In article ,
Simon Elliott Simon at ctsn.co.uk wrote:
Today being a cold day, the knocking started again. I went around the
radiators with thermostatic valves, and found one which seemed noisy.
Turning the valve slightly stopped the noise. Playing with the valve, it
seemed to be quite clunky.

Some TRVs can only be fitted on (usually) the flow side. They'll often
have an arrow on them denoting direction of flow. Fitting them on the
return can give the symptoms you describe. So check by seeing which pipe
gets hot from cold first - this will be the flow.

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*I'm not being rude. You're just insignificant

Dave Plowman London SW
To e-mail, change noise into sound.