172 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



HOT-WATER DEMAND 



The decision as to the kind, size, and system of solar water heater 

 to be installed depends upon several interrelated factors. The nature 

 of the hot-water demand is, of course, the primary question. This 

 demand involves temperature, quantity, and time of day; it varies 

 widely in individual cases, because of differences in personal habits, 

 plumbing facilities, and the relative expense of heating water. 



Temperature of hot water Tieeded for various domestic purposes. — 

 The desirable temperature of water used for various purposes is 

 known within reasonably close limits. The temperature data in table 

 1 were observed in common domestic practice. The water for a hot 

 shower is definitely too hot at 105° F. and verges on the cool at about 

 90° F. Dishwater at 120° F. requires the use of a mop because it is 

 too hot to keep the hands in; and although dishes can be scrubbed 

 clean in cold water every housewife would object to dishwater below 

 105° F. Assuming the usual temperature of the supply water to be 

 60° to 70° F., the difference in the amount of heating required to 

 obtain the upper and lower limits of temperature is only about 25 

 percent. 



Quantity of hot loater needed for various pu/rposes. — The variation 

 in quantity of water used, however, is so great that no average 

 assumption can be considered narrowly. If unlimited inexpensive hot 

 water is readily available at the turn of a faucet, its use will be almost 

 extravagant; the water will be left running while one is doing short 

 chores, tubs will be filled to overflowing, showers will be run to heat 

 bathrooms, and so on. If, on the other hand, a person must wait 

 while water is being heated or must make an effort to obtain hot 

 water or finds that hot water is expensive, he naturally will use a 

 minimum. The quantity of hot water obtainable from a solar heater 

 varies with the amount of sunshine available ; and to make up a defi- 

 ciency by starting an auxiliary heater involves a time delay, personal 

 effort, and expense so that the natural use tends to follow the available 

 supply. This elasticity of demand greatly extends the period of use- 

 fulness of solar water heaters both before the summer and afterward 

 into the autumn, often until a water coil in the range or furnace can 

 be depended upon during the winter (fig. 7). 



Despite the inherent variations in personal habits one must assume 

 some average hot-water demand in order to estimate the size of water 

 heater that will give adequate service without being unduly large. 



For late afternoon hot showers for field workers, a temperature of 

 102° F. is needed, and a minimum quantity of 12 to 15 gallons per 

 person. This is usually obtained by simple water tanks exposed to 

 the sun (pi. 2, fig. 1). 



For general domestic purposes the average rural demand is usually 

 considered approximately 40 gallons of hot and cold water together 



