A Hybrid heating and cooling systemcombines an electric space-conditioning heat pump with a fossil fuel fired furnace. When providing cooling a hybrid system operates like a conventional air conditioning system: using the heat pump to chill the air. When heating both the furnace and heat pump have roles, depending on the outdoor temperature. The heating capacity of a fossil fired furnace is relatively independent of outdoor temperature, but it's efficiency is higher at low ambient temperatures, because of less off/on cycling. Conversely the heating capacity and efficiency of an air-source heat pump are highest at higher outdoor temperatures and decrease as the outdoor temperature drops, until, at some point, supplemental heat is required to keep the house warm. This is considered the heat pumps' thermal balance point temperature. In a conventional heat pump, the supplemental heat is provided by electric resisitance strip heaters. In hybrid systems, the supplemental heat is provided by the furnace. Pairing the two technologies allows each to operate at maximu…