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Thanks for the quick response to this. I had a funny feeling I would end up having to change the valves so you have confirmed this. Only pain is because it is straight valve through floor to bottom of radiator its not so easy to drain just that radiator without fluid leaking over the place. Will just need to close off all other radiators except in adjacent room and drain from there.A few years ago I ended up just changing loads of these on a new build estate. The customers would move in and turn them off and the minute they did they leaked. I have a feeling the heads are pressed on so you would have to pry it apart with patience a hammer chisel and thin wide screwdriver. We wouldn't warranty trying to fix them so just recommended changing and the customers were happy with that.
They are ceramic discs inside so its unlikely there will be a gland to tighten, either the cartridge seal or more likely that the spline o ring has gone which means those valves are set for the bin anyway
Ok thanks for the tip. I will do that before doing the job.It's worth watching some videos on how to drain and refill a heating system because what you describe can cause problems with air locks.
Reply to the thread, titled "Lockshield valve removing cap" which is posted in UK Plumbers Forums on Plumbers Forums.
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