My Baxi 105e combi boiler works correctly on water and central heating. Five minutes after I turn the CH off, water is expelled through the external vent and the water pressure drops to almost nothing.
If I then add more water and turn the system back on, it works perfectly until the next time I turn it off.
The hot water system is unaffected,

Arbeston, August 2009

get a qualifed engineer out to re-pressurise the expansion vessel & replace the pressure relief valve.

Ian, August 2009

No. When I have added water to bring the pressure back to 1.5, the CH works correctly at the correct pressure of below 3 bar for as long as I want it. The relief valve only opens after I switch the CH off.

Arbeston, August 2009

It seems that your pressure vessel is in doubt. I assume that you are only setting the pressure to 1.5 bar.(when cold) If this is the case, then the hot water system will work. Once you turn on the heating side of the boiler the pressure builds to above 3.0 bar and the safety valve opens? If this is the case then the diagphram in the pressure vessel has ruptured and the vessel needs replacing.

townteck, August 2009