glow worm fan keeps running?

hi ,i have just moved into a house ,it has a glow worm comi boiler , the fan never seems to turn off it just keeps running and only goes off when the boilers electric is turned off .ive been to a few sites 1 mentions it need to be on, as it required for the pilot fumes to be removed. other says the thermostatic control is jamed and needs replacing
can any one help