Traditionally a heat pump system requires supplemental heating capacity in areas with colder climates and temperatures fall well below freezing. Supplemental heating can come from a variety of sources but most typically come from electric resistance elements in the system air handler, or from a gas furnace for a dual fuel system. Supplemental heating is require because as the outdoor temperature drops the amount of heat required to maintain indoor temperatures increases, and the output capacity and efficiency of an air source heat pump decreases. The temperature at which the heat loss of the structure exceeds the output capacity of the heat pump is called the thermal balance point. For cold weather ductless heat pumps, and ground source heat pumps designed within an efficient building envelop, a supplemental heat source may not be required because the structure never reaches the thermal balance point, even in the coldest winter weather.