Retrofitting MVHR

[Why Not Indeed]

[Not sure if this is the right subforum but it seems the closest]

I am considering installing MVHR in my semi.

Hopefully relevant information:

- Built in mid-1960s, 130m^2
- Brick cavity walls, not insulated
- Gas central heating
- Unit to be installed in loft
- Not really sure how air-tight it is. Modern double glazing, sealed loft hatch, no bare floorboards

Outline of a plan:

- Unit to be installed in loft
- Upstairs rooms will be easy to "plumb in"
- Downstairs...can get to the kitchen via the back of some ghastly fitted wardrobes, wet room will require some boxing in of pipes from the room above.


Does anybody have any tips, advice, experience or useful resources for this type of project?

Does it make sense to ventilate hallways? Upstairs would be easy, downstairs could be down the back of an airing cupboard.

 

[Roddy Young]

Hello,
we built a new house in 2011 with MVHR system. As long as you can draught proof your house, then your project should work.
Halls and "dead space" do not need ventilating but if you really do dry clothes in the airing cupboard then it would be sensible to have an extract vent there.
I believe the "Heat Recovery Ventilation" system, as it was called then, really does work.
It keeps the whole house surprisingly dry.
It also saves energy by not throwing warm air out of the windows either through trickle vents or a daily "we need some fresh air in here...."
I collected my experience of the unit and wrote a couple of webpages : https://hrv.yolasite.com/
Have a read and I will happily help answer any questions you have.
Roddy

 

[Why Not Indeed]

Halls and "dead space" do not need ventilating but if you really do dry clothes in the airing cupboard then it would be sensible to have an extract vent there.

I hadn't considered literally ventilating the airing cupboard, what I meant was, it's an unobtrusive route down to the ground floor ceiling from the loft.
Over the stairs, we do dry clothes from one of those cast-iron-and-wooden-slats things [the name of which escapes me]. It works well due to the convection of air up the stairs. But I am not sure if that means it would make sense to extract air there?

Can I drain condensate into the soil stack? That conveniently comes up through the loft. I've no idea if foul air from the soil stack could make it into the fresh air supply through the condensate pipe - I guess that's a design detail of each MVHR unit.
I don't think I am worried about freezing, I don't recall it ever being below 0 up there, 12 month low is 0.5°C.

 

[Roddy Young]

The moisture from the drying clothes pulley will migrate around the house. It's not a bad idea to extract the damp air near where it is produced, so if the clothes are hung up in a utility room or kitchen, then the extract valve in that room would do the job. Even if the pulley is somewhere else, the damp air will be extracted - just a little slower and giving it time to condense on cold walls or windows.
We feed the condensate into a soil pipe. It isn't like a boiler with acidic condensate - it's pretty much distilled water. We have a wee u bend in the 20mm pipe to stop smells.

 

[Why Not Indeed]

u bend in the 20mm pipe

D'oh - obvious now you say it. Yes, that's going on the parts list.

 

[Why Not Indeed]

- Not really sure how air-tight it is.

It probably doesn't matter that much. Passivhaus Trust paper reckons that the effects of air-tightness [or should I say, -looseness] are over-stated:

https://www.passivhaustrust.org.uk/UserFiles/File/research%20papers/MVHR/2020.04.27-The%20Case%20for%20MVHR-v7%20new%20cover.pdf