NOVELTY - The system (10) has a tank (12) for storing heated water. A first heat exchanger is associated with the tank, and includes a conduit system to separate heat transfer fluid from potable water in the tank. The first heat exchanger is in fluid communication with a second heat exchanger (14-1) to scavenge waste heat energy from an heat source e.g. fuel cell (14), where the heat source produces the waste heat energy during performance of primary function. The second heat exchanger draws heat from the waste heat energy and transfers the heat to the fluid to heat the water in the tank.    USE - Water heater system for scavenging heat from a heat source e.g. electrical domestic appliance such as dishwasher, dryer, range hood, heater, oven, stove and fireplace, waste hot water and fuel cell, in a domestic environment, and for scavenging heat from a gas central heating furnace, generator such as stationary plant and equipment which generate heat as a by-product, and geo-thermal source, in an industrial setting, to heat potable water in residences and commercial facilities.    ADVANTAGE - The system is configured such that expansion of heat transfer fluid or increase in pressure due to the closing of the valve can cause heat transfer fluid to flow back into the first heat exchanger from the second heat exchanger, so that the amount of fluid can increase or decrease depending upon whether heat transfer fluid in a gaseous state is present due to closing off of the valve. The fluid at a base of a column provides a head of heat transfer liquid for the pump such that an amount of heat exchange fluid in the circuit, which can be gasified when the valve is closed, can be limited to the volume of the heat transfer fluid inside the heat exchanger, so that heat transfer fluid that can gasify and the amount of pressure or back pressure to force contraflow is limited by the volume of heat transfer fluid in the second heat exchanger and the pressure in the first heat exchanger during stage of stagnation of the heat transfer fluid in the circuit can be able to be maintained below the pressure of water mains pressure in the water storage tank, thus preventing entry of steam from the return pipe from entering into the hot water delivery system.    DETAILED DESCRIPTION - The first heat exchanger is selected from a group consisting of a thin film or falling film heat exchanger (16), an external coil heat exchanger, an immersed coil heat exchanger, a mantle heat exchanger and a heat exchanger integrally formed within confines of the tank. The second heat exchanger is gas to liquid heat exchanger or liquid to liquid heat exchanger. The primary function is electricity generation.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a water heating system, which uses a falling film heat exchanger in which a pump associated with the water heating system is switched on and a valve in the heat exchanger in a waste heat unit is open.    Water heater system (10)    Hot water storage tank (12)    Fuel cell (14)    Heat exchangers (14-1, 16)    Heat transfer fluid return pipe (28)    Control valve (30)