Tight, super-insulated houses need some kind of mechanical ventilation to keep indoor air healthy. Installing exhaust fans in the kitchen and bathrooms is the simplest option. More often, energy-efficient builders install either a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV).
Both systems incorporate a heat exchanger that moderates the temperature of incoming air, which helps ease the energy penalty in both summer and winter. But ERVs also are designed to transfer some of the humidity in the air stream. And that’s what’s puzzling Donald Lintner.
Lintner is building a house in upper Michigan with R-40 walls and an R-60 roof. Winters are certainly chilly (9,500 heating degree days per year), but Lintner doesn’t expect to need air conditioning.
“Most sources seem to suggest an HRV in this situation,” he writes, “but I have had several people recommend an ERV as the better choice. What would I gain with an ERV in my situation? I thought ERVs were for air-conditioning climates?”
Either one will work
ERVs may help a little, says GBA senior editor Martin Holladay, but it probably won’t make a lot of difference. Either option should work just fine.
“Some cold-climate homes get too dry during the winter,” Holladay says. “For such homes, an ERV may help a little — by transferring a small amount of moisture from the outgoing stale air to the incoming fresh air.
“Although ERV manufacturers are strong believers in the value of humidity transfer between air streams, most energy researchers have concluded that the results are not very significant,” he adds. “That said, most purchasers of HRVs and ERVs are happy with their equipment.”
Still, says Robert Riversong, the drying potential during humid weather shouldn’t be overlooked. “The primary advantage of an ERV is not the minor latent heat savings in cold climates,” he says, “but the dehumidification energy savings in a humid summer climate.”
Maybe in theory, Holladay replies, but actual performance is another story. He cites research by Max Sherman at the Lawrence Berkeley National Laboratory suggesting the dehumidification potential of ERVs in hot, humid weather is minor — so minor it’s “hard to measure.”
“The only problems with the idea are math and physics,” Holladay adds. “In areas of the country where air conditioning is common, one of two situations applies. Either (1) Air conditioners work pretty well at keeping indoor air dry, or (2) air conditioners can’t keep up with latent removal, leading to periods of high indoor humidity.
“Situation 2 occurs in Houston. If you are somewhere where the outdoor air is so humid that you have this problem, an ERV ain’t going to solve it. There’s just too much moisture involved for the ERV to make much of a difference.”
It’s really about energy
You’re missing the whole point, Johan tells Holladay.
Yes, air conditioners can reduce indoor humidity, he says, but the question becomes one of energy consumption. Evaporating one liter of water with AC requires the same amount of energy as leaving a 60-watt light bulb on for 11 hours.
“The conclusion is impossible to misinterpret,” Johan says. “If your AC condenses 2 liters in a night, you could have had your outdoor lights on for this time instead if you used ERV technology… This is not fiction or guesswork, but math and physics at its simplest.”
Johan adds that ERVs work on the “principle of vapor pressure difference,” meaning the higher the difference in humidity and temperature in the two air streams, the more efficient ERV becomes. “The truth is, the more humid the climate the more energy is saved by an ERV.”
One caveat: ERVs aren’t designed to work as stand-alone units. They should be paired with a properly sized air conditioner to keep temperature and humidity levels down indoors.
“An ERV was never meant to be a stand-alone unit,” Johan writes. “It is a typical case of synergy between two technologies, since an ERV is inherently a passive unit. It CANNOT lower the humidity by itself, because it needs both a humid AND a dry air stream. Thus, whereas you’d spend $50 on air conditioning without an ERV, you would have spent about $25 if you had used an ERV WITH the air conditioner. That’s the whole point of the ERV.”
He directs readers to the Web site of the Air-Conditioning, Heating, and Refrigeration Institute, where the performance differences between ERVs and HRVs can be compared.
It may be a case where two sides just can’t agree.
“In a hot, humid climate, either an air conditioner or a dehumidifier can be used to lower indoor humidity levels,” Holladay says. “However, an ERV will not lower indoor humidity levels. In fact, ventilating with an ERV will raise indoor humidity levels. The more you ventilate, the worse the effect. About the best thing you can say about ventilating with an ERV in a hot, humid climate is that (in terms of helping indoor humidity levels) it isn’t quite as bad as an HRV. “
Which type of ERV to choose?
Assuming, for the sake of argument, there’s some merit to an ERV in a hot/humid climate, there’s still the question of whether they are affected by pollen and mildew, says GBA advisor Michael Chandler.
Chandler says he never liked the type of ERV that uses a dessicant wheel, such as models from Honeywell. He was more impressed with the technology from Venmar, which uses a moisture permeable separator.
But, he asks, “Have you heard of any adverse issues with the moist membrane and imperfect air filters leading to dust accumulation and mildew growing inside the permeable membrane ERVs? I’ve seen mildew growing on my stainless-steel potting bench and even vinyl soffits down here.”
Not a problem with Venmar ERVs, Holladay says. Just make sure to follow the maintenance schedule from the manufacturer and change or clean the filters when called for.
We asked GBA advisor Peter Yost for an expert opinion. Here’s what he had to say:
â— Martin is spot-on with this one.
â— ERVs and HRVs work great for sensible heat exchange between outgoing and incoming air, and ERVs can boost performance with some supplemental latent exchange.
â— ERVs are NOT active dehumidification systems; they work to help maintain the moisture content of indoor air (or reduce the drying out of indoor air) in cold climates but can accomplish very little in hot-humid climates where both the indoor and outdoor air streams are damp, especially during shoulder seasons when you might need dehumidification and your AC system is not accomplishing any because it’s not running.
â— Donald Lintner is unlikely to be unhappy if he selects a good HRV or ERV for his climate. But I would put my dollars into the most reliable HRV or ERV with the highest cfm/watt rating. With mechanical ventilation systems such as these running 24/7, their energy efficiency is pretty important.