An air source heat pump consists of two parts, and outdoor unit and an indoor unit, it functions in the opposite way to a fridge. They can either work as a heating only device or a heating & cooling device. The outdoor unit of the air source heat pump will take energy (heat) out of the ambient air by pulling the air through a fan. This outdoor air heats up a refrigerant to the point that is boils and turns into gas, thanks to its extremely low boiling temperature. Once boiled, the refrigerant is pushed through a compressor, which heats it further to temperatures of around 60-70degrees. This is then used to heat things like your hot water and radiators and underfloor heating.