Siting MHVR extract/inlet wall or roof

[larry]

Hi, 

I wonder if I could please seek advice.

We are installing a whole house MVHR system as part of a wider scheme of works.

We have created a first floor 'plant cupboard' where a gas boiler will be located plus the MVHR unit and two of the manifolds (using a 75mm radial ducting system).

I need to choose a location for the main inlet and outlet to the MVHR. 

I have a north facing outside wall which I could use but given the soffits and the height of the flashing of a ground floor lean-to I reckon I could only get 1200mm between the centres of the holes. That would also then mean a vertical flue for the boiler.

I could put the boiler on the wall and the MVHR inlets on the roof, but I'm a bit worried then about prevailing winds blowing the boiler exhaust over the top of the MVHR inlets.

Alternatively I could place the MVHR inlet on the wall underneath a wall mounted boiler and the MVHR outlet on the roof. However this then puts the inlet and outlet at different planes. I'm not sure how problematic that is.

If none of these options work then I'd have to put the MVHR inlet and outlet on a different bit of the roof, possible but not really desirable for other reasons. 

Advice welcomed! 

 

[MikeGrahamT21]

One thing I would note is to say be careful of the MVHR sucking in foul drain gases from the soil stack.

 

With regards the boiler, you are quite right to be concerned. My gut instinct would be to go with boiler through roof, and MVHR through wall, but there would likely need to be some distance between the two. I have around 4m between my intake and the boiler flue, and the boiler flu is also downwind from the prevailing winds.

[larry]

25 minutes ago, MikeGrahamT21 said:

One thing I would note is to say be careful of the MVHR sucking in foul drain gases from the soil stack.

 

With regards the boiler, you are quite right to be concerned. My gut instinct would be to go with boiler through roof, and MVHR through wall, but there would likely need to be some distance between the two. I have around 4m between my intake and the boiler flue, and the boiler flu is also downwind from the prevailing winds.

Of course I now realise I've put the red dot on the photos in the wrong place... Should be a bit to the left!

[larry]

Just now, larry said:

Of course I now realise I've put the red dot on the photos in the wrong place... Should be a bit to the left!

Thanks Mike though. I'd go with MVHR through the wall if a 1.2m gap between inlet and outlet was sufficient. Are there views on that? As you suggest that creates the biggest gap with the boiler flue.

[PeterW]

1.2m between MVHR inlet and exhaust is fine. Can you get the flue higher up the ridge by swinging the boiler onto the stud wall to the left ..?? Ideally you want 1500 vertically between the inlet of the MVHR and the flue - if you can offset them even better. 

[larry]

6 minutes ago, PeterW said:

1.2m between MVHR inlet and exhaust is fine. Can you get the flue higher up the ridge by swinging the boiler onto the stud wall to the left ..?? Ideally you want 1500 vertically between the inlet of the MVHR and the flue - if you can offset them even better. 

 

Thanks Peter. That's helpful to know and reassuring. If I put the MHVR inlet on the left hand floor level (or at least as low as I can get it) and the MVHR exhaust as high as I can get it to the right that would give an offset between the MVHR inlet and the gas flue, and I reckon that would easily get me 1.5m height between inlet and flue. I could actually put the boiler on the right hand stud wall also going up to a vertical flue. Just need to find a gas engineer who's happy to put in a new boiler and tank and is not too expensive in winter!!

 

In another thread I've just found I came across this https://www.heatspaceandlight.com/space-mvhr-ducts-tight-combined-intake-exhaust-grille-solution/ but it sounds like in any case trying to get 1.2m physical distance between them will be a better solution 

[larry]

One other option - I've got a disused chimney in the loft that could potentially be used for either a boiler flue vent or extract/inlet for the MVHR, though it would introduce a few bends and a bit more ductwork. Worth considering?

[PeterW]

16 minutes ago, larry said:

One other option - I've got a disused chimney in the loft that could potentially be used for either a boiler flue vent or extract/inlet for the MVHR, though it would introduce a few bends and a bit more ductwork. Worth considering?

Nope ..! For the flue it would need lining and restructuring, and for the MVHR it would need ducting installed and a way to balance the “other” outlet, whether that was inlet or exhaust. 
 

MVHR straight through the wall reduces condensation risks and makes it easier, boiler through the roof is a standard method any plumber should know how to do. 

[larry]

2 hours ago, PeterW said:

Nope ..! For the flue it would need lining and restructuring, and for the MVHR it would need ducting installed and a way to balance the “other” outlet, whether that was inlet or exhaust. 
 

MVHR straight through the wall reduces condensation risks and makes it easier, boiler through the roof is a standard method any plumber should know how to do. 

 

Thanks again Peter

[larry]

On 15/12/2020 at 11:05, PeterW said:

Nope ..! For the flue it would need lining and restructuring, and for the MVHR it would need ducting installed and a way to balance the “other” outlet, whether that was inlet or exhaust. 
 

MVHR straight through the wall reduces condensation risks and makes it easier, boiler through the roof is a standard method any plumber should know how to do. 

 

Hi all, slight update here, or at least more a vent on my part! Arranged to get the boiler done first. Got a few quotes and had a Viessmann 200-w combi was due to be installed today. Plumber showed up this morning as promised. However, the central office hadn't told the plumber he'd be doing a vertical flue, and he confessed he'd never done one before and basically balked at the job saying he couldn't do it and we'd need a roofer in. Am super annoyed with the central office (who shall remain nameless for now) as I'd sent (in these COVID times in lieu of a visit!) loads of photos and videos of the layout in advance and... arrahhgh! 

[Seeoda]

I heard heard that inlet ducts should ideally not be on the north side of a house? is that good advice?

[larry]

10 hours ago, Seeoda said:

I heard heard that inlet ducts should ideally not be on the north side of a house? is that good advice?

Ah, I had the opposite in my head! But I can't tell you why and have no confidence I'm right. An alternative option would be pretty impractical in my situation though

[larry]

1 minute ago, larry said:

Ah, I had the opposite in my head! But I can't tell you why and have no confidence I'm right. An alternative option would be pretty impractical in my situation though

https://www.greenbuildingstore.co.uk/mvhr-dos-donts-where-to-locate-mvhr-unit/ suggests north is preferable to allow a better summer bypass function.