Water heater capacity and recovery
The amount of hot water that your family requires depends on the number of people, their bathing and washing habits, and the number of tubs and showers available for simultaneous use. Do not judge whether a water heater is adequate by the capacity of the tank alone. Equally as important is its recovery rate-the volume of water that will have its temperature raised by 100° F in one hour. The capacity of residential water heaters typically varies between 30 and 82 gallons. The following example, although somewhat exaggerated, illustrates the point.
Let us take an average home. Assuming normal water pressure and flow, the typical faucet delivers about 5 gallons of water per minute. Let us further assume that there is approximately a sixty-minute demand for hot water flowing at the rate of 2 gallons per minute. This hot water will usually be mixed with cold water for washing and bathing. If the home is equipped with an electric water heater that has a 60-gallon capacity and a 10gallon recovery rate, after about thirty minutes there would be no more hot water. The low recovery rate of 0.17 gallons per minute is less than 10 percent of that required and would be totally ineffective in satisfying the hot-water demand. On the other hand, if the home had been equipped with an oil-fired water heater that had a capacity of only 30 gallons (half the capacity of the electric heater) and a 120-gallon recovery rate, the hot water would never run out, and the demand would be completely satisfied.
What is the capacity and the recovery rate of the water heater that you are inspecting?
The capacity of all water heaters will usually be stamped on the data plate. Gas-fired units might also have the recovery rate on the plate. If yours doesn’t, you can get an approximate value by dividing the number of input Btu (on the data plate) by 1,000 and multiplying the results by 0.85. Oil-fired water heaters might not indicate their recovery rate. This should be of little concern because oil-fired units have a high recovery rate, often on the order of 120 gallons per hour. Electric units will not have their recovery rate on the data plate. However, the wattage of the upper heating element, the lower heating element, and the total wattage will usually be stamped on the plate. You can easily compute the approximate recovery rate for an electric water heater by using the following formula: Each 250 watts heats about 1 gallon of water 100°F in one hour. Thus a water heater rated at 4,500 watts has a recovery rate of only 18 gallons per hour. This is a typical recovery rate for an electric water heater and is quite low in comparison to a gas-or oil-fired unit. To compensate for the low recovery rate, electric units should, and often do, have large-capacity tanks.
I have found the following rule of thumb effective in determining the adequacy of a water heater for a house, regardless of whether the unit is gas- or oil-fired or electric: For a house with one full bathroom (sink, bowl, and shower-tub), the sum of the capacity plus the recovery rate should total about 70. For two full bathrooms, the total should be about 90; for three full bathrooms, about 105; and for four full bathrooms, about 115. This also assumes that there is a washing machine in the house that might be operating while all the showers-tubs are being used. Of course, your requirements might be different. If you buy a house with three full bathrooms and there are only two people in your family, you obviously do not need a water heater whose capacity and recovery rate total 105.
From an energy-conservation point of view, a small tank with a high recovery rate would be better than a large tank with a low recovery rate. In addition, it is more costly to maintain a large tank of hot water, especially during periods when there is no hot-water demand.