Tuesday, December 19, 2017
HRV? You need one like you need a hole in the wall.
An air-to-air heat recovery ventilator (HRV) can be found in most Canadian houses built in the last 10 years. The theory behind their use is that natural air ventilation rates are not sufficient for good indoor air quality, and an air exchanger without heat recovery wastes energy. Although the second part is true, the first part is not. HRVs add $1,500-$2,000 to the cost of a new home, and are often a source of additional heat loss, even when not in use.
For many years I've been saying air tightness is of utmost importance in homes. According to research done in 2007 (pdf), Canadian homes built after 1991 had an average air tightness of 3.6ACH@50Pa. Even with improved construction practices in the past ten years, most new homes being built in Canada today would be better off without a HRV. Most new houses have enough natural air ventilation to maintain good air quality during the coldest parts of winter. Many actually have too much natural ventilation, causing unhealthy low levels of humidity. The supposed need for HRVs is based on the ventilation rates in CSA standard F326. The ventilation levels in F326 seem to be based on bad assumptions and wide margins of error, rather than basic science. That's despite the fact that NRCan published reports analyzing indoor ventilation requirements as far back as 1969.
What I find a bit surprising is that building engineers are aware of this issue. Several years ago a senior ASHRAE member told me, "It is widely acknowledged that continuously ventilating houses at F326 rates can results in the houses being over ventilated". Perhaps what is not as widely known is that even "tight" houses with air infiltration rates of 2ACH@50Pa will have high enough natural ventilation rates during the coldest parts of winter. This is not just based on theory, but also indoor CO2 and humidity testing done by myself and others.
I suspect this is not a concern for most people in the HVAC industry since homeowners can just turn off their HRV in the winter. Besides the unnecessary cost of the HRV, what that ignores is the heating loss from a HRV, even when it is turned off. The ducts installed for the HRV often go in and out of attic spaces, which are sources of air leakage unless they are perfectly sealed. Standard HRV designs use only a single damper to block off either the exhaust or fresh air intake when the HRV is not running. This means the HRV adds a six-inch unobstructed hole to the building penetrations. A thermal infrared scan I recently performed clearly shows the heat loss from an exterior HRV duct.
Since removing HRVs is not a viable option, homeowners should at least turn them off during the winter. To avoid heat loss through the outside vents, I tape over the hood opening. I might even leave the vents taped off all year long, and just use a bathroom exhaust fan. Although I'll loose the benefit of heat recovery, when the outside temperature is only 10-15C different than the inside, that heat loss rather modest.