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Old 27.03.2020, 18:58
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Re: Radiator valve reducing temperature

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It might be jammed open. I think what is common here is that pressure on a pin causes the valve to open. The thermostat provides this pressure. So removing the thermostat entirely should allow the pin to come out a bit and then the valve is closed.

In the case that it is jammed, the pin will stay depressed, and then the valve stays open.
Unfortunately this is incorrect. The pin is depressed to close the valve, so removing the head unit allows it to fully retract, which leaves the valve wide open.

The rest of the info about unsticking it is valid, but is used to fix a non-opening valve, of which I find two or three every season. The valve pins sometimes get a deposit that jams them and needs to be cleaned off carefully.

None of which is relevant to Phil's question though, which should not be possible with a thermostat head unless the mechanics of the thermostat itself are broken, such that it's no longer putting any pressure on the pin.

Edit: I should point out that this is true of the 30+ year old valves in our French house, the various replacements or additions I've fitted over our 20 years there, and of the brand new system in our Swiss house, so I'm fairly confident that this is the way they all work.
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