Does an HRV system actually "dry" the air?

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kenkeniff

628 posts

Ultimate Geek

#151346 22-Aug-2014 09:35

I had an HRV salesman over the other day as I'm interested in getting some sort of home ventilation and heat transfer system installed.

HRV is a system that takes air from your ceiling cavity and pushes it down into your living spaces, thereby forcing the old sale air out through cracks in windows, doors etc. Air in the ceiling cavity is naturally replaced by outdoor air flowing in under roof eaves etc.

The salesman told me this system would "dry" out my house and wouldn't need to have de-humidifier running (currently set to maintain <=60% humidity).

I'm sceptical however as the HRV system has no compressor, condenser or drain pan of any kind; that it is just going to be pumping 70-90% humid outside air directly into the house?

When I questioned how it would dry the house when outside humidity is ~90% (rainy day) and that is getting pushed into house which I want to be around 60% I was told that the moisture is caught in the filter which is replaced once a year....which sounds like a very extraordinary claim to me.

Is there some trick I'm missing?

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timmmay

20575 posts

Uber Geek

Trusted

Lifetime subscriber

#1113303 22-Aug-2014 09:40

There are at least three huge discussions on HRV systems on geekzone, I suggest reading them for more information, especially this one. I bet this thread will turn into another of those, so you might as well read up first.

Short answer: positive pressure ventilation systems can reduce the moisture in your home as the air in the roof space is warmer and drier, but at the expense of putting things in the air you probably don't want there, like fiberglass dust. They will claim the air changes in the roof space regularly which avoids this, but if the air turns over that quickly is it really warmer or drier? Of course pumping cold air from the roof space in at night will make your house a lot colder. Also consider anything said by a salesman as a lie until independently verified.

Consider a unit with a heat exchanger that pulls fresh air in from outside, prewarmed using the outgoing air.

kenkeniff

628 posts

Ultimate Geek

#1113365 22-Aug-2014 11:00

So I guess the short answer is; it doesn't. It essentially brings inside humidity into equilibrium with outside which in Auckland averages 70-80% and up to 90% which in my opinion is not very dry (we prefer ~60%).

Temperature and humidity is not currently a general problem in the house as we have a heater on timer & thermostat set to 19 degrees and dehumidifier (both in hallway) set to 60% which it easily maintains and even has a mildew mode which drops it to 50% every now and then.

When we sleep with the bedroom doors (x2 couples) wide open the atmosphere is perfect however that is not ideal due to noise and privacy.

The problem is that when we close the bedroom doors the dry / warm air from the hallway is unable to circulate with the rooms resulting in cold damp rooms during the night (<14 degrees / 80%) + CO2 level rise quite considerably.

I thought of just installing a 2 room heat transfer kit to circulate air from the hallway to the bedrooms however I also like the idea of adding some fresh air to the mix with some sort of ventilation system as we tend to leave doors / windows closed because of heater & dehumidifier.