First check that the filling loop is properly closed and not trying to top up the boiler continually (braided hose from cold water supply to central heating return usually just beneath the boiler with one or two black taps to allow water into system). The other usual cause for increasing pressure is a fault with the boilers expansion vessel, either there is a blockage in the pipe to the vessel, the diaphragm in the vessel is broken or there is simply no air in the expansion vessel. The vessel is usually red round like a big lolly pop with a schrader valve near to the top (like a car tyre valve) the vessel is at the very back of the boiler. Best way to check is to remove the cap and try letting air out as you would with a car tyre, if there is a good gush of air it is fine, if it is a bit weak attach a footpump and inflate it to approx 1 bar. This is the most common fault with pressure increase as the hot water in the system needs somewhere to expand and to do this it needs air within the vessel
RAWPLUMB
December 2009
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