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To stop the pipework bursting, the boiler or pipework is fitted with a 3 bar pressure relief safety valve (usually with a red cap).If the pressure goes above 3 bar this valve is forced open and water passes out of the system along a safety pipe and is dumped outside the building.
In a sealed system you have to allow for this expansion.
This is done by adding an expansion vessel to the system pipework and it's often inside the boiler.
Though a failed or de-pressurised expansion vessel is the most likely culprit when water passes out through the safety pipe it is not the only possible cause.
If the plate heat exchanger (secondary heat exchanger) fitted to most combi boilers is leaking internally water will probably be forced out through the safety valve and the pressure gauge will read consistently around 3 bar.
The air in the radiator will now act as the expansion pocket.
(This won't work if the radiator is piped from the top but most aren't.)Finally, check that the system is not losing pressure through the safety valve.
First you must lose the pressure on the central heating water side by draining from a drain cock until the system pressure gauge (boiler gauge) shows zero.
Don't use the 3 bar pressure relief valve to dump the pressure because, once operated, the seating may become contaminated and it may leak water through the safety pipe afterwards.
If, however, the vessel pumps up correctly, you re-fit and tighten the Schraeder valve cap and then fill the system water side again to increase the pressure.
Typically you want between 1 and 1 bar in the heating system when the system is cold.