Have a 28CDi Worcester boiler, 5 years old. When the heating side is on things work fine till the heating is switched off. As the boiler cools down the pressure drops towards 0. Start at 1.5 rising to 3.5[this I think is quite high] then down to 0 depending on how long the heating has been on. Water collects in the bag tied to the PRV overflow pipe. New PRV, new pressure gauge fitted, by gas fitter. Problem remains. Hot water side works perfectly. Any help would be gratefully recieved.

cookiesan, October 2009

From cookiesan, found the answer to my pressure loss problem in the end. The Expansion Vessel was defective. Hope this helps others.

cookiesan, November 2009

I have a 9year old Vaillant boiler that has losed it water
pressure. I want to find out if I could top it up my self

Keith, October 2009

Thanks for the help so far. Forgot to mention expansion vessel was charged to 0.5 bar as per instruction booklet, no leak there kept the pump with gauge connected for 15 mins. to check. No water coming out of the schraeder vavle. No pressure loss at all whilst using hot water side of boiler through summer months. No pressure increase when boiler is dormant , if filling loop was slightly open as John suggests would pressure increase say overnight?

cookiesan, October 2009

i totally agree with ian, first check!

ste, October 2009

expansion vessel needs re-pressurised, gas fitter should know this if he's even remotely good at his job!!!

Ian, October 2009

There are two reasons why the pressure rises on your boiler: 1) The filling loop is slightly open 2) The DHW heat exchanger is bleeding into the CH circuit.
However, check that the expansion vessel has 10 psi of air in it - if you press the schraeder valve on the top no water should come out. You may need another PRV after this!

John, October 2009