Misconceptions About HRVs and ERVs
Misconceptions About HRVs and ERVs
A heat-recovery ventilator isn’t a space heating appliance, makeup air unit, dehumidifier, or energy-saving device
Since refrigerators have been around for almost a hundred years, most Americans know what a refrigerator is used for. But heat-recovery ventilators (HRVs) and energy-recovery ventilators (ERVs) have only been around for about 30 years, and many Americans still don’t know much about these appliances.
GBAGreenBuildingAdvisor.com regularly receives questions that show that some homeowners are confused about the purpose of these appliances, so it’s worth examining and debunking common misconceptions about HRVs and ERVs.
Not a space heating appliance
Journalists who try to describe HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment in a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. building are sometimes confused about the distinction between ventilating equipment and space-heating equipment. That’s why they write sentences like these: “This home doesn’t have a furnace or a boiler. A heat-recovery ventilator supplies all of the heat needed to keep the house warm.”
In fact, HRVs and ERVs are ventilating appliances designed to introduce fresh air into a house and to exhaust stale air from the house. (While an HRV has a core that transfers heat from one air stream to the other, an ERV has a core that transfers both heat and moisture. For more information on HRVs and ERVs, see the GBA article titled HRV or ERV?)
These ventilating appliances aren’t designed to provide heat. On the contrary: during the winter, operating an HRV or ERV will tend to lower, not raise, the indoor air temperature. While it’s true that these appliances have a heat-exchange core, the efficiency of these heat exchangers is necessarily less than 100% — so operating an HRV or ERV during the winter inevitably sends some of the home’s heat outdoors.
The confusion arises because some Passivhaus builders install a heater — for example, an electric resistance heating coil or the condensing coil from an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. — in the supply air duct of their ventilation system. This (rarely performed) modification transforms an HRV into a combination ventilation/space heating system.
Not a makeup air unit
Every HRV and ERV includes a duct that brings outdoor air into the building. The existence of this outdoor air duct leads some homeowners to wonder: “If I have a big range-hood fan in my kitchen — an exhaust fan that is causing depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. problems — can I use an HRV to provide makeup air, thereby correcting the depressurization?”
The answer if no, for three reasons:
- Every HRV manufacturer and ERV manufacturer clearly states that these appliances are not intended as makeup air devices.
- Most residential HRVs have low airflow rates — generally 200 cfm or less. Their fans move much less air than a powerful range hood fan, many of which are rated at 500 to 1,200 cfm. Moreover, HRV ducts are also designed for relatively low airflow rates.
- HRVs and ERVs are balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). appliances that exhaust stale air at the same rate that fresh air is introduced into the house. A makeup air appliance, in contrast, is by definition unbalanced: it is designed to introduce fresh air in order to balance an exhaust fan that would otherwise depressurize the house.
Not a dehumidifier
Many homeowners are under the mistaken impression that an ERV is a dehumidifier. In fact, operating an ERV during hot, humid weather doesn’t reduce the indoor relative humidity; instead, it actually introduces more moisture into a building.
During the winter, when outdoor air tends to be dry — cold air can’t hold as much moisture as warm air — operating any ventilation system (an HRV, ERV, exhaust ventilation system, or supply ventilation system) will tend to lower the indoor relative humidity. But during humid summer weather, when the outdoor air tends to be more humid than the indoor air, operating a ventilation system makes things worse.
Of course, humid days during the summer are precisely when homeowners want dehumidification. During this kind of weather, don’t look for your ERV to help. In fact, it’s a good idea to minimize ventilation rates during hot, humid weather. Under these conditions, the only appliances that will provide relief are air conditioners and dehumidifiers.
The best that can be said in defense of ERVs is that, from a moisture-load perspective, operating an ERV during hot, humid weather is less bad than operating an HRV — assuming, of course, that the home is equipped with an air conditioner or a dehumidifier. (If the house has neither an air conditioner nor a dehumidifier, the indoor relative humidity is likely to be close to the outdoor relative humidity during the summer, and neither an HRV nor an ERV is going to change that situation.)
Not an energy-saving device
While it’s true that an HRV or ERV can recover heat that might otherwise have escaped to the great outdoors, operating an HRV or an ERV will not lower your energy bills. The opposite is true: the more hours per day that you operate your HRV or ERV, the higher your energy bills will be.
Operating a ventilation system always incurs an energy penalty. Ventilation systems rely on fans that use electricity; moreover, it always takes energy to condition any outdoor air introduced into a home — whether you’re talking about outdoor air that needs to be heated during the winter or outdoor air that needs to be dehumidified and cooled during the summer.
So, if you want to lower your energy bills, install a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array, not an HRV.
So what good is an HRV or an ERV?
Just because an HRV isn’t a space heating appliance, makeup air unit, dehumidifier, or energy-saving device, doesn’t mean that you shouldn’t have one.
HRVs and ERVs are designed to provide ventilation — that is, to introduce fresh outdoor air into your house and to exhaust stale air from stinky, humid rooms. Because these appliances have a heat-exchange core, they cost less to operate than any other type of ventilation system.
HRVs and ERVs perform very well, but they are expensive appliances to install. The high cost of these systems can be justified in very cold climates, but builders in milder climates (where the energy penalty incurred by simpler ventilation equipment is lower than it would be in a colder climate) will probably prefer to install less expensive supply-only or exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. equipment.
Martin Holladay’s previous blog: “Saving Energy With an Evaporative Cooler.”