Hi, I have a Vaillant VCW-Sine 18 T3 WF H combi boiler. The last 2 days I have been contiunally filling up the water pressure as it drops to zero in around 5-10 minutes when heating is off and 30-60 minutes when heating is on. I have bled all of the radiators and they all now seem air free but the problem is still occuring. I have checked for leaks and have not come across any. Does anyone have any ideas?

Mr Mehan, February 2008

im haveing the same problem however i ahve just had the vessel replaced and it is still happening help

Sam, December 2011

Had the same problem I checked all the radiator valves both thermostat and stop valves for leaks also checked heating pipe manifolds for leaks I have a small bore system, found a couple with a weeping type leaks.
Isolated radiators and re sealed mating surfaces ptfe tape plenty of it , stopped the leaks and stopped the pressure dropping on the boiler

C A Cooper, January 2010

have you looked out side where the P.R V Pipe is? Look for a water stain on the side of the hose where the boiler is

David Sargeant, August 2009

To answer John Harpers Q. Mine has 2 tap valves underneath that allows topping up of the pressurised system. One is connected to the mains water, the other to the central heating side, it's due to regulation that you have some separation between mains water & central heating to prevent feeding additives back into the water mains system.

Paul N, February 2009

I've had a similar problem for a while, but tonight it's given up all together. I think it's the pressure relief valve gone, it's dumped all the water out the overflow. Not good in this weather.

Paul N, February 2009

Firstly Check the CH expansion vessel. There is a shraeder valve which is the same as you see on a car tyre on the vessel.With the heating off press it in and if there is no air comming out it may need re-pressurising check with the MI instructions to what pressure. If water comes out of it then it will need to be replaced or you can fit a external expansion vessel and leave the original expansion vessel in place.

gaso, January 2009

Firstly Check the CH expansion vessel. There is a shraeder valve which is the same as you see on a car tyre on the vessel.With the heating off press it in and if there is no air comming out it may need re-pressurising check with the MI instructions to what pressure. If water comes out of it then it will need to be replaced or you can fit a external expansion vessel and leave the original expansion vessel in place.

gaso, January 2009

Hi, I have a water tap that I use for topping up. It has been behaving itself recently still loses pressure but over a couple of days rather than minutes. I am getting it serviced this week so will see what the engineer thinks.

Mehan, March 2008

Hiya, Its most probably the expansion vessel passing water from the waterside over to the air side as the diaphram is split. This provides the ballance for an unvented system or combi boiler. This happen in time to most combi boilers and is just wear and tear.
Im having a problem trying to locate the knob to re-pressurise one of these boilers. I see you have topped yours up several times, so cld you tell me where to locate this as I do not have a manual.
Kindest Regards Jon
PS good luck!
Email to jonnythesock@yahoo.co.uk

Jon Harper, March 2008

expansion vessel lost pressure or pressure relief valve leaking

paul, February 2008