This graph shows the balance point for a very efficient, state-of-the-art Air to Water CCHP. This example also assumes a tight, well-insulated conventionally-constructed building envelope. In this example you would begin to need supplemental heat at around 15º.
The two variables of heat pump heating capacity—the red line in the graph, and building heat loss*—the blue line in the graph. This will determine the temperature at which your particular heat pump can no longer keep up with the demand for heat.
* (as measured by a heat loss calculation )
Some heat pumps have built-in auxiliary heat. At the point that the heat pump can not keep up with the heat loss it will turn on the backup electric, resistance heater. Resistance heat is two to four times less efficient than the heat pump’s typical performance. This can lead to high electric bills during the coldest part of the winter. Consider these other forms of auxiliary heat:
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