Boiler radiators warm at the bottom but cold at the top (all brands)

A radiator that’s warm at the bottom but stays cold across the top few inches almost always means trapped air — one of the few central heating symptoms with a genuinely simple, well-known DIY fix.

What’s safe to check yourself

• Bleed the affected radiator using a radiator bleed key (widely available cheaply): with the heating on and the radiator warm, turn the bleed valve at the top corner slowly anti-clockwise until you hear a hiss of escaping air, then close it the moment water starts to appear instead — have a cloth ready to catch drips.
• Work through radiators from the ground floor up, and from the one nearest the boiler outward, if several are affected — trapped air tends to collect at the highest points of the system.
• Check your boiler pressure after bleeding several radiators — releasing air can drop the system pressure, and you may need to top up via the filling loop afterward.

When to call an engineer instead

If you’re regularly bleeding the same radiators every few weeks, or if bleeding doesn’t fix a radiator that’s cold at the bottom rather than the top (a different symptom, usually sludge build-up rather than air), that points to a system needing a professional power flush or a corroding radiator — worth a heating engineer’s assessment rather than repeated bleeding.