This is almost certainly short cycling caused by a sticking diverter valve. The hot water for the central heating is being partially or wholly contained within the boiler and therefore rapidly overheats, causing the boiler to shut down, only to repeat the cycle when it has cooled sufficiently. There are two part to the valve - the actuator which works on the expanding wax capsule principle and the valve itself which "switches" between domestic hot water and central heating as the demand changes between the two. The wax capsule can be changed easily (and cheaply about 25GPB for the part)but the problem may be the valve itself which, with actuator, will unfortunately cost 80-100GBP.
Peter the Plumber
February 2008
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