When central heating is hot, the bar guage goes into the high pressure red section and water comes out of the overflow pipe. When the heating cools down the gauge slowly goes all the way down to 0 bar red zone on the dial so I have to refill the tank. Potterton help line suggests that expansion tank is blocked or failed.
The expansion vessel is lacking in pressure here - often coloured red and sitting inside the boiler, it is a 2 chamber device with air on one side, water on the other, rubber diaphragm in between. There is a normal tyre valve connection for the air side and the average pressure for this is 10 psi. Check for water leaks around the central rim, too.
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