MVHR vent locations. Opinions...

[Gareth J]

Just fitted an MVHR unit for the house.

Not a well sealed, proper job, a lot of compromises but I decided the benefits of controlled fresh air, with an amount of heat recovery were worth the investment of time and money.

I have, 3 bed, 2 bathrooms upstairs, kitchen and lounge downstairs with hall/stairs between. Downstairs rooms have for sure a lot of leakiness via the suspended, vented timber floors and solid fuel stoves burning room air (more on that later)
-Supply vents in all bedrooms
-Extracts in both bathrooms and kitchen
-Supply vent opposite side of kitchen.

I have realised I can put a vent into the lounge, far away from the door, via a built in wardrobe upstairs to the loft (where pipework is). I feel like this room also deserves a vent. I could, reasonably easily, make it a supply or extract.... but which?

Things I'm taking into consideration;
It would be considered "normal" to make it a supply vent
The room has a WBS in it using room air. A supply would maybe be safer so as to not to risk negatively pressurise the room. Air requirement is small in operation though, vent holes, about the size of two 15mm diameter holes and I suspect that suspended timber floor more than compensates.
I'm also considering extracting because when its on, the WBS makes the ceiling area very warm - some of that through the MVHR would be valuable to transfer to farther away bedroom/s
The loft pipework would be a bit easier as an extract.

Cheers!

[Stinsy]

I’d go supply in the living room. Just make sure the airflow from the vent to the fire doesn’t pass where someone might sit or they’ll feel the draught.

[Countrypaul]

I would also go with supply vent to the lounge.

When I fitted our MVHR system, all 4 bedrooms had supply vent and the bathroom and 2 ensuites had extracts (all on first floor). On the ground floor the Kitchen, utility & WC had extracts whist the lounge, dinning room, study and far end of the kitchen had supplies. I used 70mm semi flexible piping, the ensuites, bathroom, kitchen and utility had 2 pipes to each extract vent and the WC one, the master bedroom had 2 pipes, the other bedrooms one each, the lounge and dinning room also has 2 whilst the kitchen and study have 1 each. The cooking area in the kitchen s close to the utility & WC so when cooking if we put the MVHR on boost to clear smells is pulls air right across the length of the kitchen which helps (kitchen is 6 x 12m).

I did have the "luxury" of open web joists on the first floor, no ceilngs or plaster board inplace when I fitted the pipework though, Iirc I used 200m of 70mm pipe as each room was piped back to the utility where the MVHR & Plenum boxes are.

The piping layout was designed by the supplier based on our plans.

[Gareth J]

Thanks for the input.

Yes, I'm sure the fresh input to living spaces is convention for good reason.

I'm trying to imagine how air flow/extract will impact the house but, despite spending a good bit of time involved with fluid dynamics once upon a time, I'm quite uncertain.

I *think*;

As an extract;
- It would draw air off the hottest portion of room air, great for the mvhr and moving heat around the house (no central heating)
- Air would be replaced by cold air, sinking down the stairwell into the lounge doorway at the foot of the stairs. Even more draughty - not so great.
- if the fire/extract demand was high, probably increase subfloor air entry, also not ideal.

As a supply;
-It would bring in a flow of relatively cool air, which would quickly sink down the wall, to the floor, due to the relatively big temperature differential. I don't think it would push that pillow of warm ceiling air out the door effectively.
-It would not be so likely to contribute to a negative room pressure situation, definitely a positive.

Could always try one then the other but wrestling pipes in the loft isn't fun!

[Stinsy]

Thanks for the input.

Yes, I'm sure the fresh input to living spaces is convention for good reason.

I'm trying to imagine how air flow/extract will impact the house but, despite spending a good bit of time involved with fluid dynamics once upon a time, I'm quite uncertain.

I *think*;

As an extract;
- It would draw air off the hottest portion of room air, great for the mvhr and moving heat around the house (no central heating)
- Air would be replaced by cold air, sinking down the stairwell into the lounge doorway at the foot of the stairs. Even more draughty - not so great.
- if the fire/extract demand was high, probably increase subfloor air entry, also not ideal.

As a supply;
-It would bring in a flow of relatively cool air, which would quickly sink down the wall, to the floor, due to the relatively big temperature differential. I don't think it would push that pillow of warm ceiling air out the door effectively.
-It would not be so likely to contribute to a negative room pressure situation, definitely a positive.

Could always try one then the other but wrestling pipes in the loft isn't fun!

I agree with all that. You also want the warm air from your WBS to travel as far as possible through your house before it enters an extract vent.

[Gareth J]

I agree with all that. You also want the warm air from your WBS to travel as far as possible through your house before it enters an extract vent.

Better to let it heat exchange with building infrastructure as much as possible before putting it through the mvhr I guess. It's not like I dry washing in front of the fire.

[Countrypaul]

Remember if the HE is working properly the fresh air coming in through the vents should not be that much cooler than the existing air in the house. Most MVHR systems seem to claim an efficiency of around 85% which even if outside is 0 C and inside is 20C should be around 17C - cooler yes but probably not as cool as you would get from DG windows.

We never notice any cool air from the MVHR even when the outlet is above the bed.

[jonc_uk]

I would also go with input.

..in the hope that a slightly positive pressure in the room would reduce the input of unheated air from under the floor or via the door from other downstairs areas. To me, extract would give a colder, draughtier feeling room.

I am also not convinced that the low volume of air moved around via MVHR, coupled with the low heat capacity of air, would make it of much use for shifting heat around the house.

[Stinsy]

Remember if the HE is working properly the fresh air coming in through the vents should not be that much cooler than the existing air in the house. Most MVHR systems seem to claim an efficiency of around 85% which even if outside is 0 C and inside is 20C should be around 17C - cooler yes but probably not as cool as you would get from DG windows.

We never notice any cool air from the MVHR even when the outlet is above the bed.

Surely if it was 100% efficient with an inside temp of 20℃ and outside temp of 0℃ then the air from the inlet would be 10℃?

[Sim_C]

Surely if it was 100% efficient with an inside temp of 20℃ and outside temp of 0℃ then the air from the inlet would be 10℃?

I agree, assuming the supply volume flow rate is the same as the extract.