Radiator cold at the top? What it means
A radiator that’s cold at the top and warm at the bottom has air or gas trapped in it. Bleeding the radiator lets it out and usually fixes it straight away. If you find yourself bleeding the same radiators again and again, corrosion inside the system is likely producing gas, and that corrosion is also what makes sludge.
How to bleed a radiator safely
Turn the heating off and let the radiators cool. Hold a cloth under the bleed valve at the top corner of the radiator and turn it slowly anticlockwise with a radiator key. You’ll hear a hiss as the air escapes. When water starts to come out, close the valve.
On a combi or system boiler, check the pressure gauge afterwards. Letting air out lowers the pressure, and your boiler manual tells you the right level and how to top it up.
Why does air keep getting in?
Once in a while is normal. If it keeps happening, the usual cause is corrosion: as the steel inside the system corrodes, it gives off gas that collects at the top of the radiators. The same corrosion makes the black sludge that settles at the bottom.
On older open-vented systems with tanks in the loft, problems with the feed and vent pipes can also draw air in. An engineer can check that.
Cold at the top and the bottom?
If a radiator is only warm in the middle, it probably has both: air at the top and sludge at the bottom. Bleeding sorts the top, but the bottom needs the sludge cleared. Read our guide to radiators cold at the bottom.
When to call an engineer
Call an engineer if you’re bleeding the same radiators every few weeks, the water that comes out is black or brown, the boiler pressure keeps dropping, or bleeding doesn’t make the radiator heat up. Those are signs the system needs cleaning and protecting, not just more bleeding.
In Kent?
Kent Powerflush, Gas Safe 660156, covers Faversham, Canterbury, Whitstable, Herne Bay, Thanet, Sandwich, Deal, Dover, Folkestone, Ashford and Sittingbourne. Call 07540 335 087 or get a free quote.
Local pages: powerflush in Dover, powerflush in Deal, powerflush in Margate, or see all the areas we cover.
Questions people ask
What does it mean if a radiator is cold at the top?
Air or gas is trapped at the top of the radiator, pushing the hot water down. Bleeding the radiator lets it out.
Why is my radiator warm at the bottom but cold at the top?
Trapped air or gas. Bleed the radiator. If it keeps coming back, corrosion inside the system is the likely cause.
My radiator is still cold at the top after bleeding. What next?
Check the boiler pressure, and check both radiator valves are open. If it’s still cold, the system may have a circulation problem or sludge, so give us a call.
Is a radiator that’s cold at the top dangerous?
Not in itself, but it’s a sign the system needs attention, especially if it keeps happening.
Related guides
Hot radiators across East Kent
Faversham, Canterbury, Whitstable, Herne Bay, Thanet, Sandwich, Deal, Dover, Folkestone, Ashford and Sittingbourne. Tell us about your system and Pete will come back with a price.