As we all work to make our homes more airtight, we also have to be conscious of our families’ need for fresh air — oxygen above all! If our homes were completely airtight, we not only would risk suffocation, we would also be more susceptible to the toxic gases and fumes emitted by our paint, our carpeting, our gas appliances, and more.
The outgassing from our carpet and other building materials are known as “volatile organic compounds” or VOCs.
An appliance which you’ll be hearing more about as homes become better insulated and therefore more airtight is displayed schematically in the third column. It’s called an Energy Recovery Ventilator (ERV) or a Heat Recovery Ventilator (HRV).
Before I explain this appliance’s operation, let me tell you what it replaces: the exhaust fans in your bathrooms and above your kitchen stove. Those exhaust fans simply pump air out of your house, which causes fresh air to be sucked into your house via the gaps around your doors and windows and multiple other gaps you are not aware of.
That air which enters your home is not preconditioned in any way. It is whatever the temperature is outdoors, and in midwinter it could make your furnace work harder heating the cold air which naturally enters your home, whether or not those exhaust fans are operating.
Thus, the primary job of the ERV or HRV is to use the heat from the air being exhausted from your home to preheat the air that is entering your home without having those two sources of air mix with each other. This is done through a heat exchanger. In the above diagram, the heat exchanger is in the middle of the device. The unit runs at low speed, taking the stale air from your bathrooms and kitchen (typically), through a metallic heat exchanger which then adds that heat to the air which is passing through the adjoining passageway from the outdoors into your living spaces. That fresh air replenishes the oxygen in your home. The picture below is of a typical HRV installation. Both images are sourced from homes.winnipegfreepress.com.
What I have described above is the function of the HRV, which only handles the transfer of heat from one air source to the other. The ERV also performs the transfer of humidity. Thus, if the cold air outside your house is very dry (typical of Denver’s climate), the ERV will transfer some of the moisture from the indoor air to the incoming air.
Neither the ERV nor the HRV measure or react to the presence of toxic gases in your home. That’s the added value of a third device, the CERV or Conditioning Energy Recovery Ventilator. If you’re concerned about indoor air quality, this is the device you’ll want to consider installing in your home.
Whereas the ERV and HRV may operate on an as-needed basis, the CERV is intended to run 24/7, constantly monitoring the level of CO2 and VOCs in your indoor air as it is drawn through the unit. If the levels of these or other pollutants are high, the unit’s fan will run faster. A recent update of the unit has the addition of a virus-killing UV light.
Also, a CERV contains a heat pump, so it can actually perform the function of a furnace, preheating the air which is drawn from outdoors (or recirculated within the house), not merely transferring the room temperature heat from the exhausted air to the incoming air.
I have written in the past about the Geos Community in Arvada. None of the homes in that community use natural gas. Instead, the townhomes are heated and cooled by a combination of a heat pump/mini-split system and a CERV which provides additional heating or cooling. The detached homes at Geos also have CERVs to complement the ground source heat pumps which provide the primary year-round heating and cooling of the homes.
Learn more about CERVs at www.BuildEquinox.com. The local vendor is Todd Collins of AE Building Systems, 720-287-4290.
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