176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



Valves between the solar absorber and the tank are necessary to 

 permit draining the absorber in cold weather without disturbing the 

 rest of the hot-water system. These valves do not create a danger 

 from overheating because the heat losses from very hot absorbers 

 usually balance the solar-energy input at a temperature below the 

 boiling point. If the absorber coils are filled with water that is con- 

 fined by closed valves, solid expansion by heating might produce 

 serious breaks if there is no entrained air; but this is not dangerous, 

 as is the explosion of a closed dry boiler heated by a fire. 



Whenever the water freezes in the pipes between the storage tank 

 and furnace or range coil, there is another serious danger of explosion 

 when a fire is started. The use of tees instead of elbows at the upper 

 coimections to heating coils makes it easy to inspect by loosening the 

 plugs and draining enough water to notice whether the pressure 

 remains constant. 



With gravity-flow water-supply systems, the highest pipe in the 

 solar-heater system must be considerably lower than the bottom of 

 the water-supply tank to insure full lines for thermosiphon circulation. 



Installation of extra emergency side-ann heater. — The combination 

 of solar heater and furnace or range coil works w^ell during the sum- 

 mer and the winter. During a few days in the spring and fall the 

 sunshine is not sufficient to produce water temperatures high enough 

 for clothes washing. On such occasions washing is postponed until 

 the morning following a bright day, or else an emergency side-arm 

 heater (fig. 7) is used. 



An extra heater of this type when connected in the furnace coil line 

 often leads to unsatisfactory operation of both heating coils unless the 

 hot-water pipe from each heater is carried separately into the storage 

 tank or connected together very near the tank. This high comiection 

 satisfies the requirement that the thermal density differential head in 

 each heater riser be greater in proportion to the total circulation head 

 than the ratio of heater and riser-flow friction to the friction in the 

 complete circuit. One should remember that the flow friction in the 

 heating coil often increases rapidly because of scale formation, thus 

 requiring a higher connection point. 



Combination of solar heater with automatic water heater. — ^When a 

 continual supply of hot water is essential during cloudy weather and 

 the use of a furnace or range coil is not desirable, an automatic heater 

 can be installed in the same manner as the side-arm heater. Since an 

 automatic heater is completely adequate by itself, the only advantage 

 of the solar heater combination lies in the possible saving of fuel. 

 Often the fuel cost for an automatic heater is less than the carrying 

 charges on the investment for a complete solar heater system with 

 glass-covered pipe absorbers. 



