There are several variables that influence the point at which the fans are shut off, such as outdoor air temperature, return air temperature, whether the unit is an HRV or ERV, system size and cfm, use of boost mode, etc. The unit is programmed to monitor all of these variables, and depending on the combination of factors it' s possible for the fans to be shut off anytime the outdoor temperature is below freezing. But in general, an HRV with an internal pre-heater can function normally down into the 20' s (F) and an ERV down into the teens. It' s possible for the units to function at even lower temperatures depending on the variables mentioned earlier.
Zehnder offers more robust pre-heater options for customers in especially cold climates. Please contact your Zehnder technical sales representative for more information.