Vaillant boiler no central heating.

[Phil B]

I have a Vaillant VCW-sine T3 gas boiler - the domestic hot water works fine but not the central heating.

The pump is free to turn with a screwdriver through the drain cover. When CH is 'demanded', the pump has power applied and if I listen with a screwdriver as a stethoscope it's whirrs so I assume it's okay. The boiler also fires up but shuts down again after a few seconds when the sensor heats up. It can be restarted (carefully) by pulling the wire off the temperature sensor.

The pump gets hot, the pipework at the top of the boiler gets hot but the flow pipe remains cold. I've bled at the top of the boiler and at the pump. May not be relevant but it seems to have developed a slow pressure leak in recent months.

Views appreciated. Is this a symptom of divertor valve failure? Any insight into how these work?

Thanks Phil

[Awesometoday]

Is there sufficient pressure in the system?

[Phil B]

Well there wasn't initially but that was fixed. 1.2 bar. Phil

[Ed Sirett]

Yes, I think it is the diverter valve. I'm not so familiar with this older model (20+ years). See if a replacement part or service kit is available, but think carefully if it's expensive as you might spend a lot to restore it and you'll end up with just an old boiler.