A supply and extract system comprises a central air handling unit (AHU) typically containing separate supply and extract fans, air filter and heating coil. A ductwork system is used to supply and extract air from around the building. The heating coil can provide the main source of heat for the building.

Some systems can be re-designed to re-circulate a portion of the warm extracted air back into the supply. This reduces the amount of fresh air that must be heated. An additional option to this is that a heat recovery device can be incorporated in to the AHU. The heat recovery device takes the exhaust air and heats the new supply air without mixing the new stream. There are various ways of doing this which include a plate heat exchanger (recuperator), run around coil, thermal wheel or heat pipes.