Vaillant VCW GB 221 Turn off problem

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Staffordshire
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United Kingdom
On my Vaillant VCW GB 221 when the hot tap is open then hot water is provided. On turning off the tap instead of the boiler going silent it continues with the usual "working noise" and the main flame will then switch on and off intermittently even though no hot water tap is open. Any hints gratefully accepted.
 
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right in the front middle is a servo valve that has a cam on it(at back) that comes up and hits micro switch , when water demanded it does not always return to rest position and keeps microswich activated
 
Not the most efficient boiler to run, but must have the prize for longevity in the boiler class; must be best part of 20 years old, and still plenty of them around
 
right in the front middle is a servo valve that has a cam on it(at back) that comes up and hits micro switch , when water demanded it does not always return to rest position and keeps microswich activated
Many thanks. Can this be externally lubricated (as on cam pivot) or would your experience suggest replacing the servo?
 
Personally when they start seizing i would replace servo valve and at same time strip out the water section under neath change the diaphragm and clean and lubricate the push rod .

Due to small pipes to servo it can be a sod to get air out of boiler on refilling after changing servo.

BUT before any of that make sure it is the cam sticking uo onto the HW microswitch

You can move cam back and forth several times and that sometimes frees them off for a considerable time afterwards
 
Personally when they start seizing i would replace servo valve and at same time strip out the water section under neath change the diaphragm and clean and lubricate the push rod .

Due to small pipes to servo it can be a s** to get air out of boiler on refilling after changing servo.

BUT before any of that make sure it is the cam sticking uo onto the HW microswitch

You can move cam back and forth several times and that sometimes frees trds
Brilliant practical advice. Many thanks.
 

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