Radiator cold at the top, warm at the bottom
A radiator that's warm at the bottom but cold at the top is one of the most common heating complaints we see, and the good news is it's usually straightforward to fix. This happens because air gets trapped inside the radiator — a natural occurrence as your heating system operates — and prevents hot water from circulating properly through the entire panel. It's particularly common after your system's been drained for maintenance, or if you've recently added new radiators. Most homeowners can tackle this themselves in under 10 minutes with just a bleed key, though it's worth understanding what's happening and when you might need to call in a professional, especially if the problem persists across multiple radiators or your boiler pressure keeps dropping.
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Most likely cause & what to check
A radiator cold at the top has trapped air inside — the easiest radiator fix there is.
Turn the heating on and wait until the system reaches temperature.
Insert the bleed key and turn anti-clockwise by a quarter turn — you'll hear hissing air.
Once water trickles steadily, close the valve firmly but don't overtighten.
After bleeding, check boiler pressure and repressurise to 1–1.5 bar if needed.
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Frequently asked questions
Why do radiators get air trapped inside them?
Air naturally enters your heating system when it's been refilled after a repair, or gradually accumulates over time through microscopic leaks and chemical reactions in the water. Once trapped, this air rises to the highest point in each radiator — usually the top — creating a cold spot that prevents efficient heating. Regular bleeding of your radiators helps keep your system running smoothly and maximises heat output.
Is it safe to bleed a radiator myself?
Yes, bleeding a radiator is one of the safest DIY plumbing tasks you can do — there's no gas involved and minimal water escapes. Just make sure your heating system is up to temperature before you start, keep a cloth handy to catch any drips, and don't force the bleed key or overtighten it when closing. If you're uncomfortable or the valve is stuck, that's when to call a professional.
What should I do if my boiler pressure drops after bleeding radiators?
A small pressure drop is normal after bleeding — you've released a tiny amount of water along with the air. Check your boiler pressure gauge and repressurise to between 1 and 1.5 bar using the filling loop underneath or behind your boiler; your boiler manual will show you how. If pressure keeps dropping significantly, you likely have a leak elsewhere in the system and should contact a plumber.
What if multiple radiators are cold at the top?
If more than one or two radiators have this problem, it suggests a wider air issue in your system — possibly from recent work, or a circulation problem. Bleed each radiator in turn, starting with the one closest to your boiler and working outward, then check your boiler pressure carefully. If the problem persists after bleeding all radiators, or if you notice your pressure dropping fast, it's time to have a Gas Safe engineer investigate for leaks or circulation issues.