170 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 19 3 9 



For the quick heating of small quantities of water the single pipe 

 runs should, be longer or a valve used to restrict the rate of circula- 

 tion. Copper tubing is often used for this service and is advantage- 

 ous on partly cloudy days because of its relatively low heat capacity. 

 Some domestic waters attack copper. 



In many houses there is no room in the attic above the top of the 

 absorber coils for the storage tank. In such cases a false chimney 

 can be built around the storage tank above the roof. 



NONFEEEZING SOLAR-ENERGY ABSORBERS 



A solar heater used in combination with an automatic heater can be 

 operated to advantage throughout the winter if the danger of pipe ab- 

 sorber freezing can be avoided. There is not much danger of freezing 

 during ordinary nocturnal radiation frosts on calm, clear nights 

 when the air temperature remains only a few degrees below 32° F. 

 for a short time (fig. 5) because of the high heat capacity of the pipe, 

 water, and insulation. Freezing temperatures with wind, however, 

 are almost sure to do damage unless there is considerable reverse cir- 

 culation at night drawing warm tank water through the cold pipes. 

 Such reverse circulation occurring with low-level storage tanks is 

 ordinarily very objectionable in wasting heat previously absorbed. 

 Usually the ordinary pipe absorber must be drained during periods 

 of freezing temperatures if damage is to be avoided. If frost warn- 

 ings are not received regularly, it is good practice to drain the absorber 

 as soon as the weather is cold enough for house heating. 



When it is desirable to operate a solar water heater on bright days 

 during the winter, the danger of bursting absorber pipes during cold 

 nights can be avoided by separating the absorber circulating fluid 

 from the usable water in the storage tank, and using a nonfreezing 

 solution in the absorber circuit. Figure 6 shows a commercial storage 

 tank with separate heating-fluid jacket which is the only significant 

 difference from the ordinary system of pipe-coil absorber with cir- 

 culation directly through the storage tank. Standard tanks with 

 internal heating coils or with external heat exchangers might also 

 be used. 



Gorrnnercial nonfreeze type solar water heater. — The usual com- 

 mercial type solar heater is of the type in which a nonfreezing solution 

 is used in a separate circulating system (fig. 6). In one of these de- 

 signs ^^ the absorber fluid flows through the jacket space around the 

 storage tank and does not mix with the usable hot water. This cir- 

 culating fluid merely transfers heat from the pipe absorber to the 

 storage tank, and being separate from the usuable water in the storage 

 tank it can be any suitable nonfreezing solution. Standard tanks 



^Day and Night Water Heater Co., Day and night solar water heater (advertising 

 circular). Monrovia, Calif., 1932. 



