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Baxi 310fi loses pressure
Have a Baxi 310 that's only used for heating.
Has been in service over 10 years and always held pressure.
2 years ago it started gradually losing pressure to the point it would drop from 1.5 bar down to zero over the course of 2 weeks. I could see no internal leakage anywhere other than some slight water/corrosion staining coming from the auto vent located on the pump housing.No evidence of active water coming from it though.
However after comissioning the boiler this year the pressure drop is happening much quicker.Drops to zero in a matter of 3-5 days.
I did notice the high pressure relief valve was dribbling at times so replaced it. However I am still experiencing the drop in pressure.
After replacing the safety valve I refilled the system and watched thru several cycles observing a constant venting of air from the auto vent so am questioning if I am losing the pressure thru a possibly faulty auto vent.There's no collection of water anywhere under the unit that would indicate the vent is leaking though.
Also on observation 3 days later after replacing the safety valve the system pressure had dropped to .5 bar and after refilling the system at cold temp with system not running the system seems to hold pressure.
The heating system itself (piping) is buried behind the walls so next to impossible to see if there's a leak somewhere in the system loop.
Could soneone explain how to test the auto vent? Or any other possibilities I should be checking?