In contrast to thermosiphon systems, an electrical pump can be used to move water through the solar cycle of a system by forced circulation. Collector and storage tank can then be installed independently, and no height difference between tank and collector is necessary. Two temperature sensors monitor the temperatures in the solar collector and the storage tank. If the collector temperature is above the tank temperature by a certain amount, the control starts the pump, which moves the heat transfer fluid in the solar cycle; ‘switch-on’ temperature differences are normally between 5°C and 10°C. If the temperature difference decreases below a second threshold, the control switches off the pump again.