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, because trapped air rises. It will not fix a radiator that’s cold at the bottom — that points to a circulation problem rather than air, and needs assessing. It’s a safe DIY job and usually a quick one. 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. 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 isn’t trapped air, so bleeding won’t help. Settled debris is one likely explanation, but a restricted valve or poor circulation can look the same. Skip to the section below if that’s you.
What you’ll need
- A radiator key, available from any hardware or DIY 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 commonly used approach is to start with the radiator furthest from the boiler and work back towards it, doing downstairs before upstairs, so you’re working through the system methodically rather than at random. Whichever order you choose, the important parts are that the system is off and cool, and that you re-check the pressure once you’ve finished.
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 → a circulation problem rather than air. Settled debris at the base of the radiator is one common cause, but a restricted or partly closed valve, a failing pump or a system that needs balancing can produce the same symptom. Bleeding can’t fix any of them. The right next step is assessing the system and its water, then cleaning proportionately if contamination is confirmed, with 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. Possible causes include a leak drawing air in, a pump issue, or corrosion generating gas inside the circuit. This needs a central-heating assessment rather than an appliance service: a boiler service follows the manufacturer’s schedule for the boiler, and diagnosing the wider system is separate work unless you book it. Our central heating services team investigates exactly this.
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 these two are the usual starting points for diagnosis.
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, because you risk rounding the pin. Radiator keys are inexpensive and stocked by any hardware or DIY shop, so it’s worth having the right one.
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 trapped air, and bleeding only removes air from the top. Cold at the bottom points to a circulation problem: settled debris in the radiator, a restricted valve, a failing pump or poor balancing can all look similar. Have it assessed, then cleaned proportionately if contamination is confirmed.
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 point to a circulation fault of some kind — fixable once it’s properly diagnosed, and worth sorting 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.
