A heat pump uses the outdoor unit for both heating and cooling. Often the heat pump also has some sort of backup or auxiliary heat for when it is cold outside. The backup or auxiliary heat could be electric heat strips or a fossil fuel furnace. Thermostats that control heat pumps with backup or auxiliary heat typically have an Em Heat (Emergency heat) position. When the system switch is moved to the Em Ht. position, the auxiliary heat is activated on a call for heat. The compressor is disabled. If replacing a thermostat on a heat pump system, verify your thermostat is compatible.