Central heating fixed by replacing synchronous motor

I mentioned the other day that our (gas-fired) central heating system had stopped working. The symptoms were these: the boiler would fire up and heat the hot water, but wouldn't heat the radiators if the central heating was "on". If CH alone was selected nothing at all happened. The boiler just sat there, pilot light on, but otherwise lifeless.


Last night, spurred on by the prospect of there being 12 people here at Christmas, I started fault-finding. I had already suspected the synchronous motor which drives the three-port valve (routing hot water from the boiler to the hot water cylinder coil or the radiators or both), so I removed it and tested it.


Fortunately I had replaced one of these in the past, so I knew what to do, even though the fitting to the valve was a slightly different design. While removing it I noticed that the leads had become brittle with the heat, and the insulation was cracking where they entered the motor housing. The motor itself was open circuit, the winding having presumably burnt out. Not good! It's designed for a high-temperature environment (obviously), so the manufacturer must have been saving money by using lower quality lead-out wires.


The motorised valve in the photo is the one I removed four years ago after it had done 16 years' service and disintegrated internally. During that time I had had to replace its synchronous motor: it was that (relatively new) replacement which I re-installed yesterday. Now the system is up-and-running again.