Direct Circulation System

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