Gd question. On a morning like this when not a leaf is stirring outside, there won't be any 'natural' ventilation except by stack effect - so MVHR would work usefully.

Any ideas whether MVHR would actually helpfully upset the weak process of stack effect?

Not trying to play with semantics, it depends what you define as worthwhile.

To make it economically worthwhile in that running the MVHR costs less than the savings made on the heating bills, it seems an airtightness of about 3 ACH or better is required. My sources are the PHPP calculations our architect did for our house, a lot of hearsay, the assessment from a very experienced airtightness tester (without any calcs to back this up) and a shoddy SAP based study linked to this forum ages ago :-)
Needless to say the economic case depends on the heating system, and the efficiency of the MVHR unit. An important figure of merit is the energy it takes to shift a m3 of air, or the W/m3hr number.

My house is probably not airtight up to 3 ACH yet. It is a small-ish semi with a high surface/volume ratio, which does not help with the ACH, or m3/m2 AT rating.
Having installed and lived with MVHR for a couple of years now, I am (would be) happy to pay extra to run it.
The following benefits make it very worthwhile in my opinion, but do not translate in a price tag easily:
* ventilation is near the nominal level, independent of outdoor wind speeds and without feeling uncomfortable
* CO2 levels are kept below 1000ppm day & night
* humidity is no longer unhealthily high.
* indoor temperatures are now a lot higher in winter and much more stable throughout the year
* after fitting G4 & F7 inlet filters, one household member could stop taking anti allergen medication
* a fixed, defined flow pattern is maintained throughout the house. No more lingering cooking smells outside the kitchen.
* no more condensation, mould formation etc in the bathroom

I think there are lots of opinions on Google because 'it depends'! Been working on this for our houses and the picture has changed a lot recently, as grid electricity has decarbonised.

Are people thinking "is MHRV financially worthwhile" -in that case, generally no. The cost of running the fan is cheaper than the value of the heat recovered, but not enough to payback the installed cost of the MHRV system during its lifetime. Depends a little on how you heat the air, direct-electric heat is more costly to waste than gas heat or solar gains or heatpump, and on how gold-plated or bargain-basement the MHRV system is, and whether installed during new-build or retrofit. Also depends how efficient the fan and exchanger are, and whether the flow is controlled as needed by a humidistat, or just left on at 0.5ach all year. Also depends how you would ventilate if you don't have MHRV, eg do you have dMEV or just open the windows. But generally it won't save money.

Are people thinking "does MHRV save carbon or natural resources" - in that case generally yes. The electricity to run the fan is rapidly decarbonising. Again it depends on how you heat the air, MHRV saves vastly more carbon in a gas-heated house than in a heatpump or solar heated house. (All houses are solar heated, for several months each year).

Are people thinking about other benefits such as Bhommels list - well that depends how much you value those, but most people think it's worthwhile to spend some money to have a more comfortable house.

Edit to add but maybe it's obvious: if the house is so leaky that it is already adequately ventilated just by the leaks, then just bolting on a MHRV will just increase airflow, cost and resource-usage. You do need to reduce leakage to the point that some deliberate ventilation is needed, before considering MHRV. All of the homes I have ever had, had some rooms which were that leaky when I moved in to them, all pre-1980ish houses. Initially it's only the kitchen and bathroom which need deliberate ventilation during heating season, takes some work before the living rooms also need deliberate ventilation.

Posted By: an02ewintermittent extraction working as required (probably less than every use of the room) would be more benificial.

This seems to me to be a very important aspect, I want to ventilate my bathroom, kitchen, spare bedroom all at different times of day depending on usage. I want different temperatures in different rooms at different times.

Mainstream MHRV design has a single fan unit, so if you increase the ventilation rate due to humidity in a shower room, the ventilation rate for all the other rooms increases unnecessarily.

For space reasons, I am working on having several smaller MHRV units serving different parts of the house, this does have the benefit that I can boost the ventilation in (say) the kitchen/utility without boosting the upstairs bedrooms unnecessarily. And the supply/extract can be different temperatures for different rooms.

The extreme of this idea would be one micro-MHRV for each room, maybe in the window frame, which would ventilate just that room depending on how/when it is used. Not seen that yet, probably expensive, although we do have one single-room MHRV which was cheap, effective but noisy.

Posted By: WillInAberdeenI want to ventilate my bathroom, kitchen, spare bedroom all at different times of day depending on usage. I want different temperatures in different rooms at different times.

