In a closed hot water circuit, the water cannot be compressed so any increase in volume, created by an increase in temperature, has to be accommodated by an expansion vessel.
When water is cold, the pre-charge pressure forces the bladder to collapse until the pump is started when the bladder starts to inflate.
As the temperature in the system increases, with the associated increase in pressure and volume, the expanded water enters the bladder creating additional volume and lowering the pressure.
