The HW constant switch will turn the boiler on but I have no central heating as the CH constant switch has no response to the boiler could it be the programmer or something else?
That suggests a stuck valve - they often have a lever you can operate to push it fully over against the return spring to the CH only position. Somtimes you can latch the lever to keep it in the CH position.
Thanks for your help Peccavi, think I'll call a plumber to try and sort it out, also the other day the HW was put on constant and it heated all the radiators in the house when the CH was off.
John, April 2010
Probably something else - the route of the control signal to turn the boiler on in CH only mode is shown here...
From the timer/programmer to the room stat and then to the Mid Position Valve - the Mid Position Valve motors over to the CH position and makes a microswitch. The fire-up signal is then routed to the boiler from the microswitch on an orange wire - but only when the motor has moved the valve and made the microswitch.
If the motor in the MPV fails or the valve sticks or the Microswitch fails then CH only will not work.
When Both CH & DHW are called for together the circumstances are different.
