Why does the Hawk II Optima keep turning on central heating without request?

I have a Hawk II Optima combi boiler which is around 10yrs old. I've pushed the selector switch to the disable heating position. We have the hot water on. I've used the dial to turn the heating temperature all the way down and I've turned the room thermostat all the way down. However the boiler keeps circulating water around the radiators - most of the time it doesn't heat up the radiators but sometimes it does. Any ideas why the cetral heating is being called when it is supposed to e deactivated and the room thermostat is turned all the way down?

Jeremy
September 2007