@Heacol, I didn't misunderstand but we may be talking at cross purposes even so.

If an MHVR deals with 10m3 air, it can only reclaim the heat that's in that air. As far as I can see, in order to deal with condensation you only have to extract a specific amount of air (the volume of the bathroom) and that contains a finite amount of heat.

Now I don't dispute that this means the MHVR is therefore going to be sitting idle for most of its life, only becoming useful when someone goes in and has a shower. If it was running constantly (and there was a need for it to do so) I can see that it would reclaim far more heat thereby saving far more money, and given it still cost the same its return on the investment becomes massively greater. However, there isn't the need for it to run all the time, so the sums just don't seem to add up.

If, as I think you're suggesting, a house had all its ventilation stopped up and then replaced by holes managed by a whole-house MHVR system, suddenly this changes the amount of air being processed and therefore the amount of energy being reclaimed. I can totally agree, and if I were to contemplate building my own house that's one of the things I'd want to design in right from the start. As with anything, if you have a tool with a fixed cost and can use it all the time to save money, the original cost quickly becomes insignificant compared with the savings.

@majordennisbloodnok MVHR fans should run continuously 24-7 365. A person exhales about 1 cup of water per day, that is probably more than you produced in a single shower, that is where most of the moisture in a house comes from. Moisture, like any gas, will move from a high concentration zone to a low concentration zone. Therefore, if your MVHR in the bathroom is on, it will remove moisture from the entire house. A dry house is healthier to live in and cheaper to heat.

I have personal experience of this. My bathroom used to have mould prior to installing one of these units. I only have one in my house, and my gas usage went down by about 30% when I installed it and the living environment improved.

PS: Rooms in a house do not live in isolation, air and heat moves between them. Your insulation and barriers are on the outside of your house, not between your internal rooms. Installing a MVHR is not all about savings, it is also about improving your quality of life. A stuffy, oppressive house is not a pleasant place to be in.

Posted by: @heacol

If you install any form of PIR (kingspan, celetex ect), do not expect your building to be airtight or suitable for and MVHR system.

I'm confused. How do Passive Infra Red sensors stop a building being airtight? Do they need draughts to work? I didn't think so.

@mjr PIR stands for polyisocyanurate, which is a type of rigid foam insulation that's used in building construction.

And this is, I think, where we're converging on a point made quite some time ago on this thread about new builds. Our house is not a new house (circa 1930s, I believe) and has had recent upgrades to insulation in preparation for the now complete ASHP installation. Our largest room has a lot of glazing, so even in the winter a sunny day helps dramatically with heating, and during summer we generally have windows open so it certainly isn't a stuffy or oppressive house. By the standards of the forum, our ASHP is a small one, yet it is perfectly able to cope with sub zero outdoor temperatures. As a result we're not using large amounts of energy to heat the house or water, and it demonstrates in practice that by whatever route our heat loss is pretty low - particularly for the type of house.

But it's certainly also not an ideal candidate for all-house MHVR. The bathroom is a relatively cool room and the condensation issue has less to do with the amount of moisture and more to do with short term increases in air humidity meeting still cold walls. I have little doubt that an MHVR unit would help clear the damp air quickly but there would be little point in keeping it running constantly since there is no other humidity or quality of life problem needing to be solved, and we especially don't need more ventilation. Under these circumstances I've yet to be shown an MHVR would be worth the cost.

@heacol right-o. Might be a good idea to expand that abbreviation on first use because those of us coming from "smart home" will be using Passive Infra Red sensors more than builders' insulation.

@heacol While we're talking initials: What's your opinion of PIV ? Positive Input Ventilation. Is it just a way of losing heat without having the windows open, or is it better than that?

you could of course just open the bathroom window and have the ventilation on when with the door closed when showering. Then the only room cooling down is the bathroom, the majority of the airflow will come via the window as gas moves via the path of least resistance. Then close the window, open the door and shut down the fan. Get a fan with self closing vents.

Brendons system will work, in a closed house which is very air tight. I would imagine doing this kind of air tightness as retrofitting is a large cost (or your own time)

• seal loft hatch
• seal any external pipework exits
• seal doors (exterior)
• seal/replace windows with non-opening ones
• no trickle vents
• no extractors in the bathrooms or kitchens
• seal any gaps between exterior walls or windows/doors around the edges
• seal any opening into/around lofts e.g. light fittings

you cant just seal them to an "ok" standard. They need to be well sealed.

@balalto - that's all necessary for MVHR but what about PIV ? Where heat recovery doesn't seem to be a thing ?

@heat-pump-newbie from my understanding PIV is about positive ventilation e.g. where you over pressure the house. This means air cannot enter from outside bringing in nasties. The air loss would be the same and it's not dependent on losses as it is in essence, just a fan and filter; it's cheap. MVHR is about recovering heat. You can't recover heat in PIV because it's not drawing air out, it's only pushing it in

@batalto Ah ! that would help keep out the pong of our neighbours' wood burning stove then !! Definitely worth investigating 👍 

@heat-pump-newbie that would depend if the filter gets out the smell. Otherwise you're pumping in the smell