The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California

Hot Water Suply

34. Selecting Tank of Proper Capacity.—A fair assumption in selecting the size of a storage tank is that at maximum demand not more than 75 per cent of its contents will be warm enough to be useful. The less the heating capacity of the water heater the greater should be the capacity of the hot water storage tank.

As an example, let it be required to select a hot-water storage tank for a building having 250 occupants, with an estimated per capita demand of 40 gallons per 24 hours and a total demand of 1,000 gallons in the peak hour or the hour during which the use of water is greatest.

The total demand for the 24 hours will be 250x40=10,000 gallons, and the average hourly demand will be 10,000:24 = 417 gallons. Since the demand during the peak hour is 1,000 gallons, the amount of hot water that must be supplied from the storage tank during that period is 1,000—417=583 gallons. To allow for the fact that only 75 per cent of the tank capacity is useful for storing hot water, the required capacity of the hot-water storage tank is 583:0.75 or say 780 gallons.