UFC 3-440-01
14 June 2002
temperature limit to the load or to supply the load with a specific hot water temperature.
It is important to ensure that the cold-water leg between the mixing valve and the cold
water supply to the solar storage tank is not used for connection to any other fixture.
Experience has shown that backflow through the storage tank can occur which sends
solar heated water to a cold water user. Although a check valve can be used in the
cold water supply to prohibit back flow, it is best to avoid this situation whenever
possible.
3-2.2
Direct Circulation System. The direct circulation system is the most basic
active solar energy system recommended for adoption by the Services. It should be
limited to use in locations where there are no freezing days, and where the water supply
is of sufficiently high quality (i.e., not highly scaling). The entire system operates at
existing water supply pressure and circulates potable water through the collectors
directly to storage. Figure 3-2 is a schematic of a direct circulation system.
Figure 3-2. Direct Circulation System
3-2.2.1
System Operation
3.2.2.1.1
Collector Loop. The collector loop pump is activated when the collector
temperature is large enough for energy to be collected and transferred to the solar
storage tank.
3.2.2.1.2
Storage Loop. The solar storage tank is used as a preheater for a
conventional water-heating unit, which is placed in series between the solar storage
tank and the load. When a demand for hot water occurs, cold water is drawn into the
3-3
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