Abstract: Compact, easily installable apparatus is disclosed for attachment to a conventional domestic hot water heater tank using only the preexisting threaded drain valve opening normally provided, in order to convert the heater into a simple storage tank and effect water heating externally of the existing heater in an indirect heat exchanger coupled to, for example, an existing boiler forming a part of a space heating system. In this fashion energy savings can be realized inasmuch as boiler water is used for heating the domestic water supplies, as opposed to separate firing of the hot water tank.