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Andrew Smith
Andrew Smith, Plumber
Category: Plumbing
Satisfied Customers: 3756
Experience:  City and Guilds qualified plumber
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I have a oil fired condensing boiler with a S plan valve

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Hi. I have a oil fired condensing boiler with a S plan valve system. No demand for either hot water or heating and both valves off but boiler continues to run and get V hot. Problem with micro switches in motorised valves?
Yes or faulty frost stat if you have one fitted.
To make sure go to the junction box and pull out a grey wire at a time from the valves
Customer: replied 2 years ago.
Definitely not the frost stat. Have bypassed it while snagging this problem. The fault only seems to happen when my tank is very hot. (I reclaim waste heat from my power generator as well as using the oil fired boiler as I live off grid) The valves operate normally most of the time. Do you think the problem could be in the actuator alone or do ou reckon it is worth a drain down and replace the entire valve?
You could start by taking the heads off, using WD40 on the pin and give it a wiggle and see if it cures it.
9 times out of 10 it is the valve preventing the head from closing.
It sounds like your hot water valve is faulty if the cylinder is getting that hot/
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