fault code f.25?

yes, locate your system water filling loop (usually a flexible stell braided hose with a small black handle) slowly open this, while keeping an eye on the boiler system pressure, raise it to 1 - 1.5 bar, if you exceed it drain of excess water through a radiator bleed vent.

Most of the vaillants come with a filling cycle you can run through afterwards to clear any air that maybe in your heat exchanger, run this filling & venting cycle before running your boiler.

T
April 2008