The heat source for an air source heat pump is the ambient air. Compared to ground source heat pumps for brine or water, an air source heat pump requires relatively little space. It can be installed outdoors or even inside the building. Moreover, it does not require a permit, as is the case for constructing a well or drilling a borehole for a geothermal probe. These factors mean that air source heat pumps can be installed anywhere and are a particularly useful option where older buildings and heating systems are being modernised.