look at the (prv) this is the pressure relief valve that vents water outside when excessive pressure is reached inside boiler usually around the 3 bar mark.
with NO pressure in the boiler see (what air pressure is in the expansion vessel) (tyre pressure gauge needed) may be a big fat ZERO
Note
(if water comes OUT when the valve is depressed the internal diaphram has gone, a replacement expansion vessel will be needed)
if NO WATER comes out of the valve(good sign )just RE INFLATE using a car pump back up to 1 bar pressure.
now repressurise the central heating on the water side via the filler loop to about 1.2 to 1.5 bar COLD
wirecutterman, January 2010
