HRVs vs. ERVs

August 5, 2015

Some contractors may be hearing or reading mixed messages about the application of ERV technology vs. the older HRV technology. Much of this conversation centers around the appropriate technology for cold weather applications. A quick review is in order. The history of air exchangers, air to air heat exchangers or heat recovery ventilators in residential applications was very clearly tied to excessive winter moisture and window condensation problems as houses became more energy efficient and tighter. HRVs were developed specifically for the removal of moisture in R-2000 and other energy efficient housing programs. Introducing cold, dry fresh air to replace the high humidity exhaust air has the ability to quickly bring down the relative humidity in cold weather. For example, when the outside air is at 0 0C (32 0F) and 100% RH, just 30 L/s (60 CFM) of ventilation can remove approximately 15 liters (3.24 gallons) of water a day. Using an ERV with a latent effectiveness of 50% would cut this moisture removal capability in half.

An average family of 4 with normal activities will generate approximately 6 liters (1.3 gallons) per day and a new house may release up to 12 liters (2.6 gallons) per day in the first year after construction and in the order of 5 liters (1 gallon) per day after the first year. Fairly quickly, it can be seen that the goal is to create a nice moisture balance in the home, enough for occupant comfort but not so high as to result in condensation problems on windows or walls in cold weather.

ERVs then become an excellent choice for houses with lower occupancy levels, older houses where more natural ventilation supplements moisture removal or in cases where people want more fresh air than the minimum needed for moisture removal. Moreover, since HRVs can't remove moisture in hot, humid weather, ERVs are a great choice in any application where people use an air conditioner for extended periods of time.

Finally, the issue of longevity or durability of ERV cores in cold weather applications due to freeze/thaw cycles is sometimes brought up by those with less expertise with ERV technology. The first residential ERV technology was introduced 25 years ago in Canada and they quickly delaminated in cold weather testing. Since that time there have been new materials, new methods and better control strategies that have allowed leading manufacturers like Venmar to offer ERV technology with the same reliable and durable operation and backed by the same rigorous warranties of even the best HRV core technology.

Feel confident in assessing the individual needs of your clients and offer ERVs as a great option for the best overall control of moisture in homes both summer and winter.

By Gord Cooke, President