Worcester 9.24 RSF.E fan runing all the time?

hello i have a worcester 9.24 boiler we have just moved in to a house and the heating and hot water work fine but the fan is runing all the time even when the heating is not calling for demand only way to stop the fan is by turning the electrics of to the boiler any one help here i have had several people round to check it but no one can seem to fix it and alos there is no manual