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MVHR is Largely Bogus


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I haven't been on here for a few weeks so I am late to this party.

 

I have my system up and running now. Against a lot of good advice I went ahead with my installation in a fairly leaky, 1830's stone-built cottage. For me the primary goal was to reduce the moisure level within the house while I get around to addressing the root causes over the coming years. I am staggered by the difference the system has made to our home. We now have zero condensation on the windows in the morning (vs. having to use the Karcher Window Vacuum 4 or 5 times per window every morning!), a much drier, healthier air quality, reduced condensation on walls after showering etc and an unexpected bonus that fresh food in the pantry now lasts longer than it used to.

 

As I get around to making the house more airtight I hope that the benefits will prove to be even more profound. For me MVHR has been an enormous game changer and I wouldn't be without it.

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48 minutes ago, Barnacles said:

I haven't been on here for a few weeks so I am late to this party.

 

I have my system up and running now. Against a lot of good advice I went ahead with my installation in a fairly leaky, 1830's stone-built cottage. For me the primary goal was to reduce the moisure level within the house while I get around to addressing the root causes over the coming years. I am staggered by the difference the system has made to our home. We now have zero condensation on the windows in the morning (vs. having to use the Karcher Window Vacuum 4 or 5 times per window every morning!), a much drier, healthier air quality, reduced condensation on walls after showering etc and an unexpected bonus that fresh food in the pantry now lasts longer than it used to.

 

As I get around to making the house more airtight I hope that the benefits will prove to be even more profound. For me MVHR has been an enormous game changer and I wouldn't be without it.

Would be interesting to know if it does have any impact on your space heating bill, often hard to compared though as no two days are the same, but you should know if the average use drops quite a bit. Obviously not expecting any drop at all, but just interested to know.

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23 minutes ago, MikeGrahamT21 said:

Would be interesting to know if it does have any impact on your space heating bill, often hard to compared though as no two days are the same, but you should know if the average use drops quite a bit. Obviously not expecting any drop at all, but just interested to know.

I too am keen to see if there's any difference in the heating fuel consumption. It'll be difficult to gauge as we haven't quite had a full year in the house yet but my initial impression is that we may consume less energy due to the temperatures in the house being "evened out". As we still have a fairly primative single thermostat arrangement we had hot and cold spots throughout the house (dispite use of TRVs and balancing radiator flows carefully etc). Now the house maintains a much more even temperature, again another unexpected benefit. We also feel the benefit of the wood-burning stove in other parts of the house as the air is moved around the building so hopefully that will contribute to any savings in fuel oil.

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I would expect the heating bill to rise a little, all MVHR reduces the internal temp a little but less than opening windows and letting colder air in. I would imagine any (small) increase would be worth it fir the lack of damp/mould.

 

interesting to hear the temps are more even as it’s widely said that MVHR is poor at moving temps/air within a house. Really glad you are pleased with the outcome ?

Edited by joe90
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One downside of the low humidity in our house (as a result of MVHR, passive standard build etc) is the poor old Christmas tree shrivelled up within a few days, as did the sprig of mistletoe.

 

Upside is orchids seem to thrive, usually they die off after the initial flower but we have a few that are still going strong years later.

 

 

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1 hour ago, Barnacles said:

I too am keen to see if there's any difference in the heating fuel consumption. It'll be difficult to gauge as we haven't quite had a full year in the house yet but my initial impression is that we may consume less energy due to the temperatures in the house being "evened out". As we still have a fairly primative single thermostat arrangement we had hot and cold spots throughout the house (dispite use of TRVs and balancing radiator flows carefully etc). Now the house maintains a much more even temperature, again another unexpected benefit. We also feel the benefit of the wood-burning stove in other parts of the house as the air is moved around the building so hopefully that will contribute to any savings in fuel oil.

I look forward to hearing the results, I have a pending project to put in MVHR (my need isn't as desperate as yours, certainly not for damp), I intend to buy a CO2 monitor, air quality is probably most important, as I spend probably more the 90% of my time in the house, even my work is home based, so could make a huge difference to health.

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1 hour ago, joe90 said:

I would expect the heating bill to rise a little, all MVHR reduces the internal temp a little but less than opening windows and letting colder air in. I would imagine any (small) increase would be worth it fir the lack of damp/mould.

 

interesting to hear the temps are more even as it’s widely said that MVHR is poor at moving temps/air within a house. Really glad you are pleased with the outcome ?

