ravenheat boiler keeps dummping water

[K•I•L™¬®©]

raven heat/ (modal?) combi / gas
just bright an house with central heating in it.
its's a raven heat boiler.
the pressure was too low and i filled it up to 1 bar the first time.
it stoped working ie the ch and h/water don't work so i fill it up to 1 bar and it come back on and works fine.
until the pressure starts to clime up to 3 1/2bar and starts to dump the water out then works ok till we or the timer turns it off
or on its own and the pressure is back to 0 bar
i have tryed turning the temp down as it climes 3 1/2 bar to 2 bar stable but its turned down so the rad's are warm.... and not hot
but works for about two days then pressure retuns back to 0 bar but no dump as i look out side to see if any water has been dumpt.( overflow)
there is no air in the expansion tank or when the nozzle is pressed there is no air and no water come out of it?? is this any think to do with the problem pressure loss

 

[ChrisR]

Do a search on Pressure relief valve, and/or Pressure vessel.

 

[pipe]

expansion vessel has lost pressure remove "car tyre " cap press valve if water comes out diaphragm has split.Try pumping up any way, .6 bar with car footpump (boiler off, bleed screw open on nearest rad.