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water heaters

Home water heaters are of two types: with and without storage tanks. The latter are more expensive and are currently fairly uncommon in the United States, except among the energy conscious.

Capacity

The capacity of the storage tank is often used as the size of water heaters with tanks. Capacities of home water heaters range from about 10 to 80 gallons.

Recovery rate

More important than capacity is the recovery rate, which is the number of gallons of water the heater can raise in temperature by a certain number of degrees Fahrenheit in one hour. Usually 100°F is used, but sometimes 60°F. Beginning with a full tank of hot water, a heater with a storage tank can deliver in one hour roughly the number of gallons given as its recovery rate plus about 70% of its tank capacity.

Thermostat setting

Manufacturers sometimes recommend the heater thermostat be set to 140°F. There are two reasons not to do this, and one reason why you should.

The bigger the difference between the temperatures of a hot object and its surroundings, the faster the object will lose heat. The hotter the water in the pipe, the more heat is lost on the way to the faucet.
High water temperatures increase the risk of scalding the very young and the very old.

The only reason for a 140°F setting is that automatic dishwashers formerly required this temperature for proper operation. Modern dishwashers, however, have their own built-in water heaters. Even though these dishwashers use a very costly way of providing heat (electric resistance), they heat so little water that overall energy (and money) can be saved by using them and turning the water heater thermostat down to 120°F.