heating won turn off

[bigAL61]

i have a freind who has a glow worm ultracom 12 hxi boiler,which she says has 1 tank in the loft which feeds her hot water cylinder,so i am assuming she has an expansion vessel close to the boiler somewhere,as she cannot see an f and e tank anywhere in the loft.it is a heat only boiler,a condensing model ,she cannot switch off her central heating or it wont go off when it reaches temperature,she says she turns the room stat down or off and it still wont turn off,the only way she can turn the heating off is turn the boiler off by the switched spur.the hot water mode is fine,any suggestions?thanks,bigAL61

 

[Tipper]

There's probably a motorised valve that responds to the thermostat which in turn fires the boiler. This is either one of two, two port valves or a single three port valve. Either way it is jammed, the motor failed or the gears/auxiliary contacts failed.

You probably need a new valve or just the motorhead, which is cheaper. It is also possible just to replace the motor inside but in my experience this seldom lasts long.