Why the towel rail is cold at the top
Air, usually. Here is how to bleed it without flooding the floor.
A towel rail or radiator that is warm at the bottom and cold at the top has air in it. Water cannot get past the air, so the top stays cold.
How to bleed it
Turn the heating off and let the system cool
Hold a cloth under the bleed valve at the top corner
Open it a quarter turn with a radiator key until the hiss stops and water follows
Close it, then check the boiler pressure
If it comes back
Air that returns every few weeks is getting in somewhere, usually at a failing expansion tank or an automatic vent. That is a visit, not a key.
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