Hi, I have a Baxi Combi 133 HE Plus boiler. The heating has started sporadically started coming on this week when it should not. The timer is set to off position, the thermostat is at its lowest position and the central heating control knob is turned to its lowest point. Why is the heating coming on occasionally? Many thanks in advance for any guidance.

Richard, September 2011

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