Like combustion heating systems, you control heat pumps using thermostats. If you leave and return at regular times everyday, you'll save money by using automatic thermostats, which minimize energy use during the times the home is unoccupied. However, choosing an automatic thermostat's reactivation time requires considering the duration of heat-pump operation necessary to restore a comfortable temperature. During the heating season, some homeowners also set their thermostats back 10°F, manually or automatically, when they leave home or go to bed.

A two-stage thermostat controls the heating. The first stage activates the refrigeration system. If it's too cold outside for the refrigeration system to counteract the home's heat loss, then the thermostat's second stage activates the electric resistance coils. An outdoor thermostat will prevent the less efficient electric resistance heat from coming on until the outdoor temperature falls below 40°F. An outdoor thermostat also will prevent auxiliary heat from activating when an automatic thermostat is warming the house after a set-back period. Use setback thermostats that are only for heat pumps.

A defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil during the winter. During the 2-to-10-minute defrost cycle, auxiliary heat takes over, reducing the heat pump's overall efficiency up to 10 percent. The two most common types of defrost controls are time-temperature and demand-defrost. Time-temperature defrost controls activate defrost at regular time intervals for set time periods, whether there is ice on the outdoor coil or not.

A demand-defrost control senses coil temperature or airflow through the coil, and only activates defrost if it detects the presence of ice. Obviously, choosing a heat pump with demand-defrost will pay a significant efficiency dividend.

For greater efficiency, don't locate a thermostat near a heat source or cold draft because they can cause a heat pump to operate erratically. This includes shading thermostats from direct sunlight. Also, do not turn the thermostat beyond the desired temperature. It will not make the heat pump heat or cool your home any faster. It will only waste energy. Residents who duel one another over the thermostat settings, moving it up and down to suit their different comfort levels, cause heat pumps to operate erratically and inefficiently.

http://www.eren.doe.gov/erec/factsheets/airheatpump.html