A radiator that stays cold at the top is usually air
I put a hand on the sitting-room radiator in October. The bottom was hot. The top was cold. I thought the rad had failed. I got the bleed key, a cloth, and a mug. I put the cloth under the valve at the top corner, turned the key a fraction, and air hissed out. Then water came, a brown trickle. I shut it. Ten minutes later the top was warm. The rad was full of air, not broken.
Air sits at the high point. Water cannot heat a space that is dry. You get a cold band, or a cold top, and a noise like water moving in a pipe. People drain a system for that. I bleed one valve. A TRV that will not shut is usually the pin is the other fault on the same rad. If the whole rad stays hot with the head on zero, stay with the pin. If the top is cold and the bottom is hot, stay with the air.
I use a key that fits. Square, the small one, not a random bit from the drawer that rounds the pin. I hold the cloth so the first spit hits that, not the wall. I turn a little, not a full unwind. The bleed valve is a small screw. If I take it out, I have a hole I cannot shut. I listen. Air first. When it goes to water I close it. That is the stop. I do not keep it open because the water looks dirty. Dirty water is still the system. I am not draining it.
If the key will not sit on the square, I stop. A rounded bleed pin is a leak waiting. I do not force a spanner until I have a fountain. If water will not stop when I turn back, I press the cloth on, I try the key again, and if it will not shut I stop and I get help. I do not invent a drain-down on the carpet.
I bleed when the heating has been on so the air has risen. I do one rad, then I feel it. I do not walk the house opening every valve because I have the key in my hand. A house of cold tops after one bleed is further than this job. I stop.
Skip a new radiator because the top was cold. Skip opening the bleed with no cloth. Skip a screwdriver that almost fits. Skip leaving the valve open while I answer the door. Skip oil in the bleed hole. Skip balancing lockshields to fix air. Free the air first.
I keep the key where I can find it. A hook in the cupboard, not in a tin I emptied last spring. I wipe the valve after. A crust of scale on the square is why the next key will not fit.
This shows in autumn, the first week the heating is on, and after any work that has let air in. I now feel the top of the rad I care about on that first cold evening, before I decide the room is just like that.
I do not need a new radiator because the top stayed cold. I need a key that fits, a cloth, air out, water, shut, and a stop if the valve will not close.
If I later add a buy link here, it will be a paid promo.