CorrectThe collar is smooth inside so assuming this is just a cap for the lock shield valve on that side?
YesOn both?
Yes ,and it may need bleeding of air.Heating on now so will see if heats up. Should both sides remain fully open?
Close the TrV' s on all other rads ,do not touch their lockshields. Bleed air from the problem rad. It should then get to full temp very quickly. If your system is pressurised ensure pressure is 1 bar after bleeding.Seems to be warming a bit but still way behind other rads
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