An ERV Is Not A Dehumidifier
A common misconception about the energy recovery ventilator (ERV) is that it’s good for humid climates because it helps to dehumidify a home. It’s usually the better choice for a humid climate when you’re trying to decide between an ERV or an HRV (heat recovery ventilator) but not because it’s a dehumidifier. It is not a dehumidifier. Here’s why.
How ERVs and HRVs work
The photo above shows the inside of what could be either an ERV or an HRV. They both work on the same principle. The difference is what happens in the recovery core.
Basically, this mechanical ventilation device is just an insulated box with two fans, four ducts, and a recovery core. It pulls in outdoor air (red arrows) and sends it into the house. It also exhausts indoor air to the outdoors (blue arrows).
The two air streams both pass through the recovery core, shown above and below. Ideally, the two air streams do not mix at all but do exchange heat (both ERV and HRV) and moisture (ERV only).
Exchanging heat is definitely a good thing. It means that when you need to bring in outdoor air to ventilate a home in the winter, you can both preheat the incoming air and rescue some of the heat in the outgoing air. In the summer, you precool the incoming air.
The humidity confusion
It’s easy to get confused over humidity, of course, because the mixture of dry air and water vapor exhibits such interesting behavior. (See two of my favorite books, Understanding Psychrometrics by Don Gatley and Water in Buildings by Bill Rose, if you’d like to crack that nut yourself.)
One point of confusion is the difference between relative humidity (RH) and absolute humidity. You might think that 100% RH is humid, but if the air is cold, there’s really not much water vapor in it because cold air is dry air.
There are different ways to talk about absolute humidity, but I like dew point. You can’t tell which is the more humid air by their relative humidities without also knowing the temperatures. Dew point, however, is a single number describing a volume of air that does tell you what might happen when you exchange indoor air and outdoor air.
An ERV doesn’t dehumidify
The confusion over ERVs and dehumidification stems from their ability to reduce the amount of water vapor introduced into the house when the outdoor air has more moisture than the indoor air. But reducing the amount of moisture coming in is not the same at all as dehumidifying the air in the home.
In fact, the ERV will usually raise the humidity in the home when the outdoor air is more humid than indoor air because it doesn’t exchange water vapor between the two air streams perfectly. Just as only some of the heat is transferred between air streams, only some of the moisture is exchanged. So the incoming air will be more humid than the outgoing air on a humid day.
That’s not a bad thing because the ERV is still reducing the amount of humidity that would have been brought into the home. If you ventilate with an HRV, supply-only, or exhaust-only ventilation sytem, you’re bringing all the humidity in.
The ERV brings in less but doesn’t dehumidify. If you want to ventilate and dehumidify at the same time, you need some type of supplemental dehumidification, such as a ventilating dehumidifier. That’s one way to deal with the higher ventilation rates of ASHRAE 62.2-2013 in airtight, low-load homes in humid climates.
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Posted In: Building Performance, Residential Buildings
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