I suppose that the losses associated with the heat exchanger alone should amount to a 10%ish loss in energy (plus ducting losses etc especially as most of the system is located in the cold roof space) but it feels like the heating system is kicking in less often than it used to. Of course that's not scientific, it could just be that we're having milder weather at the moment.

 

I don't think that the MVHR is good at evening temps by extracting the warm air and redistributing it's energy through the vents, I rather suspect that air is being pushed and pulled through areas of the house that had no significant airflow before. Again, not scientific, more just a 'feel'.

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On 06/01/2020 at 16:11, Barnacles said:

I suppose that the losses associated with the heat exchanger alone should amount to a 10%ish loss in energy (plus ducting losses etc especially as most of the system is located in the cold roof space) but it feels like the heating system is kicking in less often than it used to.

 

Any ducting between the MVHR unit and the plenums running through cold spaces should be lagged or under the insulation.  Air exchanged through the MVHR has a 90% heat recovery; air exchanged through other leaks has a 0% heat recovery.  Also look up the concept of latent heat of evaporation.  Air condensed out by the MVHR heat exchanger and vented as water through the condensate drain recovers additional heat is also lost in non-MVHR systems.

 

Yes, the efficiency will drop considerably in a leaky house if there are external winds which cause pressure differentials and increase the % of non-MVHR exchange, but IMO even so the overall benefits to the freshness of the living environment and removal of damp / mould remain.

 

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  • 2 months later...

I have run my system and reviewed the numbers. My heat flow numbers come out the same as Jeremy's spreadsheet, in fact we have very similar houses. I've worked out the cost/benefit of my MVHR and it's not pretty. See calculations below. The numbers show savings at 0.3 ACH which are bad enough but in reality 0.1 is more than adequate most of the time. These systems will never pay back.

 

My numbers assume a 100% air tight house so the numbers below are a best case scenario. My house isn't 100% but its pretty close. No amount of air tightness improvements will improve the underlying issue. 

 

I stand by my first post. Unless you have an air filtering requirement or a damp problem (in which case it would really be better to fix the damp) then don't bother with MVHR, just go with well designed trickle vents and maybe some strong extractors with valves for occasional use.

 

HX-Savings.png.f1fdd912ce2bb606628f5d842cc567cf.png

 

 

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You're still missing the point - many (most?) of us are not using MVHR for financial reasons but rather the air quality benefits it provides over and above a passive system (such as trickle vents). Inicdentally, £13.50/yr sounds pretty amazing value to me! 

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Frankly I'd pay a lot of money to run the thing just to get the air quality benefit.  It makes such a massive difference over just passive ventilation that there's no contest.  We can even dry clothes inside the house quickly, avoiding the need to use the tumble dryer, with no condensation problems.  MVHR probably saves me over £100 a year in hay fever medication alone.  I need to work out what the saving is from not running the tumble dryer, probably a fair bit.

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Just had a shower, MVHR came on boost automatically to reduce RH and it will go off automatically when it gets back to normal. No trickle vents to create cold draughts, what’s not to like.

 

edit to add, like Jeremy I have found clothes dry quite quickly with no condensation and I banned a tumble drier in our new build.

Edited by joe90
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I would be REALLY surprised if your real world energy saving is only £81 per year with the mvhr.

 

Are you taking account of the fact that if you don't fit mvhr you will have to fit trickle vents in EVERY window.  You will have to fit an extract fan in the kitchen, utility and every bathroom.  Unless you spend a lot of money on those fans, they will be leaking air all the time even when not on.

 

So with all the ventilation you will NEED I suspect your ACH will be a lot more than 0.3 just by fitting all the ventilation that building regs require.  Sealing the house up and not fitting mvhr is not an option unless you make other ventilation provision.

 

But do carry on and when it's finished let us know your real life energy bills once you have made all those holes in your house. And let us know if you get any condensation or lingering smells anywhere.

 

For me, mvhr is worth it, not to have anti back draught flaps clattering in the wind, not to have internal doors blow shut when you open an exterior door etc (with a sealed house with mvhr you can open 1 door or 1 window even on a windy day and not have any through draught)

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The basic assumption in Part F is that trickle vents and extraction fans will produce the same degree of ventilation as the background ventilation rate when using a continuous mechanical ventilation system, so the heat loss difference will be just that from the MVHR heat exchanger efficiency (typically about 85%).

