Heat pumps

Heat pump is a term for a type of air conditioner in which the refrigeration cycle is able to be reversed, producing heat instead of cold in the indoor environment. Using an air conditioner in this way to produce heat is significantly more efficient than electric resistance heating. Some home-owners elect to have a heat pump system installed, which is actually simply a central air conditioner with heat pump functionality (the refrigeration cycle is reversed in the winter). When the heat pump is enabled, the indoor evaporator coil switches roles and becomes the condensor coil, producing heat. The outdoor condensor unit also switches roles to serve as the evaporator, and produces cold air (colder than the ambient outdoor air).

Heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 40-55°F (4-13°C), because heat pumps become inefficient in more extreme cold. This is due to the problem of the outdoor unit's coil forming ice, which blocks air flow over the coil. To compensate for this, the heat pump system must temporarily switch back into the regular air conditioning mode to switch the outdoor evaporator coil back to being the condensor coil so that it can heat up and de-ice. A heat pump system therefore will have a form of electric resistance heating in the indoor air path that is activated only in this mode in order to compensate for the temporary air conditioning, which would otherwise generate undesirable cold air in the winter. The icing problem becomes much more prevalent with lower outdoor temperatures, so heat pumps are commonly installed in tandem with a more conventional form of heating, such as a natural gas or oil furnace, which is used instead of the heat pump during harsher winter temperatures. In this case, the heat pump is used efficiciently during the milder temperatures, and the system is switched to the conventional heat source when the outdoor temperature is lower.

Some more expensive window air conditioning units have the heat pump function. However, a window unit that has a "heat" selection is not necessarily a heat pump because some units use electric resistance heat when heating is desired. A unit that has true heat pump functionality will be indicated in its literature by the term "heat pump".

Professional bodies American Society of Heating, Refrigerating, and Air-Conditioning Engineers

ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) is an organization devoted to the advancement of indoor-environment-control technology in the heating, ventilation, and air conditioning (HVAC) industry. ASHRAE was founded in 1894 to serve as a source of technical standards and guidelines. Since that time, it has grown into an international society that offers educational information, courses, seminars, career guidance, and publications. The organization also promotes a code of ethics for HVAC professionals and provides for liaison with the general public. It is based out of the town of Washington D.C.