source heat pumps take heat from the air outside, increase its temperature and use it inside the home. There are two types – air- to- air pumps and air- to- water pumps. The former produce warm ( and also cool) air and circulate it through fans. Air- towater pumps supply your home’s ( wet) central heating system. Air source heat pumps can work at temperatures as low as - 15 C outside, but can be less effective when it’s colder than - 5 C, so, again, another form of heating may be needed in winter.