Whole House Ventilation: ERV and HRV

2010 February 13
by Otto

Ever since we started building our house, I knew I wanted to have mechanical ventilation. The reason for that is that you want to build a house as leak-free as possible in order to conserve heat. The problem with that, though, is that the leaks in a standard house serve the purpose of removing stale air from the house and bringing fresh air in. It’s not just stuffiness that’s the danger, but also moisture control and even your health: radon, carbon monoxide, and VOCs from building supplies all need to be ventilated out to protect the inhabitants.

Mechanical ventilation allows you to control the air flow in and out of the house. But the key to achieving the best of both worlds is a Heat Recovery Ventilator (HRV) or an Energy Recovery Ventilator (ERV).  They both extract the heat (or in the summer, the chill) from the stale air being exhausted and then pre-heat or pre-cool the incoming fresh air. With a transfer of heat efficiency in the 70-90% range, you can have consistently fresh air throughout the home without paying for it on your heating and cooling bills.

Read on to check out all factors involved in the decision:

The difference between an HRV and an ERV is that an HRV only extracts the heat from the air, while the ERV extracts moisture from the air flows as well. An ERV can be a good choice for two reasons: 1. If you live in a very humid climate, you want to keep moist air from entering the house. 2. There is latent heat in the airborne water. If you keep the humidity in the house from leaving, you can capture even more of the heat in your house.

our ERV: Pretty much the definition of "Black box techology"

our ERV: Pretty much the definition of "Black box techology"

We live in a very dry climate, so most websites will tell you that we should get an HRV. We ended up with the UltimateAir 200DX ERV because my research indicated that it was the most efficient unit out there. Even with limited moisture content, extracting the latent heat from the water is still important. I also decided that I wanted to protect the additional indoor humidity that we might have (eg from a humidifier, or cooking spaghetti – we live in such a dry climate that I didn’t want to lose all that).

Since we live in a cold climate, there was also one more important consideration. When the temperature outside drops below freezing, frost on the HRV/ERV unit becomes a big problem. You can easily imagine why – cold air is coming in, warm air is going out, and wham! Frost. There is no particularly graceful way of dealing with this. Most units simply run in a recirculation mode perhaps five minutes an hour when it gets below freezing. So the incoming air shuts off and the system just blows the warm indoor air around the heat exchanger. As it gets colder and colder, the unit spends more time in recirc mode. I think they get up a point where they spend 30 minutes an hour recirculating air.

That would seem to work fine, but at that point you’re spending valuable energy on simply running a fan. Even worse, I could imagine unfortunate bathroom smells being recirculated through the house.

An Electric Preheater: Basically a glorified hair dryer

Our Electric Preheater: Basically a glorified hair dryer

The other approach is to preheat the incoming air to a point where frost isn’t an issue. I liked that better, imagining that I could blow the incoming air over a heat exchanger coupled with my solar hot water. That proved to be easier said than done (it would require a separate water loop as well as a separate pump to move the water through it). The other option is to use an electric pre-heat. This sounds like a terrible idea–electrical heating!–but UltimateAir is fairly clever about it. The 200DX unit can handle air down to 10F without frosting up. So you only need preheating for several days a year, and avoid all of the wattage otherwise wasted on the recirculating cycles of other units.

Then there’s my in-ground air tube (read about it here). I don’t know how much it’s going to pre-heat my air, but if it manages to heat up the air by 5 degrees, then my electric preheat on my ERV only kicks on then it gets below 5F. I’m hopeful that will only be a few days, maybe one week, a year.

Designing the system

An important consideration is designing the duct placement. The broad idea is simple: You want to extract air from the “dirty” areas of the house (bathrooms, kitchen, laundry) and pump fresh air into the “clean” areas (bedrooms, living room). But one design goal I had was that I didn’t want to have separate bathroom fans in addition to my ERV. And to avoid that, you want to have the system powerful enough to clear out a bathroom after a big shower. The more air extract tubes you put in the house, the more that outgoing air flow is divided up between them. So we decided to put in only four extraction spots: the three bathrooms and the kitchen. I just put in a regular bath fan into the main-floor powder room, and I just skipped the laundry room.

ERV ducting through the attic

ERV ducting through our attic

The 200DX can exchange 200 cubic feet per minute. If I have four extract points, that means 50 cubic feet per room. That’s not that wonderful jet-engine room-clearing power you can get out of a serious bathroom fan, but it seemed good enough to allow me to avoid extra bath fans. I wanted to avoid them for three reasons: 1. Bath fans cost money. 2. Installing the exhaust hose of the bath fan means yet another hole in the thermal envelope of my house. 3. All the air blown out through a bath fan contains valuable heat I want to capture.

I skipped the powder room because I figured that the room would get only occasional use. I put in the bath fan there so guests could “clean up” after themselves should the need arise.

HRV/ERV systems run most efficiently at low continuous flows. I expect to run our ventilation at about 70CFM. To clear out bathrooms, we’re also installing buttons in the bathrooms that you can push t0 boost the system into 200CFM mode for about 20 minutes.

Placing fresh air returns is much easier. Here the idea is simply to bring air to each bedroom and to each floor. The key is to avoid blowing air onto the inhabitants of the house. So hallways and behind doors is a much better place to bring in air than above the couch or a bed. It’s not very much air volume – at 70CFM that’s only just more than one cubic foot a second – so it’s more a dribble than the blowing we’ve become accustomed to with forced-air heating systems. Still, you don’t want that washing over you while you’re trying to sleep.

The ducts in the room are tiny because the system is very low-flow (compared to traditional forced-air heating systems)

The ducts in the room are tiny because the system is very low-flow (compared to traditional forced-air heating systems)

Both fresh and stale air tubes run through our laundry chute to reach the upper floors

Both fresh and stale air tubes run through our laundry chute to reach the upper floors

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