Vaillant Boiling sticking in DHW mode

[AMR]

Hi
I have a Vaillant VCW GB 221H combi boiler. I've had a problem with the boiler sticking in DHW mode for a few weeks . When I turn on the hot water tap the pin on top of the diverter valve rises and the boiler fires up OK. When I turn off the hot water tap about 50% of the time the pin on top of the valve does not lower and the pump keeps running. I then have to push the pin back down. Does anybody have any idea why the pin is sticking and how to cure the problem. Could it be the diaphragm?

Thanks
Andy

 

[kevplumb]

do you mean diverter valve or servo valve?

 

[AMR]

Hi
I think I must have got diverter valve and servo valve mixed up. When I turn on the hot water tap a pin rises on top of what I presume now is a pressure sensing unit. This pin pushes agains a plastic lever which has a spring return. This lever than pushes against the DHW switch and also changes the direction of what I think might be a servo control unit ( black plastic unit with 4 thin copper pipes). The servo control unit then connects to the diverter valve. If I have identified all the parts correctly it is the pin on top of the pressure sensing unit that does not always lower when the hot water tap is switched off. I normallly press the lever down where the spring connects to get the pin to lower.

 

[ChrisR]

The diaphragm works to raise the pin, it doesn't pull it back. Strip down, clean, grease, change any worn O rings etc.