Does an HRV system actually "dry" the air?

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kenkeniff

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#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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#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.

raytaylor

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#1113345 22-Aug-2014 10:45

Yes it does work - we have had an HRV brand system in our house for the last 3 years and it works very well.
I dont feel that it brings fibreglass etc down as I change the filters regularly.

Basically it doesnt dry the are itself, it just brings in dry air from outside, and pushes the moist inside air out.
This constantly happening means that items such as furniture which absorb moisture will be dryer so in the mornings it doesnt condense on the windows etc.
So sure you get the overnight moisture from things like people breathing in the room, which creates some humidity, but its only a real tiny amount and not enough to be noticable in terms of condensation.

Because of the continuous air circulation, it is constantly pushing moise air out of the house, and bringing in dry air from outside.

On a warm day, or when sun is shining on your roof, the air warms up in the attic space before it is pumped down into the house.

Its 10.30am here in napier on a cold but sunny day - few puffy clouds but mostly blue sky.
9.8 degrees outside with a bit of wind.
22 degrees in the attic
21 degrees in the house

I like to leave the HRV set to 27 so if it does warm up during the day, the warm air is pushed into the house to get it as hot as possible while we are at work.
This heat is then absorbed by things such as furniture, into the walls etc. Then when we get home at night, the heat pump doesnt need to be used as much as the house is already warm.

Running the HRV for 24 hours is the equivalant of 10 to 15 minutes of heatpump power usage. (480 watts .14c a day vs 2000 watts .56c per hour)

Overnight it stops when its colder in the attic than in the house so its not pushing cold air down into the house. Unless you set the goal temperature to be lower than the room temperature. Which is real good on a hot hawkes bay summer night because i can take the vent cover off in my bedroom and it will bring a nice cool breeze into the room without the noise of a fan keeping me awake.

The only downside I have found is that if you have the outdoor vent kit added - it will blow air from inside to outside when its both hotter in the attic and room than what you set your goal temperature to. So if you are running your heat pump on cooling mode, you need to turn off the HRV.

Edit: I am not sure if this is the best way to describe it, but rather than using a condenser to dry the air, think of it more as forced osmosis. Why run a condenser when there is dry air outside that you can just bring in?

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kenkeniff

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#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.

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#1113375 22-Aug-2014 11:07

Ours has been running for about 9 years. Bottom line is yes, the air inside is most certainly dryer than it would be if we weren't using the HRV system. Dryer air is easier to heat than damp air, so our heating cost is significantly lower than it was prior to installing the system. Prior to installing the system we required multiple dehumidifiers to be on during the winter months. We haven't used a dehumidifier since.

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mattwnz

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#1113424 22-Aug-2014 13:05

Or you could just open a window. The air pumped in from the roof space is the same air that comes from the outside, and during winter it is around the same temperature as the outside air, unless you use a heat exchanger.

mattwnz

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#1113426 22-Aug-2014 13:15

raytaylor: Yes it does work - we have had an HRV brand system in our house for the last 3 years and it works very well.
I dont feel that it brings fibreglass etc down as I change the filters regularly.

From what I have been told the fibreglass particles can be so small that they need a very very fine filter. Not sure there has ever been any testing on the effectiveness of these filters. The new fibre glass is bio soluble, but the old stuff that many people have installed may not be.

timmmay

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#1113432 22-Aug-2014 13:37

mattwnz: Or you could just open a window. The air pumped in from the roof space is the same air that comes from the outside, and during winter it is around the same temperature as the outside air, unless you use a heat exchanger.

To be fair, a motorised system probably puts more air through, and the roof cavity while dirtier is generally warmer than outside air.

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DarthKermit

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#1113445 22-Aug-2014 14:12

timmmay: 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.

That really depends on your house design. Ours is very drafty.

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mattwnz

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#1113451 22-Aug-2014 14:20

timmmay:
mattwnz: Or you could just open a window. The air pumped in from the roof space is the same air that comes from the outside, and during winter it is around the same temperature as the outside air, unless you use a heat exchanger.

To be fair, a motorised system probably puts more air through, and the roof cavity while dirtier is generally warmer than outside air.

Totally depends on the house. Some old houses are very drafty with big gaps in the windows and doors, where the house may never pressurise enough for the system to be effective. I believe it works best on newer houses with newer windows that are better sealed. Also would depend on the size of the house, as the larger it is, the more windows and cracks it would have making it less airtight, meaning you would probably need a bigger more powerful fan to pressurise it, which I presume would be noiser. I believe you can do pressure tests on houses to see how sealed they are, which I think would be a must if you are setting up such a system. Unless you know how leaky the house is, you don't know how large a fan you will need.

kenkeniff

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#1113468 22-Aug-2014 14:43

timmmay: 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.

You were right ;-)

redjet

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#1113490 22-Aug-2014 15:16

Purchased an HRV system over five years ago and it works really well. It completely eliminated the crying windows we had in winter.

mattwnz

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#1113526 22-Aug-2014 16:04

redjet: Purchased an HRV system over five years ago and it works really well. It completely eliminated the crying windows we had in winter.

I solve that too by having the window open a crack to let out the moist warm air. I never have had a problem with condensation when the window is open. But it all depends on the house, and there is no one solution that fits all situations.

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#1113538 22-Aug-2014 16:35

i'd just spend the money on heatpumps. but that's me.

timmmay

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#1113558 22-Aug-2014 16:44

Ventilation and heating are complimentary. A dry home is easier to heat, gets less condensation, and is probably generally nicer. A heat pump in a damp atmosphere will make condensation worse.

Niel

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#1113579 22-Aug-2014 17:14

No, the air will certainly not be dry (except for some/few parts of the country). The reason why I say this is because as an example in Auckland the humidity virtually never drops below 80%. In December it never drops below 90%, typical is over 95%. This is far from dry, and without removing moisture there is no way that air will become dry. Slightly dryer yes, or rather less humid, but certainly not dry. Actually this makes product warranties interesting, because e.g. LCDs are specified to be used below 70% humidity.

If you want the house to be truly dry, get it mostly sealed up (e.g. sealed down lights and draft stoppers, especially for bathroom fans and range hood extractor fans as it is just a 125mm hole in the ceiling ducted to the outside) and install an air circulation system to prevent stale air and get a dehumidifier with a humidistat. We have done this and added an aircon, we use the dehumidifier only every 3rd day or so, and it is comfortable with temperature 20-22 degC (with aircon set to 18 degC day, 20 degC night) and humidity 50-60% day and night. Suddenly for the first time in 14 years in Auckland I have no more ear and sinus infections...

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