Hi Veronica,
That pipe os the overflow pipe, when the boilers water pressure exceeds 3bar (safety limit) it lifts and reduces pressure by allowing water to live the system via this pipe.
When your system is totally cold, check your boiler system water pressure, should be between 1 - 1.5 bar, if it is much higher than this, make sure your filling loop is turned off, and drain water from your system via a radiator bleed valve until the pressure reads 1 - 1.5 bar.
If it reads 1 - 1.5bar, but when the system is running it increases to 3bar or above, then your system expansion vessel may need replacing, call a gas service tech. out to do this for you.
T
March 2008
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