I'll assume that you've checked the obvious settings on the timer/room thermostat. Given that its a good chance that its the 3 port valve (diverter) that you'll usually find adjacent to the hot water tank or boiler. It will be stuck on the hot water position. To gain heating you can temporarily wedge the actuator in the mid position, allowing both the heating and hot wter to function, whilst you arrange to have it repaired.
If you're lucky its usually just the actuator head that is jammed so this can be obtained and renewed. Make a note of which contacts the wires go to before removing the old head.
Stephen, November 2007
