A TRV that will not shut is usually the pin
I set the bedroom radiator to frost in April and it stayed hot. The head clicked to zero. The rad did not care. I thought the head had died. I unscrewed it. Underneath, the little pin in the valve was stuck up, proud, and would not move under a finger. The head had been pressing nothing. The valve had sat open all winter. I put a manual cap on so I could shut it, then I freed the pin. The rad went cold. The head was fine. The pin had seized.
A TRV head is a wax pot that pushes a pin. The pin opens and closes the valve in the body. If the pin rusts in the up position, the valve stays open and the rad stays hot whatever you set. If it sticks down, the rad stays cold. This note is the hot one, the one that will not shut. People buy a new head. They screw it onto a pin that still will not move.
I take the head off and I look. The pin should be proud and it should give a millimetre if I press it. Stuck solid is the job. I cap the valve with the little manual cap that often comes in the pack, or I hold the pin shut with care so the room can cool. Then I work the pin. A drop of penetrant, a gentle tap on the side of the body, a press and a release, not a hammer on the pin into the valve. I want it to move, not to vanish.
I do not drain the system for a stuck pin. I do not open the lockshield unless I must isolate that rad and I know which way it shuts. If I cannot isolate that rad and the valve starts to weep, I stop. I put the cap on. I leave it. A wet valve on a live rad is further than this job.
When the pin moves I put the head back, set it, and I wait. A rad that still runs hot with the pin free and the head on zero may be a head that has failed. I try the cap shut first. If the cap shuts the rad, the head was the spare part, and not a drain-down. If the cap will not shut it either, I stop.
Skip a new rad because one valve stuck. Skip forcing the head down onto a pin that will not go. You will crack the plastic. Skip leaving the head off all summer so the pin seizes again. I take the head off in the off season on purpose, press the pin once a month, and put the head back in autumn. That is how I stop next April's hot bedroom. Skip oil on the head sensor. The oil is for the pin, and a film, not a bath.
A radiator that wrecks a wall is usually the one you painted around is the same rad as furniture. I do this pin job when the heating is off and the metal is cold, the same off season I would paint behind it. I do not work a hot valve with wet hands in January if I can wait.
I also look at the other end. A lockshield wound open and forgotten will keep a rad warm even with a working TRV. I do not balance a house from one stuck pin. I free the pin. If the room is still wrong, I stop before I invent a system job.
I do not need a new radiator because the TRV would not shut. I need a pin that moves, a cap I can shut, and a stop if I cannot isolate that rad.
If I later add a buy link here, it will be a paid promo.