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I have a Baxi solo boiler on an open vented pumped system. I can get the boiler to start on hot water demand but not for CH only. I have a voltage input to the diverter valve on CH demand but the pump does not run and the boiler does not fire. I was told it was the diverter valve motor controller and this has been replaced with no change. The room thermostat and immersion thermostats both function and control the diverter valve but the boiler and pump will only start when hot water is demanded together with CH. Any suggestions please?

A supplementary question; how does the boiler 'know' the pump is running and is safe to start?

Hi
This sounds like a valve fault, but could also be a wiring fault.
So I have to start with a question or two.
Is this a new fault - any history ?
Exactly which valve is this, there should be a number on the end ?
Exactly which part has been replaced ?

The fault has been there since before the summer and we just lived with it as the CH would work satisfactorily with both CH and HW demand. I meant to get it fixed over the summer when the heating was off but didn't. It is a honeywell 3 way diverter valve and the whole controller motor head has been replaced. I dont think it has anything to do with the valve itself as we tested the new motor before mounting it on top of the valve and the CH still would not come on.

Problem solved. The 5 core wire from the timer/controller had been loosely routed inside the boiler cover and had been touching the hot boiler casing and melted the cable sheath. When this section of cable was replaced everything worked as it should and there must have been a short inside the cable. A sloppy bit of work from the original installers years ago.

Thanks for getting back. Good to know of a success. Sounds like you found a fault that could easily have gone unnoticed. Well done.