I have a Vaillant Eco Tec Plus Combi boiler (5 yrs old) that works fine -but- I notice that the water pressure falls to zero when not working and when CH is on, the water pressure inreases steadily to 4 bar and stays there until CH is Off. There is also a steady dripping of water from the blow off pipe. Is this a problem with the expansion tank or the blow off valve

Ron, October 2011

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max, June 2012

It is not a problem with either, it's both, you need to re-pressure the expansion vessel to a charge of about 0.8bar (the boiler water must be able to escape the vessel while charging), then you will most likely have to change the prv.

Carefree, October 2011

As the valve is leaking, faulty. replace, that will stop your pressure drop.

SD, October 2011