PRESSURE and TEMPERATURE GAUGES
A pressure gauge shows if the closed loop system is within an acceptable range of pressure. A typical system pressure is on the order of 10 to 15 psi. A pressure gauge is used as a diagnostic tool to monitor the state of the glycol charge. A loss of pressure indicates a leak in the system that needs to be located and repaired.

Two temperature gauges in the closed loop and one in the water loop are optional, but they are valuable indicators of the system´s function. One gauge on each side of the heat exchanger in the collector loop shows the temperature rise across the collectors and the temperature change across the heat exchanger. A temperature difference of 15 to 20°F indicates effective system operation. One temperature gauge in the water loop between the exit of the heat exchanger and the entry to the storage tank will display the current temperature of solar heated water entering the storage tank. The temperature gauges should have a range of 0 to 240 or 300°F.
A hot summer day may produce water temperatures in the solar loop around 200°F, although normally 180°F is the maximum temperature attained.

PRESSURE RELIEF VALVE
Every hydronic heating system must have protection against high pressures due to high temperatures. A pressure relief valve of 50 psi is usually adequate to protect closed-loop plumbing systems from excessive pressures. Temperature/pressure relief valves are not commonly used in the closed loop because high temperatures are common. Pressure-only relief valves are most commonly used. Pressure relief valves should have a vent tube that directs escaping fluid to a bucket or floor drain. Once one of these valves opens, it is wise to replace it, since they often do not fully reseat, scale or dirt particles may allow a slow leak.

CHECK VALVE
A check valve permits fluid to flow in one direction only. It prevents heat loss at night by convective flow from the warm storage tank to the cool collectors. Check valves are either the "swing" type or the "spring" type. Swing-type check valves should be properly installed (i.e. not vertically upside-down which allows them to hang open). A swing-type check valve should be used with pump powered directly from a PV module. Low sun conditions produce lower flow rates, which may not be strong enough to overcome a spring-type check valve. For systems using AC circulating pumps, spring-type check valves should be installed. The spring resists thermosyphon flow in either direction.