Baxi Combi 133 HE Plus boiler-central heating coming on when set not to? ?

the diverter valve is probably passing. when the boiler fires for hot water some of the heat that should be staying in the boiler to heat the water will be passing into the heating.
you can check yourself by running the hot tap and holding the flow pipe under the boiler, if you feel it getting warmer then the valve is passing and will need to be replaced. you will need to make sure the system is cold before checking. the flow pipe will be one of the larger size pipes (on the far left i think), it may be labelled.

Ian
September 2011