There are several options for preventing frost in HRVs/ERVs, and the following is a sampling of the most popular options. It is important to have adequate frost prevention, and depending on your climate, there are best practices available. If you have questions about best practices for your region, contact Zehnder America for recommendations.
Option One - Utilize a ground source Pre-Heater/Pre-Cooler. The Zehnder ComfoFond-L utilizes a ground loop of glycol and a small circulating pump to pre-heat or pre-cool and dehumidify incoming air with a hydronic coil. The approximately 50F coil is an effective pre-heater in winter, and an effective pre-cooler and dehumidifier in summer. By tempering the air entering the system this way, both comfort and efficiency are dramatically improved in very hot or very cold climates. This option is being utilized in projects in Fairbanks, Alaska, where winter temperatures reach -50F, with excellent results.
Option Two - Utilize an electric or hydronic pre-heater. By warming the incoming air prior to entry in to the H/ERV, frost is prevented. This can use a relatively high amount of energy, depending on the mechanism for turning the pre-heater on and off. If it is temperature driven, the pre-heater may operate for extended periods of time over cold days, and especially cold nights. The Zehnder ComfoAir units utilize a mechanism that only powers the electric pre-heater when the unit is actually exhibiting signs of frost. This method uses significantly less energy, while providing effective frost prevention.
Option Three - Imbalance flows to allow for warm exhaust air to de-frost unit. This option either slows or stops the incoming, cold air while allowing the exhaust air to warm the Heat Exchanger's surfaces and remove frost. Many North American units use a re-circulation method, which shuts off the incoming air and re-circulates the inside air. Although this will work, it can be an issue with re-circulating the air you are trying to get rid of.