To bleed a radiator: turn the heating off and let the system cool, put a cloth and bowl under the bleed valve at the top corner of the radiator, and turn the valve a quarter-turn anticlockwise with a radiator key. You’ll hear a hiss as trapped air escapes. Close the valve the moment water starts to dribble out. Then check your boiler’s pressure gauge: bleeding lowers system pressure, so top it back up to 1–1.5 bar if needed. Bleeding fixes radiators that are cold at the top (trapped air rises); it will not fix radiators that are cold at the bottom. That’s sludge, which needs a different solution. It’s a safe DIY job that takes about a minute per radiator. If bleeding doesn’t cure it, The Happy Heating Co’s Gas Safe engineers (657972) sort circulation and sludge problems across West and North West London. See our central heating services.
A radiator with cold patches at the top is one of the few heating problems you can genuinely fix yourself in under five minutes. Here’s how to do it properly, in what order, and how to tell when the problem isn’t air at all.
When does a radiator need bleeding?
Air rises, so trapped air collects at the top of a radiator and stops hot water filling that space. The signs:
- The radiator is hot at the bottom but cold at the top.
- It gurgles or bubbles when the heating runs.
- It takes noticeably longer to warm up than the others.
One caveat before you reach for the key: if the radiator is cold at the bottom and warm at the top, that’s not air. It’s sludge (rusty deposits settling at the base), and bleeding won’t help. Skip to the section below if that’s you.
What you’ll need
- A radiator key, a pound or two from any hardware shop; many modern valves also accept a flat-head screwdriver.
- A cloth and a small bowl to catch drips, as heating water is often dirty and can stain carpets.
How to bleed a radiator: 7 steps
- Turn the heating off and let the radiators cool. Bleeding a hot system sprays scalding water.
- Find the bleed valve. It’s the small square pin inside a round nut at one top corner of the radiator.
- Hold the cloth under the valve with the bowl beneath it.
- Turn the valve a quarter- to half-turn anticlockwise with the key. You’ll hear a hiss; that’s the trapped air escaping. Don’t unscrew it further; it doesn’t need to come out.
- Wait for the hiss to stop. When water starts to dribble steadily from the valve, the air is out.
- Close the valve, clockwise, snug but not forced.
- Check the boiler pressure and repressurise if needed (next section).
That’s it. A minute per radiator, plus drying your hands.
Which order should you bleed radiators in?
If several radiators need doing, a good rule of thumb: start with the radiator furthest from the boiler and work back towards it, doing downstairs before upstairs. Air migrates through the system, and this order chases it out efficiently instead of pushing it around. In a typical West London terrace or maisonette that usually means starting in the back bedroom and finishing in the hallway.
After bleeding: check the boiler pressure
Bleeding releases air and pressure, so the boiler gauge will read lower afterwards, sometimes low enough that the boiler locks out. Check the gauge and top the system back up to 1–1.5 bar cold via the filling loop, per Worcester Bosch’s pressure guidance. We’ve covered the full routine in why is my boiler losing pressure?, including what it means if the pressure keeps falling afterwards.
Bled it, and a radiator is still cold?
Two patterns, two very different causes:
- Still cold at the bottom → sludge. Rust and debris settle at the base of radiators over the years and block circulation. Bleeding can’t remove it. The fix is a proper system clean or powerflush and, ideally, a magnetic filter to stop it recurring. That’s our territory: central heating services.
- Air keeps coming back. If you’re bleeding the same radiators every few weeks, air is getting into the system, which usually means a leak, a pump issue or corrosion generating gas internally. Worth a proper look before it shortens your boiler’s life; an annual boiler service is the natural moment to have it investigated.
Radiators that stay stubbornly cool across Queen’s Park, Kilburn and the surrounding patch are a big share of the heating calls we get each autumn, and nine times out of ten it’s one of these two.
Frequently asked questions
How do I bleed a radiator without a key?
Many modern bleed valves have a slot that takes a flat-head screwdriver; try that first. If the valve only takes the square key, don’t improvise with pliers; you risk rounding the pin. Radiator keys cost a pound or two from any hardware or DIY shop.
How often should radiators be bled?
There’s no fixed schedule; bleed them when the symptoms appear: cold at the top, gurgling, slow to heat. Checking each autumn before the heating season starts is a sensible habit. If you’re having to bleed the same radiators every few weeks, air is re-entering the system and that needs investigating.
Why is my radiator cold at the bottom after bleeding?
Because the problem isn’t air. It’s sludge. Rust and debris settle at the base of the radiator and block hot water from circulating there. Bleeding only removes air from the top. The fix is a system clean or powerflush by a heating engineer, ideally with a magnetic filter fitted afterwards.
Does bleeding radiators reduce boiler pressure?
Yes. Releasing air lowers the sealed system’s pressure, and the boiler gauge will read lower afterwards, occasionally low enough to lock the boiler out. After bleeding, check the gauge and top back up to 1–1.5 bar cold via the filling loop. That’s normal, not a fault.
Should the heating be on or off when bleeding radiators?
Off, and cooled. With the heating on, the system is pressurised with hot water. Opening a bleed valve can spray scalding water, and the pump can pull more air around the system as you work. Let everything cool for a safer, cleaner job.
Radiators still not heating properly?
Cold spots that bleeding won’t shift usually mean sludge or a circulation fault, both fixable, and both worth doing before winter.
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We’re Gas Safe registered (657972) and cover West and North West London. Heating that just works, no drama.
About the author Adam Said is a Gas Safe registered domestic gas engineer (657972) at The Happy Heating Co, servicing boilers and central heating systems in homes across West and North West London. This guide reflects what we see on real jobs every week.
Related reading
- Central heating services
- Why is my boiler losing pressure?
- Annual boiler servicing
- How often should you service your boiler?
Pressure figures reflect Worcester Bosch’s boiler pressure guidance (verified July 2026); always follow your own boiler’s manual.
