i have a vaillant 828/2e condensing boiler and i have to keep topping up the water level of the boiler too much in my opinion and it has been suggested i am leaking water in the heating system radiators etc but there is another solution i have been told the expansion vessel diaphram may be perforated.If this is the case how much is the cost of a new expansion vessel.
extremely doubtful problem is the pressure vessel if it was the pressure would be up and down like a yo yo first check pipe from pressure release valve it is usually outside then with system cold check around rad valves (take white plastic caps off and check spindles) you can have any number of tiny weeps which with the system hot evoporate with out leaving any traces
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