Your use case is quite different to mainstream MVHR usage, methinks. In the big scheme of things, the individual extract rates aren't that great or that important. The largest rate is the kitchen (it's the only room that requires a double duct in our house). The extract rates at all times have to match the supply rate and that is set by IAQ requirements. As you've said, that can be optimised somewhat by e.g. humidity control. But then you use the artificially high constant 0.5 ACH as a strawman. ISTR the building regs figure is 8 l/s (30 m³/hr) per person for excellent IAQ and 4 l/s per person minimum with a backstop of 0.3 l/s/m². Intermittent extractors are required to operate at much greater rates than continuous ventilation - double or so. And of course they usually present a severe airtightness challenge.

With two people in our house we normally run the MVHR at 50 m³/hr. It gets boosted to 125 m³/hr overnight in winter so our heating works. If we have people staying then we increase the rate appropriately. We don't increase the ventilation rate for showers nor for normal cooking; there's no need. In summer the rate varies depending on the need for fresh air to cool the house.

Most MVHR systems aren't involved with the heating system, so whether or not you have different temperatures in different rooms and/or over time is largely an independent question. It has more to do with how well insulated and airtight the house is, IMHO, since the extended response times of a well-insulated airtight house make it a somewhat different problem. In our particular case, the MVHR is part of our heating system, since it is practicable.

I'm interested to learn about any new developments that deal with the issue of wind driven infiltration/overventilation if there are multiple ventilation inlets and outlets on different faces of a building.

Posted By: djhartificially high constant 0.5 ACH as a strawman. ISTR the building regs ... backstop of 0.3 l/s/m²

0.3l/s/m² = 0.5ach, for the usual height room. No strawmen there!

It's still the minimum MHRV capacity, even in the new 2021 version of the English regs. From previous discussions on here I understand that many folks have their MHRV commissioned at that rate, and then leave it running so forevermore.

And the minimum intermittent extract rate for a kitchen with a recirculating cooker hood is 60 l/s = 216m³/h. The minimum continuous rate is 13 l/s = 50m³/h. As DJH mentioned, that much should be enough for a whole house, nevermind just the kitchen.

Edit: Today, indoor humidity here = 11g/m³ and outdoor= 4g/m³. A ventilation rate of 60m³/h would remove 60*24*(11-4) = 10kg/d of moisture, which is about right for our family of 5. That would be about 0.2ach, if it were ever possible to get our house that airtight which I doubt. If that much air (or more) is supplied by leakage, then adding MHRV would just add to ventilation losses, not help. However that ventilation needs to be mainly from the bathroom at shower time, the kitchen at teatime and the bedrooms overnight.

However in April, the outside humidity will be much greater and we’ll need much more ventilation.

Posted By: djhHmm I think your numbers are right but your conclusion is wrong.

I stand well corrected. Your numbers are quite correct. It just didn’t seem a lot when seen at a glance.

Posted By: WillInAberdeen60 l/s = 216m³/h. The minimum continuous rate is 13 l/s = 50m³/h. As DJH mentioned, that much should be enough for a whole house, nevermind just the kitchen.

But these are intermittent at best, some being never used or isolated, my auntie tapes up all extraction for fear of spiders getting inside, and doesn’t have an issue with damp or mould. Poor AT is managing any excess moister. Clearly continuous ventilation would add nothing.

Posted By: an02ewBut these are intermittent at best, some being never used or isolated, my auntie tapes up all extraction for fear of spiders getting inside, and doesn’t have an issue with damp or mould.

In our airtight house we don't really get spiders inside, and my wife tends to keep an odd one to catch the occasional fly/mosquito.

Damp and mould isn't the only concern; just the most visible. There's plenty of evidence about fresh air and decision-making ability in humans now. I think the Swedish did a lot of work in schools, for one example.

If your auntie's poor airtighness is managing her moisture, I'd be checking for interstitial condensation.

Posted By: WillInAberdeen0.3l/s/m² = 0.5ach, for the usual height room. No strawmen there!

Perhaps you'd like to show your working, to demonstrate the lack of strawmen?

Posted By: WillInAberdeenWhere's that from? AD F requires minimum 25,31,37 l/s for 2,3,4 bedrooms, or more depending on floor area. That's 133m³/h minimum for a 4-bed which is way more than we seem to need. Scottish standards require 18l/s = 65m³/h based on number of wet rooms.

ADf table 5b states minimum supply rate for 1 bed 13l/s (assumimg 2 occupants) and 4l/s for each additional bedroom OR 0.3l/s per m2 floor area whichever is greater.

however to balance the system one should concider the minimums for extract too, ofter when added together these can be larger due to the trend of en-suites for every bedroom.

Posted By: djh+1

At any level of AT? evan at numbers higher than 4 or 5 AC/hr fabric leakage?

And

Why are LABC insisting on continuous ventilation once trickle vents are removed?

