Hi , I have a vaillant boiler, the pressure drops to zero when the
system is cold. As far as im aware the pressure should always be at 1.5 bar. I have to top up the boiler everytime I switch it on.
The overflow pipe outside for the boiler is dripping -
Does anyone have any ideas as this is driving me around the bend.
I think it may be the Pressure release valve not working correctly and allowing the overflow pipe to leak.

Phil Walker, April 2005

definitly the presure relief valve what happens is if it has to release a bit of presure then particles in the water can hold the valve open drain the system the valve is held on by a clip pull out pull of the valve take a spanner just below the knob and undo anti clockwise then clean put back to gether then puss the valve on hard the insert clip refill do not for get to bleed upstairs radiators down stair should be fine as these will not drain back through the boiler

eddie, December 2009

It is either the P.R.V as u have mentioned or the expansion vessel. If the diaphragm in the expansion vessel is worn/damaged there is nowhere for hot water to expand so it blows through the P.R.V. Repressurise the system and leave cold if the P.R.V is continually running then it is the P.R.V if it only runs when it has got hot then it is the epansion vessel.

robbo, July 2008

if it only gets to 2bar then i would just change the press valve it a cheap easy start anyway hope that helps...there about a tenner

d.., April 2005

The pressure goes to 2.0 bar maximum .
I know you shouldnt turn the system on from cold but when
I do the pressure goes from 0 to 1.5 where it should be
would this harm the boiler ?
do you think I just need the pressure relief valve ?
Cheers

Phil Walker, April 2005

does the pressure really increase a lot when its running to about 3 bar?? if it does you will not only have to replace the pressure valve but also pump up the expansion vessell

d.., April 2005