Two things spring to mind. The boiler has an electrically operated diverter valve which goes between the heating and hot water positions. It also has the facility to pre-heat the hot water.
If the diverter valve is sticking in the heating position intermittently and you also have the controls set for hot water pre-heat, the heat would be able to get to the radiators when the boiler fires up to pre-heat the hot water even when there is no demand for central heating.
To test for this if you shut off the hot water pre-heat function the fault should diminish although it may still happen when you run the hot water taps.... does that make sense??? It's worth a try.
T.h.s
May 2007
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