Property How to bleed a radiator
Heating · Beginner
How to bleed a radiator
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If a radiator is hot at the bottom but cold at the top, air is often sitting in the top of the panel and stopping the hot water circulating. Bleeding lets that air out through the small valve at one top corner. This guide is for an ordinary wet central-heating radiator — common in the UK, and found in some homes in Australia and the northern US. It is not for steam systems, electric radiators, or a boiler you can smell gas from.
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At a glance
Your call
Go or no?
Fancy giving it a go, or would you rather call in a heating engineer? There’s no wrong answer.
Right then. Here’s the kit, then the steps.
No problem. We’ll show you how to find a local heating engineer.
Shop the full kit
You can pick up the items below as you need them. Prices vary by shop and country, so we have not invented a figure here — check the live price before you buy. Shop links go to a product search on a local store for your country. Until a tracking tag is in place they are ordinary store links, not tracking URLs.
All-in-one kit
There is not usually a single boxed kit for this job. Buy the items below as you need them, and skip anything you already have.
Free printable
Get a free printable bleed a radiator checklist
A one-page list of tools, prep and the order of work, emailed once this is live.
Before you start
Safety
Stop and get a pro: If you smell gas, see scorch marks or soot on the boiler, or the boiler is in lock-out and you do not know why, stop and call a Gas Safe registered engineer in the UK, or a licensed gas technician in the US or Australia. Do not loosen the big valves where the pipes meet the radiator — only the small bleed valve at the top. If water will not stop when you close that valve, get help. If the system is steam, or you have an open-vented or unvented setup you do not understand, do not experiment.
- Let the radiator cool before you open the bleed valve. Hot water and steam can scald.
- Hold a cloth under the valve so any water lands on the cloth, not the floor or your hand.
- Only turn the bleed valve. The larger lockshield and wheel-head valves at the bottom are for balancing or isolation, not for this job.
- On a sealed system, bleeding lets out a little water as well as air, so the boiler pressure may drop. Only repressurise if you already know how, and only by following the boiler’s own instructions. If you are not sure, stop and call an engineer.
- This guide does not cover any work on the gas supply, flues, or the inside of the boiler.
Prep that is worth doing
- Turn the heating off and wait until the radiators are cool enough to touch.
- Find the bleed valve: a small square spindle, usually behind a cap, at one of the top corners.
- If you have a combi or a sealed system, look at the boiler’s pressure gauge before you start so you know the usual reading (many sit around 1–1.5 bar when cold — check your manual, do not assume).
Tools and materials
Check the live price at a shop you trust. Typical prices on this site, where they appear, were checked in the UK and may be out of date. Do not invent or assume a figure from this table. Shop links go to a product search on a local store for your country. Until a tracking tag is in place they are ordinary store links, not tracking URLs.
| Item | Why you need it | Typical price | Shop link |
|---|---|---|---|
| Radiator bleed key | Fits the square spindle on most UK radiators. Some valves take a flat-head screwdriver instead — look first rather than rounding the spindle. | check price | |
| An old cloth or kitchen towel | Catches the first spit of water. | check price | |
| A small container (optional) | If you would rather catch the water than rely on a cloth. | check price |
Shop this list:
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Step-by-step
Cool the system, open only the bleed valve until air gives way to water, close it, then check the boiler pressure if you have a sealed system.
Turn the heating off and let the radiator cool
Bleeding a scalding radiator is unpleasant and harder to control. You can leave the radiator’s own valve open; you are not removing the panel.
Find the bleed valve and hold a cloth underneath
It is a small valve at one top corner, not the pipe connections at the bottom. If there is a plastic cap, prise it off and keep it.
Open the valve a fraction
Fit the key or screwdriver and turn anti-clockwise — usually a quarter turn is enough. You should hear air hissing out. Do not unscrew the valve all the way or it can come out of the radiator.
Close it as soon as water appears
When the hissing stops and a steady dribble of water follows, close the valve clockwise, snug but not with all your strength. Wipe the corner dry.
Repeat on other cold radiators, then check the boiler
Start with the radiators that were coldest at the top. On a sealed system, look at the pressure gauge when everything is still cold. If it has dropped below the range in the manual, repressurise only the way the manufacturer describes, usually with a filling loop. If you cannot find those instructions, or the pressure falls again after a day, stop and call an engineer — a persistent drop often means a leak.
Turn the heating back on and feel the radiator later
The top should warm through once the air is gone. If it stays cold, or only one end heats, the system may need balancing or there may be a blockage — that is a heating engineer’s job, not more bleeding.
General guidance only. Check our disclaimer.
Common mistakes
- Opening the wrong valve. The pipe valves at the bottom can drain the radiator or the system if you undo them. Leave them alone for this job.
- Winding the bleed valve right out. It can come free and let a lot of water out before you can get it back in.
- Bleeding while the system is very hot. You risk a scald and a messy spray.
- Repressurising again and again without finding out why the pressure dropped. That is how a small leak becomes a larger one. Get an engineer if the gauge will not hold.
- Ignoring a gas smell or a boiler fault light. Those are stop-and-get-a-pro moments, not DIY.
Upload a photo to see the finished job
When the mock-up is live, you can upload a photo and see an impression of the finished job — not a quote or an exact product match.
FAQ
Why is the radiator still cold after I bled it?
There may still be air in other radiators, the lockshield may be almost closed, or there may be sludge in the panel. Try the other radiators first. If the whole system is cool, the boiler or pump may be at fault — call an engineer.
How often should I bleed radiators?
Only when a radiator is cold at the top, or you can hear water sloshing in the panel. You should not need to do it every week. If you do, there is a problem to investigate.
What pressure should the boiler be?
Follow the figure in your boiler manual, not a number copied from the internet. Many sealed systems sit around one to one-and-a-half bar when cold, but models differ. Overfilling is also a problem.
We don’t have radiators — we have forced air or electric heaters.
This guide will not help. Do not open an electric heater, and do not bleed anything on a steam system unless a professional has told you exactly what to do.
Would rather not DIY? Find a local heating engineer
If the radiator will not heat after a bleed, the pressure keeps dropping, or you are not comfortable near the boiler, a heating engineer is the right person. Gas work is never a DIY job.
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This page contains affiliate links. If you buy through them, we may earn a commission, at no extra cost to you.