Individual air-to-air (air-source) heat pumps can
also be installed as a packaged system. A heat pump is
essentially a vapor-compression air-conditioner which
can be reversed to extract heat from the outdoor environment
and discharge it into the occupied space. A significant
drawback to air-source heat pumps is that vapor
compression refrigeration becomes inefficient when the
evaporator is forced to extract heat from a source whose
temperature is 30°F (0°C) or below.