When a heat pump is not sized to meet the design heat load of a home, a supplementary (or auxiliary) heating system is required. The temperature at which an ASHP can no longer meet a given home's heating needs is referred to as the thermal balance point. Most ASHPs switch to a supplementary heating system at temperatures below the thermal balance point. However, some ASHPs may be programmed to switch to their supplementary heating system at a higher temperature, where its more economical to operate the supplementary heating system.^e Conversely, ASHPs are sometimes designed to operate in conjunction with the supplementary heating system at temperatures lower than the thermal balance point since the ASHP may still be providing some useful heat. ASHPs might also need supplementary heat (e.g. integrated electric resistance coils) during the defrost cycle.

Supplementary heat can be supplied by any type of heating system. Central furnaces (i.e. oil, gas or electricity) are mainly used as supplementary heating systems but electric baseboards/resistance heaters and central boilers can be used as well. Where possible, supplementary heating systems are activated by the ASHP thermostat. Outdoor thermostat should also be used to lock-out the supplementary heat if the outdoor temperature is above the thermal or economic balance point. Thermostat setup and compatibility are very important to efficient use of supplementary heat and occupant comfort; further details on this subject are included in Section 3.5.

Although it is not necessary, it is recommended that heat pumps have an emergency (backup) heating system sized to meet the total heating load at design conditions for redundancy. This condition is easily met if existing heating systems are used to provide supplementary heat. In other situations, it may be cost-effective to install a larger bank of supplementary electric resistance heaters if the incremental cost of doing so is limited. To ensure efficient heat pump system operation it is critical to ensure that the system controls only engage any backup heating during a true emergency (i.e. rare system failure).

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^e Since the performance of heat pumps declines at lower temperatures, it may be more economical to heat the home with a fossil fuel fired system below a certain temperature. The point at which it is more economical to operate the supplementary heating