 

For our house, I ran the numbers both with and without MVHR, with the same level of ventilation, and these are the results:

 

2079451126_Asbuilt-withMVHR.jpg.ff2cb7b17d6070bb3afa04f1b7697f02.jpg1777883263_As-built-noMVHR.jpg.9a41076c6e8b22514c01dbe7ed8bcb80.jpg

 

 

 

 

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4 hours ago, DavidHughes said:

go with well designed trickle vents

what are they? does a trickle vent move the air where it is required to avoid the build up of condensation and therefore mould growth? mvhr supply is in the opposite corner to the extract and will move more air from across the room.

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Trickle vents have a very highly variable flow rate, from near zero on a still day (so almost no ventilation) to a very high flow rate when extraction fans are running.  My experience with them in three houses was that they are a curse, for several reasons.  Noise was one of the main problems we had.  In two of the three houses we've lived in with trickle vents they howled like a banshee with the wind in a particular direction.  Dirt was another issue, they produce dirty smears where dirt and cobwebs get pulled in around them.  We also didn't find them very effective in any of the houses that had them, and just tended to open windows to get ventilation in some rooms.

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>MJNewton

 

I agree with your comments on air quality but your comments on cost do not stack up. Please don’t conflate the two issues.

What I don’t agree with is that you say “£13.50/year sounds pretty amazing value to me!” The true cost is £13.50/day plus say (£2000 DIY + £5000 Installed) / 2, so £3500 amortised over 30 years (which is generous) = 32p/day plus running costs of 4p/day plus filter costs. It is really costing, for even a small system 35-40p/day.

>Declan52 “So not even 4p a day”

See my comments to MJNetwon

>Jeremy Harris

>Frankly I'd pay a lot of money to run the thing just to get the air quality benefit. 

You are a) paying a lot of money and b) getting an air quality benefit. My comments are so that everyone understands what they are getting for their money. Also, your spreadsheet and graphs are very good and I think accurate but nowhere do you evaluate the cost. This is my only issue.

>Joe90

>Just had a shower, MVHR came on boost automatically to reduce RH and it will go off automatically when it gets back to normal. No trickle vents to create cold draughts, what’s not to like. 

All good, but what you say has nothing to do with cost/benefit, and this is the central theme of my post. One could say “I really like my house because it has a huge central heating system for winter and a magnificent aircon system for summer”

>ProDave

>I would be REALLY surprised if your real-world energy saving is only £81 per year with the mvhr.

Those are the numbers which I have calculated and I have more than 10 years of experience running these system, that’s the simple house heat numbers, I invite you to prove me wrong with data rather than conjecture.

>Are you taking account of the fact that if you don't fit mvhr you will have to fit trickle vents in EVERY window.  You will have to fit an extract fan in the kitchen, utility and every bathroom.  Unless you spend a lot of money on those fans, they will be leaking air all the time even when not on.

Most windows which require them (like bedrooms) already have them. Fitting extra trickle vents in walls is very simple and not expensive compared to an MVHR. Most bathrooms already have extractor fans, which cost £30-50 for a good one and already have flap valves when they are off but are still a bit leaky but are not that bad. Could be improved. For the two years before I fitted this system, I had largely given up on turning the shower extractor on because there was no damp problem and the ensuite cleared within about 15 min due to the slightly leaky flap valve on the old extractor. 

>So, with all the ventilation you will NEED I suspect your ACH will be a lot more than 0.3 just by fitting all the ventilation that building regs require.

I have measured my needs based on damp control (not an issue at all in my house) and CO2 monitoring. Both are comfortably satisfied by 0.1 ACH, in fact over the last few months I have dropped it to 0.075 based on C02 monitoring. If there were a few more people I would up it to maybe 0.15 ACH, or, as I do press button 2 on the controller for 0.3 ACH with cooking and guests. My master bedroom reads 750ppm CO2 in the morning against a background of 400 ppm and a house average of 550 ppm when occupied by two people indoors all day. And for example, a school maximum recommended is 1500 ppm. Again, I ask you to present facts rather than conjecture.

>Sealing the house up and not fitting mvhr is not an option unless you make other ventilation provision.

That’s obvious, my house is, what’s your point?

>But do carry on and when it's finished let us know your real-life energy bills once you have made all those holes in your house. And let us know if you get any condensation or lingering smells anywhere.

