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In page Solar water heating:

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A differential controller senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. The controller starts the pump when the water in the collector is sufficiently about 8–10 °C warmer than the water in the tank, and stops it when the temperature difference reaches 3–5 °C. This ensures that stored water always gains heat when the pump operates and prevents the pump from excessive cycling on and off (in direct systems the pump can be triggered with a difference around 4 °C because they have no heat exchanger).[citation needed]