Residential heat Pumps

Most residential heat pumps are of the air-source type whereby heat is transferred from the outdoor air to a refrigerant in the outdoor section and then transferred from the refrigerant to the indoor air in the air handler (fan/coil). When the outdoor air temperature decreases the COP and heating capacity of the heat pump decrease while the heat loss from the heated structure increases. The outdoor temperature at which the capacity of the heat pump matches the structure heat loss is known as the ‘balance point‘. When the outdoor temperature drops below the balance point a supplemental electric resistance heater operates in an ON/OFF manner with a duty cycle that maintains the desired indoor temperature. (The heat pump operates continuously during this time.) When the outdoor air rises above the balance point the electric heater is shut off and the heat pump operates in an ON/OFF manner at its required duty cycle to maintain the desired indoor temperature.

The balance point can be used to maximize the trade-off between comfort and operating cost especially when setback is used at night and when the residents are away. An increase in the balance point over time can be used as a diagnostic to detect chronic heat pump problems, e.g., slow refrigerant leakage, and structure problems (like increased infiltration losses). The magnitude of these problems can be determined by comparing duty cycles from subsequent tests.

When the outdoor temperature drops below 45°F frost may form on the surface of the outdoor coil thereby acting as an insulator and also impeding air flow across the coil. To maintain energy efficiency and heating capacity heat pumps have an automatic defrost mode. This mode is identical to the cooling mode of the unit except that the outdoor fan motor stops and supplemental heat is turned on to continue warming the conditioned space. This heat must be included in determining the balance point.

The design outdoor temperature for estimating heat losses of structures in the State College, PA area is 5°F. The effects of supplemental heat at this temperature can be determined by comparing a supplemental COP with a conventional one.

As implemented from the above the heat loss of the structure (residence) is determined in finding the balance point. The heat loss is assumed to be a linear function of the outside temperature being zero where the outside temperature is equal to the inside design temperature (usually 70°F). Also, as implemented above the outside design temperature depends on location.