An attachment or accessory water heater for mounting on the exhaust gas flue of a gas-fired water heater such as those sold for domestic use. There is usually a considerable space above domestic water heaters as ordinarily installed and the exhaust gas flue of such heaters ordinarily traverses this space. It is proposed by this invention to use much of the heat otherwise dissipated through the exhaust flue to preheat, or superheat, the water. The accessory heater is essentially a heat exchanger conduit of considerable capacity capable of encircling the exhaust flue and having means for connection in series in the piping of the main heater, the conduit having thermal insulation to inhibit loss of heat to the atmosphere.