I've got some cold spots at the top of my radiators even though the heating's on full blast. The rest of the radiator gets warm fine, just these patches stay cold. Is this definitely air trapped in the system, or could it be something else?
Don't just assume it's air and start bleeding every radiator - sometimes cold spots point to a blockage instead, which bleeding won't fix. The difference matters because a blocked radiator needs a power flush or chemical treatment, not just a bleed valve.
That said, if it's only the top that's cold and the bottom half heats up normally, air is the most likely culprit. The trapped air rises and settles at the highest point, preventing hot water from filling that space. You can test this pretty easily: turn off your heating, let it cool completely, then switch it back on and feel the radiator again within a few minutes. If the cold patch is still there in exactly the same spot, it's probably air. If the cold area moves around or changes, you might be dealing with a sludge blockage or water flow issue instead.
To bleed it, you'll need a radiator key (dead cheap, a few quid). Turn it slowly at the bleed valve (usually at the top corner) until water starts coming out - just a dribble, you're not draining the radiator. Have a cloth ready because it'll spray. Once water flows without air hissing out, close it back up. If you've got multiple radiators with cold spots, bleed them one by one, starting with the ones furthest from your boiler. If bleeding doesn't solve it after a week or so, that's when you'd want to call someone in to check for a real blockage.
Cold patches at the top of a radiator are pretty much the classic sign of air trapped in the system. When air gets into the pipes, it rises to the highest points and blocks the hot water from circulating properly, which is exactly what you're describing. The rest warming up fine while the top stays cold is textbook air in the radiator. That said, it's worth ruling out one other thing first: if you've got multiple radiators with the same problem across your whole system, it might be a circulation issue rather than air. But if it's just certain radiators or specific spots, bleeding them will almost certainly fix it.
Bleeding a radiator is straightforward. You'll need a radiator bleed key (they're cheap and every hardware store stocks them) and a small container or cloth to catch any water. Turn your heating on and let it run for a few minutes so the system warms up, then turn it off and wait for things to cool slightly. Locate the bleed valve at the top corner of the affected radiator, insert the key, and turn it counterclockwise slowly. You'll hear a hissing sound as air escapes, and eventually water will start coming out. The moment water flows steadily, close it back up. Do this for each radiator that has cold spots.
One thing to keep in mind: if you're bleeding a lot of radiators or if the problem comes back quickly after you've bled them, you might have air being drawn in somewhere in your system (often at the pump or a loose fitting). In that case, you'd want to check your system pressure and top it up if needed, since low pressure can allow air to be sucked in. But for a one-off bleed of a few radiators, the key approach should sort you out.
Log into answer.