Posted By: an02ewWhy are LABC insisting on continuous ventilation once trickle vents are removed?

Seems odd to me. But it also seems odd to me that some self-builders like to try and pull the wool over BCOs by pretending to have the ventilation and/or removing/blocking it. Does them and their house no good at all. It also seems odd it's so often assumed you have to have trickle vents in the windows to satisfy the regs on ventilation. I think there are many BCOs who misunderstand this also.

In short, the whole thing's a bit odd...

Posted By: WillInAberdeenA) ventilation per unit floor area, AD F requires 0.3l/s/m² = 1.08m³/h/m²
B) room volume per unit floor area= 2.3m³/m² (for typical room say 2.3m high)

ventilation per unit volume A÷B = 0.5 per hour

OK, so there we have it. You think 2.3 m is a typical ceiling height! Historically, I believe the MINIMUM ceiling height was 8' (2.44 m) and then briefly reduced to 7'6" (2.286 m) before minimum heights were abolished. By a strange coincidence, standard PB, OSB etc sheet lengths are 8' now 2400 or 2440 mm. I wonder why that is? So in my opinion 2.4 m is a better estimate of the usual ceiling height.

I'd understood that ancient versions of CIBSE guide mentioned 0.5ach

Not that I'm aware of. I thought the magic number that got quoted was 0.44, which corresponds to the 8' ceiling height figure, but I can't find it now.

You mentioned running at 50m³/h - how many ACH is that? (Show working please )

Yes, scarily low. Something like 0.135 (PHPP volume is 369 m³) But for seven hours overnight it's 0.339 or 0.447 depending whether it's run at 'normal' or 'boost' setting. (which affects the heating) So about 0.2 ACH on average.

Posted By: WillInAberdeen

Posted By: djhthe building regs figure is 8 l/s (30 m³/hr) per person for excellent IAQ and 4 l/s per person minimum

Where's that from? AD F requires minimum 25,31,37 l/s for 2,3,4 bedrooms, or more depending on floor area. That's 133m³/h minimum for a 4-bed which is way more than we seem to need. Scottish standards require 18l/s = 65m³/h based on number of wet rooms.

The 4 l/s is the figure from Part F (2010-2013) and the 8 l/s is from memory but I haven't found the reference. It might be in one of the BRE documents or maybe the compliance guide; I haven't checked. Part F (2021) apparently changes the basis completely and now has 6 l/s per bedroom irrespective of how many people are in it. Presumably a compromise between one and two people and god forbid you have more than two in one room apparently. The whole thing is a bit of a mess IMHO. But I suppose they're trying to size the systems (ventilation holes or fan power) and how far you open the vents or how fast you run your fans is up to you.

If you would much prefer 2.4m to 2.3m ceiling height?

A) ventilation per unit floor area, AD F requires 0.3l/s/m² = 1.08m³/h/m²
B) room volume per unit floor area= 2.4m³/m² (for typical room say 2.4m high)

ventilation per unit volume A÷B = 0.5 per hour, still (to the conventional 1 s.f.)

RICS consider anything above 1.5m ceiling height to be habitable, apparently.


Cibse guide B2 (2005): ”a whole house ventilation rate of 0.5 air changes per hour"

Cibse guide B2 (2016): "Formerly, much use was made of expressing ventilation rate in terms of air changes per hour (ACH). This is now falling out of favour"


If building standards told me to install 2x as many front doors as I really needed, but that it would be up to me how far I opened them, I'd be miffed! Definitely a mess.

That's right, the vernacular houses round here have the first floor within the roof, chalet style, with a full-height section in the middle of the room and sloping ceilings to both sides. The ceiling height when averaged across the whole room is well under 2m (not good as I'm 1.9m tall!)

Scottish ventilation standards are different but not necessarily clearer. There is a minimum extract rate for wet rooms which is less than the rates in the English standards. There is no ACH or l/s/m² specification for continuous mechanical extract ventilation, instead it must comply with CIBSE B2 (see above).

MHRV must comply with an obscure BRE guide from 1994, which suggests that (leakage+deliberate) ventilation should add up to 0.5-0.7ach, and that leakage will be 0.7ach for a typical house, or 0.2-0.35ach for a specially detailed house. So MHRV should be sized for 0.3ach or less, maybe 0, which is hardly prescriptive!

I also noticed that the new English regs allow you to pick any ventilation rate you like, which is supported by 'expert advice'. The definition of who is an expert, is fairly broad.

Odd as air leakage is very low in calm weather, even allowing for stack effects.

When it is very windy too much unwanted ventilation causes excessive heat losses.

Air tightness is the answer, fix the problems then proper ventilation will do what it says on the tin.