One of the most frequently asked questions regarding the type of balanced ventilation devices known as the energy recovery ventilator (ERV) and the heat recovery ventilator (HRV) is: “Can I get that in red?” OK, not really. Everyone knows that most people prefer their ventilation systems in blue. The real question that gets asked a lot, however, is whether to get an ERV or an HRV. As you know already from the title, my take is that you probably need an ERV, not an HRV.
Climate and occupancy
Deciding between an ERV and an HRV should land on ERV for most people in most places. In a warm humid climate, an ERV brings in less outdoor humidity than an HRV. (Note: an ERV is not a dehumidifier. It does still add to the latent load in the house.) In a hot-dry climate, an HRV will make your already dry air even drier, sending your precious water vapor out into that desert air. In a cold climate, bringing in outdoor air without moisture exchange can result in extremely low humidity in winter because cold air is dry air. On the basis of climate, it’s only in mild climates where it doesn’t get too cold, too humid, or too dry where HRVs make sense . . . sometimes. That’s why they’re popular in the Pacific Northwest.
Occupancy, though, is another important factor to consider. The higher the density of people in a space, the more you might need to dry out the air with an HRV. A small, airtight apartment or condo with two or three people in it, for example, may be too humid indoors with an ERV. In a dry or cold climate, this works year-round. In a humid climate, it works in the winter, but an ERV would work better for the warm, humid days.
History, efficiency, and core swapping
Some people think HRVs are the way to go because older ERVs didn’t have good control over frost forming on the core. That’s not the case anymore. ERVs work fine in really cold places now. Another reason people choose HRVs is that they’re more efficient at transferring heat than are ERVs. What good is it to have high-efficiency ventilation, though, if you end up growing mold or going through 50 liters of skin lotion each year?
Another possibility is to do both. If you’re in a humid climate and don’t want to bring in a lot of water vapor with your summer ventilation, you need an ERV. But if your house is very airtight, you may get too humid indoors in winter with an ERV. In that case, you could have an ERV core for the summer and then swap it out with an HRV core for the winter. (Not every manufacturer makes models with swappable cores, so check the one you’re buying if you think you may want to do this.)
The primary way to choose between an ERV and an HRV is to understand the moisture control needs of the space being ventilated. In general, when the outdoor air is significantly drier or more humid than indoor air, you should go with an ERV.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard. Photos courtesy of author.
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