open for ventilation:

 

 

 

[Bitpipe]

47 minutes ago, Ed Davies said:

 

My view - the reason Passivhaus requires airtight houses in the first place is to reduce heat energy waste through ventilation in two ways:

 

1) through not overventilating when it's windy or there's a large stack effect and

 

2) through recovering heat for the ventilation which is needed to keep the house comfortable and healthy.

 

I suppose it's a circular argument

 

House is airtight to minimise ventilation heat losses therefore needs a ventilation system that minimises heat losses, or vice versa.

 

Edited December 19, 2019 by Bitpipe

[Bitpipe]

1 hour ago, DundeeDancer said:

Going back to bare bones to hopeful get the apartment airtight enough to get a good return on the heat recovery element of the MVHR system and to significantly add to the insulation levels as well. 

 

May be worth paying for an airtightness test before committing to the MVHR investment.

 

Remember that your external doors need to be airtight, as do windows - no letterboxes or cat flaps, bathroom / kitchen extractors etc. 

 

I think there is general agreement here that there is air quality improvement with MVHR but the degree of that is somewhat subjective, as is whether it is a worthwhile investment. The heat recovery is more empirical but is highly reliant on very good airtightness (<2ACH). 

 

The title of this thread is a bit misleading - would be better phrased as a question or a 'In my renovation experience...'

[Jeremy Harris]

There's a reasonably good argument for defining goals at the start of any project, and then gathering data to see how best those goals might be met, and finally doing a lot of "what if?" comparisons to try and narrow down all available options to get the best compromise.

 

Sadly, this isn't often how some approach major works, as it seems to be human nature to want to choose things, not because they make best sense, but just because your want them, for reasons that may not be wholly rational.  Many people buy houses, cars, choose holidays etc largely on this basis.

 

With a renovation job I think that a bit of money spent up front on seeing just what you've got to start with, and just what you might reasonably be able to do, would be a wise investment.  Even a rough and ready model of the best and worst elements of the fabric would give a useful indication as to where best to commit limited resources.  It may well be that MVHR just doesn't make sense, in terms of the energy saving it might offer for a particular project, in which case a PIV system might be a lot cheaper and easier to install and offer much the same air quality benefit.

 

It's far too easy to get suckered in by the advertising hype for products and systems, and assume that they are "must have" bits of kit.  Sometimes they may be, but often they may not.  Exactly the same arguments apply to other technologies that get discussed here, like heat pumps, or phase change thermal storage.  They can work very well for some use cases, but not for others.  It all comes down to understanding what you're dealing with initially, and what you might reasonably be able to achieve, both practically and in terms of affordability.

[DundeeDancer]

31 minutes ago, Jeremy Harris said:

If I started seeing levels as high as 900ppm then I'd be looking to find out why they had risen so high

From what I've read 800-900ppm is about ideal in that it means the room is not being over ventilated.

 

I haven’t got my figures to hand but in my current bedroom if we sleep with the door closed then by around 7am in the morning the ppm will be around the 2000 mark. Also if it is cold outside say under 4c then the windows will covered in condensation. With the door open all night it will average around 1500ppm at 7am which doesn’t feel to uncomfortable.

 

Not that long ago the wife and I went and stayed in a posh hotel up in the Highlands, was a bit of a drive to get there but when we did I had a little lay down on the bed and before I knew it I was snoozing and very doozy.  I then looked at my CO2 monitor I thought it must be faulty as was reading about 3,500ppm.  The room couldn’t have been ventilated from the night before and the lack of oxygen was making me sleepy. ?

 

[Gone West]

4 hours ago, Jeremy Harris said:

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.

We've found that we are now looking at tenths of a degree differences whereas in the old bungalow we were looking at several degrees difference. I think you are right that we are being too fussy but it's surprising how quickly the body adjusts to 'requiring' constant warm temperatures.

[Jeremy Harris]

On 19/12/2019 at 16:12, DundeeDancer said:

From what I've read 800-900ppm is about ideal in that it means the room is not being over ventilated.

 

I haven’t got my figures to hand but in my current bedroom if we sleep with the door closed then by around 7am in the morning the ppm will be around the 2000 mark. Also if it is cold outside say under 4c then the windows will covered in condensation. With the door open all night it will average around 1500ppm at 7am which doesn’t feel to uncomfortable.

 

Not that long ago the wife and I went and stayed in a posh hotel up in the Highlands, was a bit of a drive to get there but when we did I had a little lay down on the bed and before I knew it I was snoozing and very doozy.  I then looked at my CO2 monitor I thought it must be faulty as was reading about 3,500ppm.  The room couldn’t have been ventilated from the night before and the lack of oxygen was making me sleepy. ?

 

 

@DamonHD  (used to be on here for a time, but haven't seen him for ages) looked into this in depth some years ago, in the context of the school environment and the impact of CO₂ concentration on the learning performance of children.    IIRC, his findings (supported by some reasonably good science) were that learning performance started to fall off at concentrations above about 800ppm.

