Mechanical heat recovery ventilation damp problem?

Mechanical heat recovery ventilation damp problem?

10:06 AM, 2nd January 2024, About 7 months ago 65

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Does anyone have experience with mechanical heat recovery ventilation units? I have a top floor flat with 2 bedrooms facing north, and a kitchen and sitting room facing south.

The outside walls of the bedrooms mainly the north-facing walls suffer from condensation and the humidity level at the moment is reading about 67 which is lower than I expected because the air in the bedrooms feels and smells dank. A dehumidifier has been installed for the past week.

I have had a discussion with a company that sells MHRV units and they suggested extracting the air from the hallway which is in the middle of the flat and pushing it into the bedrooms.

I am confused because I would have thought it would make sense to extract the damp air from the bedrooms and push the fresh air back into the hallway. The hallway is a U shape so the fresh air would be pushed back in on the leg of the U outside the 2 bedroom doors.

The company I spoke to had a floor plan and was aware we only had a problem on the north side of the building in the 2 bedrooms.

Any assistance will be greatly appreciated.

Slooky


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Slooky

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15:19 PM, 9th January 2024, About 6 months ago

Reply to the comment left by Andrew Rudge at 09/01/2024 - 15:15
It's seeming that the location we've chosen will work for the condensate drain as it's next to the wash machine and condenser dryer which drains with wash machine. Aybe messy to drill through as it is a solid wall but ..
When people say don't make the duct runs too long, what exactly would be considered too long?

SupaSarge

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15:29 PM, 9th January 2024, About 6 months ago

I didn’t suggest no need to insulate those roof to MVHR runs Andrew, just meant their run lengths are of less significance when it comes to heat exchanger efficiency (when siting it).

Not losing the heat energy of the warm supply air from the flat to the MVHR on its way to it or the energy from the warmed infeed air between it and the flat (MVHR to rooms) being key. Keeping the distance of run lengths and bends on to a minimum and insulate well shd be the design goal.

SupaSarge

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15:48 PM, 9th January 2024, About 6 months ago

That said I would challenge the need to lag the roof to MVHR ducts considering internal guttering and all the open loft voids that causes. May be worth checking the temperature gradient between the loft and outside.

With cupboard location, bear in mind you’ll be sending moist air down vertical ductwork and back up vertical ductwork. Keeping the MVHR on the same horizontal plane as its ductwork and running the drain down that cupboard seems like a much better idea to me.

Andrew Rudge

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15:51 PM, 9th January 2024, About 6 months ago

Reply to the comment left by Slooky at 09/01/2024 - 15:19
For our 73mm radial duct it was 18M max. I've no doubt it will be longer for for solid ducting.
I also tried to centralise the unit to try and keep the duct runs a similar length wherever possible. I you end up with one duct run just 2M long and another 15M long, then you may have to put a restriction in the shorter run to force air down the long run and balance the system a bit. The adjustable end terminals probably wouldn't cope with that much, and if it did it would be noisy because of the pressure behind the end terminal. These restrictions usually go in the manifold.

If the unit is going in a cupboard then the manifolds will be in loft, I would also highly recommend attenuators which would have to go in the loft as well. There not expensive but can be bulky, but they will take the drumming noise out that you may otherwise hear through the vents.

Get yourself a cheap core drill off fleabay, there more than good enough for waste pipe.

Andrew Rudge

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15:53 PM, 9th January 2024, About 6 months ago

Reply to the comment left by SupaSarge at 09/01/2024 - 15:29
I mis understood, my apologies.

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