The Air to Water system is designed to heat water for use in the home. It works in the same way as the Air to Air pump apart from the indoor heat exchanger is inside a large cylinder full of water. This water can then be pumped around the house and either be used directly (out of a tap) or used in secondary heating systems such as under floor heating pipes and radiators. Because they require a large water tank for heating these systems are better suited to a larger house.