Valves

Valves are used in the solar hot water system for balancing flow, flow adjustment, component isolation, and for temperature and pressure control. These valves should be of the same material as the pipe. At drainage locations of the collector piping ball valves should be used.

For solar collector arrays, the piping design can create nearly equal flow, but a balancing valve may be needed for a final adjustment. Use of these valves should be minimized since their use adds to the pressure drop in the piping system. Each collector array should also have a ball valve at the inlet, and a three-way valve at the outlet, with the third port open to atmosphere for isolation. Use of these valves will allow parts of the system to be taken out service for maintenance, repair, etc. while the remaining parts of the system are operating. These valves can also be used to stop flow as part of a freeze protection plan. Both of these valves are manually operated. The balancing valve should be set during fluid flow system set-up and then fixed at that position.

At heat exchangers, fluid flow adjustment may be necessary to assure the appropriate temperature is being delivered by the solar system components. For example, a mixing valve is used to blend hot water with cooler return water so that a 140 °F (60 °C) domestic hot water is delivered to users. Another example is flow control valves that can be used to direct heated water to the proper height of the storage tank to minimize mixing of different water temperatures. Figures 3.21 through 3.25 show such arrangements.

Valves, other than seasonal or emergency shut-off valves, should be electrically operated and located out of the weather or well protected. A vent must be provided at the high point in liquid systems to eliminate entrapped air and it should also serve as a vacuum breaker to allow draining of the system. To avoid multiple venting, systems should be piped to avoid having more than one high point. Pressure relief by safety relief valves must be provided at some location in each flow circuit that can be isolated by valves. The safety valves must be sized for flow conditions that could occur under stagnation. Check valves can be added to prevent thermally induced gravity circulation. A flow-check valve (used in the hydronic heating industry) will also accomplish the same purpose.

When the solar storage tank contains hot water to be used as DHW, an anti-scald valve is often required on the leaving DHW pipe to limit the outlet temperature, which can reach temperatures of 176 °F (80 °C). Care should be taken to compensate for any pressure drop this valve adds to the DHW circulating piping system.