CH not starting on programmer (Ed,)

[Cherrywood]

I have a Horstmann H37XL 3 channel heating controller - downstairs rads, upstairs rads and hot water. The downstairs system has been intermittent for a few days - only working after fiddling with the room stat or switching into Boost mode. I wasn't exactly sure what got the system working, but decided the room stat was at fault and changed it for a new one. No different. I then connected the two stat wires together to permanently call for heat. I then called for Boost and it worked. Then next time it should have come on, it didn't. The boiler works on the upstairs system. Does the boiler start when the pump moves the water through the boiler - could the pump be faulty. Never known one be intermittent!!

 

[stem]

Depends on the type of heating system that you have. If you have motorised valves, (very likely with a 3-Channel controller) the external controls (i.e. the programmer & thermostat) only open and close the motorised valve/s. Then, when a motorised valve is fully open, it triggers a small microswitch inside that starts the boiler and pump. It is quite common for these microswitches to get temperamental so that they work sometimes and not others.

My guess would be that a motorised valve controlling the downstairs heating has a dodgy microswitch. If you have a multimeter and are competent in its use, a simple test of the microswitch wiring would confirm.