The "Overall System Efficiency" definition is not the same as the defined in the 1976 ASHRAE Handbook--Systems volume. To obtain such an efficiency, which is defined as the "ratio of the heat in the water delivered at points of use to the heat supplied to the heater," it would be necessary to fit heat meters to every faucet. Obviously, this is not a practical proposition. The efficiencies reported in this paper will consequently be somewhat higher than the "overall system efficiency" since we are ignoring the fact that the water at point of use will be at temperatures lower than that leaving the heater due to cooling in the supply pipework to the faucets. Spot tests showed that at times of limited use it was necessary to run as much as 14 L (.3.6 U.S. gal) of water to waste before the supply water temperature reached the circulating temperature.