 

That's not surprising, as we evolved to live in an environment with a CO₂ concentration of less than 300ppm.  From what I can recall of the aeromedicine stuff that was drummed into me at regular aeromedical and safety training school refreshers, the lower the partial pressure of CO₂ in the air we breathe, the better our lungs are at expelling the stuff as a waste product.  The rate at which CO₂ diffuses from our blood stream to the air in our lungs is pretty much wholly dependent on the partial pressure of CO₂ in the air, and that partial pressure is in turn proportional to the CO₂ concentration.  Arguably there is no optimum level of CO₂ as far as our bodies are concerned, it needs to be as low as possible, and the closer we can get the internal CO₂ level to that outside the better, as that global CO₂ concentration is already a lot higher than it was just 50 or 60 years ago (in 1960 it was less than 320ppm, now it's around 420ppm).

[eandg]

3 hours ago, Jeremy Harris said:

 

 

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

Thanks. I get a message that the file is corrupt and cannot be opened... I am trying to do so from work so not sure if it's an overly sensitive firewall or not. 

[DundeeDancer]

3 minutes ago, Jeremy Harris said:

CO2 concentration

Interesting post there Jeremy, after I complete my PhD into human biology maybe I’ll understand it a bit more. ?

 

Not sure what to make of the new a/c unit in my office, it’s blowing away making a din of about 52dB, the temp is a chilly 21.4c 1,300ppm Hum 42%.

Yesterday it was quite quiet, 24c, 900ppm and it felt normal to me.

 

Strange I feel cold here at work at 21.4c but feel fine at home at 21.7c without a/c. I can’t figure it out.

[Alex C]

Can anyone recommend a co2 monitor please.

I have been living in a passive build house with mvhr for over 2 years and although I have no idea how much heat it recovers I really enjoy the improvement in air quality over the old house that was on the same plot. We are next to fields and it did a great job of keeping the oil seed rape pollen out of the house. The filter collects a lot of pollen which both my wife and kids are allergic to. We only ever have the mvhr on quite low but the house volume is large for the number of occupants. The humidity is always between about 38% and 50% even on very damp days, great for quick drying of washing. I would be interested to know what the Co2 levels are like.

[Jeremy Harris]

3 minutes ago, eandg said:

Thanks. I get a message that the file is corrupt and cannot be opened... I am trying to do so from work so not sure if it's an overly sensitive firewall or not. 

 

 

Just downloaded it and it opened in Excel for me OK.

[TerryE]

We have a passive-class house with MVHR which ensures that we have an airy damp-free environment.  The HR element roughly halves the overall heat losses for the house.  We only have ground floor UFH in the slab and this heats our entire 3 storey house effectively.  Without the HR we would have had to design and install some form of secondary CH for the upper two floors.  As it we don't so no rads, no boiler, no wet CH piping, and unencumbered walls. So for us, MVHR was a cost-effective self-install and far cheaper / simpler than the alternatives.  And I strongly suggest MVHR is a no-brainer for anyone wanting a passive-class home.

[DundeeDancer]

3 minutes ago, Alex C said:

Can anyone recommend a co2 monitor please.

 

I use one called Green Eye, pretty much the one below. It's been pretty good.

https://www.amazon.co.uk/Gain-Express-Temperature-Laboratory-Classroom/dp/B00OXMVJME/ref=sr_1_51?keywords=co2+monitor&qid=1576773418&sr=8-51

[Ed Davies]

59 minutes ago, DundeeDancer said:

Strange I feel cold here at work at 21.4c but feel fine at home at 21.7c without a/c. I can’t figure it out.

 

There's a lot more to the perceived temperature than just the actual temperature of the air. Drafts and humidity make a difference as does the radiant temperature of the walls, ceiling, furniture, windows, etc. The radiant temperature is about equally as important as the air temperature so if say, at work you have a large window which is a couple of degrees cooler that'll make quite a difference to how warm it feels whereas at home in the evening you'll likely have the curtains closed at this time of year and feel the effect of the windows a lot less.

 

What confuses me is that they say that higher humidity makes you feel cooler but I'd think the increased evaporation from your skin in drier air would make it feel cooler. Odd.

[ProDave]

There are other benefits, lack of draughts for instance.

 

Our previous house I don't think was "bad" from an air tightness perspective, at least I can't think of where there might have been big accidental leaks.  However to comply with building regs, all 4 bathrooms, the kitchen and the utility all had extract fans, that we all know are not air tight when not in use.  Then it had a stove connected to a chimney. the stove drew it's air from the room so there had to be a vent near the stove to let air back into the room.  then there was a letterbox, and a cat flap. And of course every window had trickle ventilators.

 

This all adds up to a LOT of holes in the building.  The result was on a windy day, if you opened one door, somewhere in the house, an internal door would slam shut as a gale blew in (or out) through all the multiple paths.

 

The new house has mvhr, so just two vents for all the air, and a room sealed stove.  No cat flap and no letter box.

 

If you open 1 door on a windy day now,  nothing, no draught at all.

