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In a closed hot water circuit water cannot be compressed so any expansion in volume, created by an increase in temperature, needs to be accommodated by the expansion vessel. When the water is cold the pre-charge pressure forces the membrane sack, within the vessel, to collapse until the pump is started when the sack starts to inflate. As the temperature in the hot water circuit increases, resulting in the expansion of the water within the circuit, the expanded water enters the sack creating additional volume and lowering the pressure in the hot water circuit. As the water temperature drops the pre-charge pressure force the water from the sack back into the hot water circuit.