Boiler is 2 years old and has been working well. Heating comes on with timer and then goes off after about two hours. F.15 is on display - nothing about this in the manual. I have swapped a couple of radiators recently but system has got good pressure and there aren't any leaks. Cannot figure it out and though a plumber balanced and bled the system about 2 months ago no other works have been done with the heating - Any clues

NAB - Southport, November 2011

F.15 on my extended faults chart (as a Vaillant agent we're given such things), refers to on one of them:
NO INFORMATION
and secondly as:
NOT USED (Flue sensor fault)

Wherever there is NO INFORMATION is displayed, generally refers to a pcb error, before doing anything drastic, try resetting the pcb using d97 and d93 (sometimes works), if not, a new pcb will be the answer.

Carefree, November 2011