Water heating is fine but central heating will not switch on?

On my Potterton Boiler and on many others I suspect - when in Central Heating Only mode - the Fire Up signal to the boiler is routed from the Timer Programmer - to the Room Stat and then to the Mid Position valve - when the valve's motor has moved the valve to the Central Heating position a microswitch is activated which sends the fire up signal to the boiler.

If you have a MVP in your system you should hear the motor running for a few seconds when CH only is selected - there is sometimes an override lever with the ability to latch the valve permanently in the CH position I believe.

http://www.wickes.co.uk/content/ebiz/wickes/invt/443033/3-Way-Mid-Position-Valve_large.jpg

I am not qualified or experienced in boiler maintenance - my advice is worthless.

Good luck...

Peccavi
April 2010