Literature on solar domestic hot water (SDHW) control systems was reviewed to assess results of control system performance and reliability. Seven SDHW control systems underwent six laboratory tests: a sensor temperature response characterization, a sensor stagnation test, a controller function test, a controller vibration test, a controller environmental exposure test, and a controller inspection and high-potential test. Both the literature review and the testing indicate serious concerns with SDHW control system performance and reliability. The results of the laboratory tests included failure of 9 of 15 thermistors due to stagnation conditions, drifting of sensor response, differentials strongly dependent on the system temperature, electronic failure caused by high ambient temperatures and humidity, and the failure of the equipment to meet published specifications. Because of these problems, a control system could operate the pump when the storage tank is warmer than the collector. Because control systems for SDHW systems and active space heating systems are similar, many of the results presented here can be extrapolated to apply to active space heating control systems as well.

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