Abstract: An arrangement for heating a liquid including a boiler, and a condenser located at least in part within the boiler and open at its inlet end which is located within the boiler such that the liquid/gas combination heated in the boiler passes directly into the open inlet of the condenser. The condenser and the boiler may be formed as concentric tubes, the inner tube forming the condenser and the space between the tubes forming the boiler. Valves or the like are provided for determining the direction of fluid flow.