Forced hot water furnaces use water, which absorbs and retains heat, to move heat through distribution piping to heat emitters where the heat is released. In such furnaces, water is not the source of the heat, it is the means by which the heat is conveyed to its destination.
Forced hot water or hydronic furnaces are powered by gas or electricity. The boiler is more complex than that found in a forced air furnace with valves and gauges to measure water temperature and pressure and a tank that allows for the expansion of the heated water. The flow of hot water is regulated by a control value, and a pressure relief valve responds to high water pressure. A vent releases unwanted air, and a drain valve opens to drain the boiler.
Thirty-five hundred times more heat can be absorbed by a given volume of water than by the same volume of air, making it possible for hydronic furnaces to distribute heat using much smaller conduits than those required by forced air furnaces. Forced air furnaces increase air pressure, while hydronic furnaces do not, and can increase the rates of heat leakage by 26 percent and energy usage by 40 percent.