[Jeremy Harris]

3 minutes ago, Ed Davies said:

 

There's a lot more to the perceived temperature than just the actual temperature of the air. Drafts and humidity make a difference as does the radiant temperature of the walls, ceiling, furniture, windows, etc. The radiant temperature is about equally as important as the air temperature so if say, at work you have a large window which is a couple of degrees cooler that'll make quite a difference to how warm it feels whereas at home in the evening you'll likely have the curtains closed at this time of year and feel the effect of the windows a lot less.

 

What confuses me is that they say that higher humidity makes you feel cooler but I'd think the increased evaporation from your skin in drier air would make it feel cooler. Odd.

 

 

We comment on this often, as, although the house temperature is very stable, there are some days when we feel cooler, or warmer, than other days.  I've ruled out humidity, as that doesn't seem to change much, and an pretty much convinced that it's a psychological response to what we see outside.  Days like today (wet, mild, gloomy, a bit windy) seem subjectively cooler indoors than days that are very much colder outside. 

 

Part of my reasoning is that neither of us notices that it "feels" cooler until we've opened the blinds and can see what the weather's like outside.

 

What is odd, is that I don't remember noticing this before; it seems to have only been something we've both commented on since moving in to this house.  I suspect it may be related to the way we've become more sensitive to small temperature changes in some way.

[MikeSharp01]

Interesting post. Taking the big picture view and using your 90% efficiency there are two issues.

 

1. The environment - everything we can do to conserve energy is going to be a help and if we pay directly now or indirectly later is common on both sides of the equation (and you can improve the 'now' situation using standard accounting approaches). @SteamyTea can probably do ths sums on the energy required to manufacture the MVHR but your solution has a tec solution as well for the most part again creating a common factor albeit perhaps of different quantums.

2. While I appreciate your view that you don't save much it is clear that this is probably due to more factors than your air tightness and, I hope we can agree that air tighness is not, draftproofing but also the amount of the losses due to the difference between the two and the effects of cold bridges etc taking heat you pay for and giving it to the environment for free. Heat which never sees the MVHR. I am confident that a few minutes with a package like Therm and section drawings of your property will show you how the heat is leaving the house without reference to the MVHR. 

 

All this said you may be right although I feel it is situation specific.

[Pocster]

Even though I’ve yet to even turn ours on it made absolute sense . Air tight house ; have to remove moisture somehow . Trickle vents are like a cheap bodge - of some but limited use . The fact in an area one vent drags stale air out and the other pushes fresh warmed air in will create a new cycle of good air . Be hard to compare how it ‘feels’ as a before and after as it’s a new build . Just seemed a sensible thing to do . Not many negative posts on mvhr on this forum .

[DavidHughes]

Hi Jeremy,

 

I'm looking at your graphs now and can't quite make my calculations match yours though I am in the same ball park.

• Please can you confirm your mean annual continuous flow rate, mean annual outside temperature and mean house temperature.
• It looks like your MVHR is delivering about 90% efficiency, can you confirm?
• You might add the density and SHC of the air which you are assuming and your cost/kW.hr for re-heating air lost with no MVHR. This will help me align my calculations.

David

 

[DavidHughes]

Hi  Onoff,

 

> Have an air tightness test done

How much does this cost for a 100 m^2 bungalow?

 

David

 

[Onoff]

7 minutes ago, DavidHughes said:

Hi  Onoff,

 

> Have an air tightness test done

How much does this cost for a 100 m^2 bungalow?

 

David

 

 

Sorry, don't know. 

[DundeeDancer]

To clarify my understanding of when it is not worth installing an MVHR system is when there are to many gaps in the fabric of the building that allows a certain percentage of cold air to be sucked into the building through the fabric and  not via the MVHR inlet manifold.

 

So this external cold air is just seeping into the building without going past the MVHR heat exchanger.

 

In this scenario a few undesirable processes are occurring:-

1. Cold air is seeping into the building without being warmed up by the MVHR system so cold drafts occur in the dwelling.

 

2. The stale air does not move so readily between the rooms to then be used by the MVHR system to heat up the fresh air from the MVHR inlet.

 

These points mean the efficiency of the system is significantly reduced and with the volume of air that goes through MVHR system it may well be possible that the MVHR system then starts to significantly chill the dwelling.

 

The only way to stop this drop in efficiency of the system is to ensure there is a good airtight barrier in the dwelling apart from the MVHR systems inlets and outlets. 

 

The only way to get to the point of sufficient airtightness is to install an airtight barrier behind plasterboard and ceilings etc.  So this takes a deep refurbishment project to install correctly or to be installed on initial build.

 

In summary I believe installing an MVHR system will always help with air-freshness and de-humidifying a dwelling but if the air-tightness of the dwelling is not up to standard then in effect the system will be bringing to much cold air into the property via the gaps in fabric of the building and chilling the building.

 

This is my laymen’s understanding of why installing an MVHR system may be folly. I do wonder if I am missing any other points?

 

Many thanks, DD.

Edited December 22, 2019 by DundeeDancer