Emergency heat is a setting that controls a homes backup heating system. It is used when the primary heating system, such as a heat pump, is not working properly or when temperatures drop too low for the heat pump to be able to remove heat from outside. Emergency heat is meant to be used only in emergency situations, such as when there is something wrong with the primary heating system. It is not meant to be used for long periods of time, as it can be taxing on the heating system and can cause high energy bills. Emergency heat can be activated manually, but it should only be done so when the primary heating system has failed completely. The backup heating source for emergency heat can be a gas furnace, oil furnace, electric, or hot-water system.