Radiator is not heating up
If the radiator is cold at the top, you will need to bleed the radiator to release any air. The bottom of the radiator will be full of hot water, but this heat will be unable to reach the top because of the air bubble. Bleeding a radiator is a very easy task and will only take you a few seconds if you have a radiator key available. Turn the key slowly until you hear air being released from the valve. Once you can no longer hear air being released or water begins to come out, you’ve successfully removed the air and can close the valve.
If the top half of the radiator is warm but the bottom half is cold, you will have a larger job on your hands. There will be a thick layer of sludge at the bottom of the radiator, which will stop it from heating up. You will have to remove the radiator from the wall and the plumbing and rinse it through. Don’t forget that water will gush out of the radiator when you remove it, so be prepared with pans and towels.
If the entire radiator is cold, check that the thermostatic radiator valve and the second valve on the other side is open, and that the pins within the thermostatic valve are moving properly. You can check if a thermostatic valve is working properly by taking the head off and seeing if the pin moves down and up freely. If it is stuck, give it a careful tap to free it up.
If this doesn’t work, turn off all the radiators in the rest of the house to see whether or not the broken radiator heats up. If the radiator warms up, then turn your other radiators on one-by-one. You can then identify any other radiator problems and find out which other radiators may need bleeding. If your cold radiator still won’t heat up, you might need to replace the valve.