The heat pump employs the same basic principle as the common household refrigerator, extracting heat from a space at low temperature and discharging it to another space at higher temperature. Electrical power is needed to run the compressor in the system. By reversing the direction of the refrigerant flow with a valve, the system can be used, as required, for either heating or cooling. The performance of a heat pump may be obtained by dividing the total useful heat output by the electrical energy needed to operate the system. Both the heating capacity and COP of the heat pump are reduced as the outdoor temperature drops.
Heat pump capacity and building heat loss for a home in the Ottawa area.
Supplementary heaters are staged to come on in such a manner that only the portion actually required to make up the difference between the heat pump capacity and the space heating requirement is activated. Below the balance point temperature, as more stages of supplementary heaters are brought on line, the output and efficiency of the heat pump decrease. Above the balance point, the unit has surplus capacity and is made to match the building heat loss by cycling on and off. Seasonal Performance . A sample calculation for Ottawa illustrates the seasonal performance.