I have a Vaillant Ecotec 831 and when the heating is not firing the boiler loses pressure and then when fired up it gains pressure. It has been down to F22 a few times and all the way to the top a few times.
Ive had the heating engineer out to it and he suspected a leak in a pipe and put Sentinel Leak Sealer through the system but this didnt work.
To be honest the company i use are not very nice but if i go with someone else they say it will invalidate my warranty. The system has only been installed for 2.5 years.
Anybody got any clues??

Kim, April 2011

Thank you for your help Brian. Yes when i try to fill the system the pressure does rise very quickly. I am going to ring the company that installed it and if i get no joy i will ring trading standards.

Kim, April 2011

basically it's the start and finish pressure that matter the cold standing pressure should be 1-1.5 bar which will slowly rise to 2 bar approx. when the system is hot if when you try to fill the system or when you switch the boiler on the pressure rises very very quickly then more than likely you have a problem with the feed to the pressure vessel or the vessel it'self re the warrenty if it was from vailent they can't void it ring Trading Standards up and get their advice the service company may be breaking the law

brian, April 2011