In a forced-circulation SWHS, water is pumped from the storage tank through the collectors and back into the tank. An electronic controller, a small pump, valves and few other components are needed for proper operation and maintenance of the system. With a forced-flow type water heater, the collector and storage tank can be installed independently, and no height difference between tank and collector is necessary. Two temperature sensors are used to monitor the temperature of collector and the storage tank. If the collector temperature is higher than the tank temperature by a certain amount, the controller starts the pump, which moves the heat transfer fluid in the solar cycle. If the temperature difference decreases below a second threshold, the control switches off the pump again. The "switch-on" temperature differences are normally between 5 and 10°C.
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