I'm surprised that no one has mentioned that in residential heat pump installations the usual practice is to make the switch-over from heat pump to backup not based on a particular outside temperature but on whether or not the heat pump can maintain the thermostat's set temperature. Most thermostats are designed to switch to backup when the room temp with heat pump lags by a preset number of degrees.
The advantage of this approach is that it automatically adjusts for the fact that a heat pump operating in a dry environment spends less time reversing to defrost, hence it is more effective than an equivalent heat pump in a humid environment. This is a more energy efficient approach than is arbitrarily changing to backup at a fixed temp.
__________________
Joel (AKA docj)--
RV Technology Specialist
|