How to Bleed Radiators Properly (and When To)
Bleeding a radiator releases trapped air through a small valve at its top corner, restoring the full hot surface you're paying to heat. It's a five-minute homeowner job needing only a bleed key, a cloth and a container. The parts people get wrong: bleeding in the wrong order, forgetting to re-pressurise afterwards, and treating repeated bleeding as maintenance when it's actually a symptom.
When bleeding is the right answer
Bleed when a radiator is hot at the bottom and cold at the top — the classic air signature, since air rises to the highest point in the panel. Bleed also after any system work that let air in, and as a start-of-season check if radiators feel patchy. Don't bleed a radiator cold at the bottom and warm at the top: that's sludge, not air, and bleeding achieves nothing but lost pressure. And don't bleed a radiator that's entirely cold on a hot system — check its valves first.
Timing matters: bleed with the heating off and the system cooled, so the water isn't scalding and the air has gathered where you can release it. Bleeding a hot, running system risks a face full of very hot water and gives poorer results.
The method, in order
Turn the heating off and let radiators cool. Start at the radiator lowest and furthest from the boiler, then work upwards and inwards — bottom floor before top floor. Hold a cloth and container beneath the bleed valve (top corner, a small square or slotted socket), insert the key and turn anticlockwise a quarter to half turn. You'll hear a hiss; when water dribbles steadily rather than spitting, close the valve firmly — snug, not forced.
Repeat around the house in that order, then check the boiler's pressure gauge: bleeding always drops it, often below the 1 bar threshold. Top up via the filling loop to 1.2–1.5 bar, fire the heating, and feel each radiator across its face. A few stubborn ones may need a second pass once the system has circulated.
What frequent bleeding really means
A properly sealed system shouldn't generate air. Needing to bleed the same radiator every few weeks means air is either getting in — through a leak drawing air on the suction side, a failing automatic air vent, or repeated top-ups introducing dissolved gases — or being generated inside, by corrosion producing hydrogen in an under-inhibited system. That second cause is the one worth taking seriously: hydrogen from active corrosion is your radiators quietly dissolving.
The test an engineer runs is simple: collect the gas and check whether it burns (hydrogen, meaning corrosion) or not (air ingress). Either finding leads somewhere useful — inhibitor and cleansing for the first, leak-tracing for the second. Endless bleeding treats neither and dilutes inhibitor further with each subsequent top-up.
Balancing: the step after bleeding
If radiators are still uneven after bleeding — near ones roasting, far ones lukewarm — the system needs balancing rather than more air released. Balancing adjusts the lockshield valve (the plain one, usually capped) on each radiator so flow distributes properly across the whole circuit, and it's the difference between a system that works and one that merely runs.
It's a fiddly but non-invasive job involving thermometers and patience, and many engineers include it within radiator or boiler work. Worth asking for explicitly after any radiator changes: a new radiator dropped into an unbalanced system will happily steal flow from its neighbours, which is why 'we fitted a new radiator and now the back bedroom's cold' is such a common complaint.
| Symptom | Cause | Fix |
|---|---|---|
| Cold top, hot bottom | Trapped air | Bleed it |
| Cold bottom, hot top | Sludge | Flush / powerflush |
| Entirely cold, others hot | Stuck TRV pin, closed valve, balancing | Check valves |
| Needs bleeding often | Air ingress or corrosion gas | Engineer diagnosis |
| Far radiators lukewarm | System unbalanced | Balancing |
If the symptoms don't quite match
Faults read textbook-clean in guides and messier in airing cupboards: if your symptoms straddle two patterns above or match none cleanly, that ambiguity is itself useful diagnostic information — tell the engineer precisely that. Intermittent faults deserve a log (time, weather, what the boiler was doing) because patterns invisible in one visit leap out of a week's notes. And any smell of gas overrides everything here: leave the property, call 0800 111 999.
Frequently asked questions
What order should I bleed radiators in?
Downstairs before upstairs, and within each floor start furthest from the boiler and work back. It pushes air toward the points you haven't done yet rather than back into ones you have.
Do I need to re-pressurise after bleeding?
Almost always — releasing air drops system pressure, sometimes below the lockout threshold. Check the gauge and top up to 1.2–1.5 bar via the filling loop as part of the same job.
Why does the same radiator need bleeding repeatedly?
Air is either entering the system (a leak, a failing air vent, repeated top-ups) or being generated by internal corrosion in under-inhibited water. Both deserve diagnosis — persistent bleeding is a symptom, not maintenance.
Should I bleed radiators with the heating on?
No — turn it off and let things cool. Hot systems release scalding water and pressurised circulation makes the air harder to catch at the bleed point.
If you'd rather hand this over
Plenty of readers reach this point, weigh the afternoon involved, and sensibly hand it to someone who does it weekly. No judgement — that's what the network exists for. Every engineer is Gas Safe registered (plus OFTEC for oil and MCS for heat pumps), insured and locally based; prices are agreed up front; and genuine no-heat emergencies are prioritised. Describe the symptoms you've identified here and you'll get an accurate price rather than a guess — informed customers reliably get better quotes.
Carbon monoxide is odourless and colourless. Our CO safety guide covers the symptoms, where alarms belong and the emergency response.