Heating & hot water
How to bleed a radiator (properly)
Cool at the top, warm at the bottom: that radiator has swallowed air, and the cure has not changed in a century. Ten minutes, a key and a cloth will sort every radiator in the house, as long as you finish with the one step almost everyone forgets.
By the Tickover team · Published 16 January 2026 · 5 min read
Check the pattern before you start
Bleeding fixes exactly one problem: air trapped at the top of a radiator. Run the heating until the system is warm, then feel each radiator top and bottom. Cool at the top with heat below means air. Cold across the bottom with heat above is sludge, and bleeding will do nothing for it.
Air gets in with fresh fill water, through tiny weeps at fittings, and as hydrogen given off by corrosion inside the system. It rises to the highest points, which is why upstairs radiators and the far ends of the circuit collect it first. The pocket sits above the water line and stops hot water filling the top of the panel.
Once a year is the sensible rhythm, ideally in early autumn before the heating season starts, plus any time a top feels cool. Newly filled systems need it more often in the first few weeks, as dissolved air works its way out of the water.
The kit
- A radiator key, or a flat-blade screwdriver for slotted valves
- An old towel for the floor, a cloth for your hand
- A mug or small bowl to catch the dribble
- Kitchen roll for the weep check afterwards
- Two minutes at the boiler at the end, for the pressure
The method, radiator by radiator
Bleed a radiator in seven steps
Turn the heating off and let it cool
Hot radiators spit scalding water, and a running pump churns air around the system instead of letting it gather at the tops.
Start downstairs, furthest from the boiler
Work back toward the boiler, then repeat the order upstairs. This chases the air sensibly instead of shuffling it between rooms.
Set the cloth and bowl under the bleed valve
It is the small square pin inside a round brass fitting at one top corner of the radiator.
Turn the key a quarter to half a turn anticlockwise
A hiss is air escaping. Keep the cloth around your hand; the mist can carry black water.
Close when the hiss turns to a steady dribble
Water means the air is out. Snug the valve shut: firm, not forced.
Repeat on every radiator with a cool top
Skip the ones heating evenly. Dab each closed valve with kitchen roll afterwards to catch any weep.
Check the boiler pressure and top up
Bleeding lets pressure out of a sealed system. If the gauge reads below 1 bar, top it back up to about 1.2: the pressure guide walks through the filling loop.
When bleeding does not fix it
One radiator cold all over, top and bottom, is usually not air at all. Check its thermostatic valve is turned up, and that the lockshield at the other end has not been wound shut during decorating. A TRV pin can also stick after a summer of idleness; unscrew the head collar by hand and free the pin with a gentle press.
A radiator that needs bleeding every few weeks is telling you something too. Constant new air means corrosion gassing away inside the system, a weep pulling air in, or inhibitor that has run flat. Mention the pattern at the next boiler service: persistent gassing deserves a proper look rather than a growing key collection.
If the whole system heats unevenly even after bleeding, some radiators roasting while others idle, that is balancing rather than air: a plumber’s job with thermometers and patience. And anything on the boiler itself belongs to a Gas Safe registered engineer.
Bleeding rules of thumb
Do
- Bleed with the heating off and the radiators cool
- Keep a cloth around the key: the spray can be dirty
- Check the boiler gauge after bleeding, every time
- Make it an autumn ritual before the heating season
Don’t
- Do not unscrew the bleed pin all the way out
- Do not force a seized valve: that is a plumber visit
- Do not expect bleeding to fix a bottom-cold radiator
- Do not bleed with the pump running and expect it to last
Evenly heated radiators also let you run the boiler cooler, which is where the real savings hide: see thermostat settings that save money. And for every other radiator or boiler misbehaviour, the complete boiler guide maps each fault to its fix.
Radiator still cold after bleeding?
Run it through Self-Diagnose: a few questions separate air, sludge, balancing and valve faults, and tell you whether the next step is DIY or a vetted local pro.
Quick answers
How often should I bleed radiators?
Once a year in early autumn suits most homes, plus whenever a radiator top feels cool while the bottom is warm. A system that suddenly needs bleeding every few weeks has a fault brewing: corrosion gassing, a small leak drawing air in, or spent inhibitor. That pattern deserves a professional look.
Do I bleed radiators with the heating on or off?
Off, and cooled. With the pump running, air gets churned around the system rather than sitting at the radiator tops where the bleed valve can catch it, and hot water under pressure spits when released. Switch off, give it half an hour, then work around the house.
Why does nothing come out when I open the bleed valve?
Silence, with no hiss and no water, usually means system pressure is too low to push anything to the high points. Close the valve, check the boiler gauge and top up to about 1.2 bar, then try again. On open-vented systems, a stuck ballvalve in the loft tank can cause the same emptiness.
Will bleeding help a radiator that is cold at the bottom?
No. Cold at the bottom with heat at the top means settled sludge, not air, and no amount of bleeding shifts it. That pattern needs a chemical clean or power flush, plus a magnetic filter to keep the result. Save the key for radiators that are cool at the top.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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