Heat recovery ventilation units

Nov 06, 2023 Last reply: 2 years ago 8 Replies

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Tim+ 2 years ago

Anyone fitted one? We have a relatively large house but most of the time there are just the two of us knocking about in it. We’re trying to reduce heat loss in our sitting room and bedroom but those two rooms have fixed air brick type ventilators that can be major cold spots depending on wind direction.

We have experimented with temporarily blocking the bedroom one but it had a significant effect on condensation. I think some sort of heat recovery ventilator plumbed into these two rooms would have a significant effect on reducing humidity and heat loss. I don’t think that we need a particularly large unit though.

Any recommendations from someone who has fitted one? Looking at units like this.

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Tim

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Theo 2 years ago

I've not fitted one, but just to note you can get single room aka 'decentralised' MHRV units, eg

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They can be a drop in replacement for an extractor fan, so perhaps would fit in your air brick, although to drain condensate the outlet needs a slight slope.

Might be easier to manage than a whole house unit with ducting.

Not sure how they cope with wind blowing directly at them.

Theo

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Jeff Layman 2 years ago

I've been considering something like that as we have a condensation (and mould) problem, but I wonder how much difference it would make. I have a weather station, and at this time of the year the relative humidity outside is /consistently/ much higher than the humidity inside. Let's assume a room temperature of 20°C and RH of 70%, and an outside temperature of 10°C and RH of 90%. According to the table at

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, the absolute humidity of the room gives 12.1 g/m^3 of water, and the outside has 8.5g/m^3. So although it's less, it's not that much less, at around 3.5g/m^3.

How much water does that unit actually extract from the air? I couldn't see a figure; all it mentions is "Condensation Drain fitted to bottom of Unit". My Dimplex dehumidifier removes about 1.5 litres in 12 hours from a room of about 3.5 X 3.5 x 2.5 m^3, and it's rated at 440W - around 10 times that of the recovery unit.

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Fredxx 2 years ago

What it does mean is none of that 8.5g/m^3 will condense on cold surfaces as long as they're not colder than the outside temperature.

The usual technique is to remove warm humid air from places like kitchens and bathrooms and pipe in dry air into habitable rooms.

Air recovery systems may provide improved air quality as well as remove some traces of Radon.

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Animal 2 years ago

Should be able to make a decent one for much less, it's only a crossflow heat exchanger & fan.

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Animal 2 years ago

Absolute humidity is typcally lower outdoor in winter, even on a wet rainy day. The amount of heat loss of HRV is a big determinant of the economics. Homemade HRVs can be relatively good.

depends entirely on conditions

and wastes no heat.

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David 2 years ago

Have you considered blocking the bricks, then using a small dehumidifier to remove the condensation?

This might be a far less disruptive measure.

Oh, and what temperature is the room, normally? Colder temperatures will encourage condensation.

Cheers

Dave R

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RJH 2 years ago

IIUC, it's not a dehumidifier - it just extracts stale air and recovers some of the heat, which is then used to warm the air coming from outside. Moist air will condense on cold surfaces as it goes through the device, but that's not its primary function and pretty much a coincidental benefit.

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Animal 2 years ago

which dehumidifies

Condensation is not how it dehumdifies.

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