Have the flue switch checked by a competent person - it senses the fan is running.
On DHW demand the boiler ignites with a high flame, which causes a pulse of pressure in the combustion chamber. This can cause misoperation of the flue switch, leading the boiler to drop out of the ignition sequence - then it tries again . . .and again.
On central heating the flame is modulated to a low start up setting and there is less of a pulse. The boiler will frequently ignite without problems.
Anengineer
November 2008
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