Radiator Cold at the Top? Air and How to Clear It
A radiator hot at the bottom and cold across the top has trapped air sitting where it always sits — at the highest point of the panel, displacing the water that should be heating that surface. Bleeding releases it in under a minute with a bleed key, a cloth and a container. The follow-up questions matter more: re-pressurising afterwards, and asking why air keeps appearing if this is a regular event.
Clearing it
Turn the heating off and let the system cool. Hold a cloth and small container under the bleed valve at the radiator's top corner, insert the bleed key (or flat screwdriver on some valves) and turn anticlockwise a quarter to half turn. Air hisses out; when water dribbles steadily rather than spitting, close the valve firmly — snug, not overtightened.
Then check the boiler's pressure gauge. Bleeding always costs pressure, sometimes enough to drop the system below its lockout threshold, so top up via the filling loop to 1.2–1.5 bar if needed. Fire the heating and feel the radiator across its whole face; a stubborn one occasionally needs a second bleed after the system has circulated.
Where the air comes from
Some air is normal after system work — refilling, radiator changes, top-ups — and one round of bleeding settles it. Recurring air is not normal and has two very different explanations. Air ingress: a leak on the suction side of the pump, a failed automatic air vent, or repeated top-ups introducing dissolved gases that come out of solution when heated. Corrosion gas: hydrogen generated by steel radiators actively rusting in under-inhibited water.
The second is the one to care about. An engineer can test which you have — hydrogen burns, air doesn't — and the finding directs everything after it. Hydrogen means the inhibitor has failed and your radiators are dissolving slowly; the answer is cleansing, fresh inhibitor and usually a magnetic filter, before the sludge shows up as cold radiator bottoms next season.
If bleeding doesn't help
No air comes out and the top stays cold: check the radiator's valves are open — a closed lockshield or a TRV with a stuck pin starves the panel entirely, though that usually gives a radiator cold all over rather than just at the top. Air comes out repeatedly within days: see above, it's a diagnosis rather than a chore. Radiator now heats but others have gone cold: the system needs balancing, not more bleeding.
And if the whole system is lukewarm rather than individual radiators being patchy, look at the boiler side — pressure, pump, or a boiler running at too low a flow temperature for the weather. Individual cold radiators and a uniformly tepid system are different problems with different answers.
Good practice through the season
Worth doing each autumn before the heating season: run the system hot, feel every radiator, bleed any with cold tops, top the pressure back up, and note anything that needed bleeding. That last part is the useful record — a radiator on your list two years running is telling you something the single bleed doesn't.
Keep a bleed key somewhere findable (they cost pennies and vanish reliably), and don't overtighten bleed valves; the small brass screws strip readily, turning a free job into a valve replacement. Gentle, snug, done.
| Item | Detail |
|---|---|
| Symptom | Cold top, hot bottom |
| Tool | Bleed key, cloth, container |
| System state | Heating off, cooled |
| Turn | Quarter to half turn anticlockwise |
| Stop when | Steady water, not spitting |
| Afterwards | Re-check pressure, top up to 1.2–1.5 bar |
The cost of leaving it
Heating faults rarely improve with seasoning: pressure that keeps dropping dilutes the inhibitor protecting your radiators, sludge restricting flow gradually cooks the heat exchanger, and marginal components fail completely at the least convenient hour — usually a cold one. The economical moment to fix a diagnosed fault is while it's minor, on a standard-rate weekday visit rather than the emergency call it's rehearsing to become.
Frequently asked questions
How often should radiators need bleeding?
Once a year at the start of the season is typical, plus after any system work. Needing it every few weeks means air is entering or being generated — worth diagnosing rather than repeating.
Can bleeding a radiator cause problems?
Only two: forgetting to re-pressurise afterwards (leading to a lockout), and overtightening the small bleed screw. Both are easily avoided.
Why is water coming out brown or black?
That's magnetite — corrosion debris — telling you the system water is under-protected. Bleeding is fine; the follow-up is an inhibitor check and possibly cleansing before it shows up as cold radiator bottoms.
Should I bleed every radiator or just the cold one?
Just the ones showing cold tops, though a full sweep at season start is good practice. Work downstairs first, furthest from the boiler inwards, and check the pressure at the end.
When to bring in a Gas Safe engineer
Every guide on this site draws the same line in the same place: homeowner-safe checks stop at the boiler's casing. Re-pressurising via the filling loop, thawing a frozen condensate pipe, bleeding radiators, checking the thermostat and pressing reset once — all yours. Anything requiring the case to come off involves gas, and gas work is legally restricted to Gas Safe registered engineers for reasons that predate any website's advice. Our network handles exactly that side of the line, with engineers matched to the job and prices agreed before anyone starts.
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