Vaillant VCW Sine18 T3 - Hot water intermittently firing up

[Shilks]

Some while ago the boiler started to get temperamental in not firing up when running hot water. It seems linked to a slight drop in water pressure as during some times of the day things are fine – other times not. Turning on the hot tap sometimes causes the boiler to fire. Other times when pressure is slightly less – the boiler does not fire. I can sometimes get over his by quickly opening the single lever kitchen tap (fully set to hot water) – it seems to shock the boiler into firing up. If I can’t get the water to switch the boiler on – setting the heating on then enables hot water to be drawn off as the boiler seems to have hot water priority.
Is there a setting for how much water pressure will fire the boiler when hot water is run? Should I call the water co to check the pressure? Any advice / help welcome.

 

[Agile]

Its either the diaphragm failed or the pin gland seized.

Its housed in the chrome plated round part in the centre underneath.

Its not an impossible DIY repair for someone competent with water joints ( five are involved! ).

If you are going to do it yourself then replace both but make sure you fully clean the pin!

Tony