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In a closed central heating system water cannot be compressed so any increase in the volume, created by an increase in temperature, needs to be accomodated. This is done with the use of an expansion vessel, when the water in the system is cold the pre-charge pressure of the system forces a diaphram in the expansion vessel against the tank towards the inlet. As the temperature increases the expanding water volume pushes against the diaphragm creating additional volume for the water to enter the expansion vessel. As the temperature decreases the pre-charge pressure forces the water from the expansion vessel back into the central heating system. This maintains a constant pressure within the central heating system and helps to reduce energy and water consumption.