I have been running this house on MVHR for 6 months over a stormy wet winter. I have had no problems. I have now ‘let you know’ - Please comment.

>For me, mvhr is worth it, not to have anti back draught flaps clattering in the wind, not to have internal doors blow shut when you open an exterior door etc (with a sealed house with mvhr you can open 1 door or 1 window even on a windy day and not have any through draught)

These are trivial/non problems. If your flaps are banging then put a put a bit of rubber trim on them. I had the same problem on my last house in an elevated position in the Mendips and the rubber trim sorted it. Even then it needed very high winds to cause the problem. Are you seriously suggesting that one spends thousands of pounds to not have an open-door slam in high winds or flaps clatter?

>Simply Simpson

>what are they? does a trickle vent move the air where it is required to avoid the build-up of condensation and therefore mould growth? mvhr supply is in the opposite corner to the extract and will move more air from across the room. 

They are vents in the walls or the windows, commonly built into double glazing units for many years now. If you wish to use them instead of MVHR then you need to place another vent on the other side of the room or in an adjacent room through the ceiling. Thermo-syphoning (hot air rises) will do the rest. You don’t need that much flow, see previous comments. If you do need to control damp, maybe you should look at the damp problem first, MVHR is expensive.

<Jeremy

I think you are clutching at straws here, however…

<Trickle vents have a very highly variable flow rate, from near zero on a still day (so almost no ventilation) to a very high flow rate when extraction fans are running.

Then just open the windows in summer, that’s what everyone else does!

>Noise was one of the main problems we had. In two of the three houses we've lived in with trickle vents they howled like a banshee with the wind in a particular direction.

Get better vents. Also are you seriously saying that in high winds you can’t hear wind noise from anywhere else in the house? I don’t think so.

>Dirt was another issue; they produce dirty smears where dirt and cobwebs get pulled in around them. 

That’s valid but dirt accumulates all over the house from all sorts of sources, the house always needs cleaning. An MVHR will not stop the general cleaning requirements on a house.

David Hughes

 

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11 minutes ago, DavidHughes said:

<Jeremy

 

I think you are clutching at straws here, however…

 

<Trickle vents have a very highly variable flow rate, from near zero on a still day (so almost no ventilation) to a very high flow rate when extraction fans are running.

 

Then just open the windows in summer, that’s what everyone else does!

>Noise was one of the main problems we had. In two of the three houses we've lived in with trickle vents they howled like a banshee with the wind in a particular direction.

 

Get better vents. Also are you seriously saying that in high winds you can’t hear wind noise from anywhere else in the house? I don’t think so.

>Dirt was another issue; they produce dirty smears where dirt and cobwebs get pulled in around them. 

 

That’s valid but dirt accumulates all over the house from all sorts of sources, the house always needs cleaning. An MVHR will not stop the general cleaning requirements on a house.

David Hughes

 

 

 

Not at all, just relating our experience over about 25 years of living in houses with trickle vents and extractors, that's all.

 

Our house with MVHR has zero wind noise.  With the thick cellulose insulation (great for acoustic attenuation) and the 3G glazing we never hear any wind noise in the house.  All we ever hear in really severe weather is rain against the windows, even during the storms a few weeks ago.  Two of the houses we had with trickle vents were incredibly noisy in strong winds.  One even used to have some noise from the vents when they were closed (we had to close them in strong winds to stop them howling).

 

Opening windows for ventilation is OK in warm weather, but less than desirable when it's cold outside.

 

MVHR has massively reduced the cleaning requirements in this house.  The MVHR filter seems to take out almost all the airborne dirt, and, more importantly for me, the pollen.  The latter saves me a significant sum in antihistamines, as well as giving a marked improvement in comfort.  There is very little dust in the house, what does settle seems to be fluff and hair mainly.  At a guess I'd say this house only needs about 1/4 the amount of cleaning of our last house, yet it's about 30% larger.  We bought two Dyson cordless cleaners, intending to have one for use upstairs and one downstairs.  We can vacuum the whole house with half a charge from one of them, and we generally only need to do that maybe once every three weeks.

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Frankly @DavidHughes if you don’t want MVHR then don’t have it, mine cost relatively little to instal, unit from Ebay, self made manifold, terminals, silencer. DIY instal so the costs were minimal. Regarding benefits my air quality is very good and consistent so in my opinion well worth the costs and effort, if (and I am not likely to at my age) I did another new build it would be on my “must have “list.?

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