To bleed a radiator, turn off the heating, hold a cloth under the bleed valve, and turn it with a radiator key until air hisses out and water appears, then close it. A radiator with a cold top needs bleeding. If you’re bleeding the same radiators every few weeks, air is getting in somewhere, and that can point to a leak or a failing part in the system.
In This Guide
Why radiators need bleeding
Air gets trapped at the top of a radiator and stops hot water reaching that space. You end up with a radiator that’s warm at the bottom and stone cold at the top, even with the heating on full. Bleeding lets that trapped air escape so water can fill the whole radiator again.
Some air is normal. When you first fill or refill a central heating system, a bit of air always ends up in the pipework and works its way to the highest points. Most homes need a bleed once, maybe twice, a year, usually at the start of autumn when the heating goes back on after months of standing idle.
What’s not normal is a radiator that needs bleeding every month, or one that hisses again within days of you doing it. That’s a different problem, and it’s the one worth paying attention to.
How to bleed a radiator, step by step
You need a radiator key (a small square-socket key, sold cheaply at any hardware shop), an old cloth or towel, and a small container to catch drips. Turn your heating off first and give the radiators a few minutes to cool, since bled water can come out hot.
Step 1: Find the cold radiators
With the heating running, feel each radiator from top to bottom. Any radiator that’s cold at the top but warm lower down has trapped air and needs bleeding.
Step 2: Locate the bleed valve
It’s the small square-socket fitting at one top corner of the radiator, opposite the valve you turn to control heat output.
Step 3: Open the valve slowly
Hold your cloth under the valve, fit the key, and turn it a quarter-turn anticlockwise. You’ll hear a hiss as air escapes. Keep the key steady, don’t force it further than needed.
Step 4: Close it the moment water appears
Once the hiss stops and a thin, steady stream of water comes out, close the valve firmly (clockwise) straight away. Water under pressure will spray if you’re slow.
Step 5: Repeat on every affected radiator
Work through the house, bleeding any radiator with a cold top. Start with the radiator nearest the boiler and work outward, which helps air clear the system in the right order.
Where the filling loop comes in
Bleeding a radiator releases a small amount of water along with the air, which drops the pressure in the whole sealed system. The filling loop, a flexible braided pipe usually fitted under or beside the boiler, is how you top that pressure back up. Most combi boilers show pressure on a small gauge on the front panel.
Topping up pressure afterwards
Bled off enough water and your boiler’s pressure gauge will drop below the normal range, typically 1 to 1.5 bar when the system is cold. Below that, you may see an error code or the heating simply won’t fire properly.
To top up, connect the filling loop (some are permanently fitted, others need a hose attached), open both valves slowly, and watch the gauge climb. Close both valves the moment it reaches the middle of the normal range. Overfilling causes its own problems, so go slowly and check often.
If you’ve bled several radiators and the pressure keeps dropping again within a day or two, that’s not simply a bit more air working its way out. Something is losing water from the system faster than normal, and that’s worth investigating rather than repeatedly topping up and hoping.
When re-bleeding points to a leak
A sealed central heating system shouldn’t need topping up more than a few times a year. If you find yourself bleeding the same radiators repeatedly, or topping up pressure every week or two, air and water are both escaping somewhere they shouldn’t.
The usual suspects are a pinhole leak in a pipe run, a weeping joint behind a wall or under a floor, a radiator valve that’s slowly seeping, or corrosion inside the system generating hydrogen gas as a by-product (which collects at radiator tops just like trapped air, and re-forms no matter how often you bleed it out). We see this most weeks in winter, when systems that have sat half-neglected all summer get switched back on and every weak point in the pipework gets tested at once.
Our guide on finding a leak in a central heating system covers the DIY checks worth running before you call anyone out. If pressure loss is showing up as a fault code on your boiler display specifically, our piece on boiler fault codes that mean pressure loss lists what the common brands display.
If your radiators need bleeding constantly and you’re also topping up pressure more than once a month, don’t keep pouring water in and hoping it settles. A closed heating system losing water needs to go somewhere, and left long enough it causes damp patches, stained ceilings, or damage to floors and joists that costs far more to fix than the leak itself.
Cold top vs cold bottom
The pattern of cold on a radiator tells you which problem you’ve actually got, and it’s worth knowing the difference before you reach for the bleed key.
Cold at the top, warm below
- Classic trapped air symptom
- Bleeding usually fixes it in seconds
- Normal after refilling or seasonal start-up
Cold at the bottom, warm above
- Usually sludge or debris settled in the radiator, not air
- Bleeding won’t clear it
- Points to inhibitor breakdown or corrosion in the system
If a radiator is cold at the bottom, bleeding it does nothing, because there’s no air to release down there. That pattern usually means sludge has built up inside the radiator and needs a power flush, and it’s often linked to central heating inhibitor that has broken down or was never dosed correctly after a system was drained and refilled.
Radiators that never stay bled?
Constant air, dropping pressure and radiators that won’t stay hot are classic signs of a hidden leak in the system. We can find it without lifting a floor.
When to call in a professional
Bleeding radiators is a job any homeowner can do safely with a £2 key and ten minutes. Finding out why a system keeps needing it is a different job entirely, and it’s not one you can do with a radiator key.
We use central heating leak detection methods, including thermal imaging and acoustic listening equipment, to trace exactly where a sealed system is losing water, without opening up walls or lifting floorboards on a guess. That matters because central heating pipes commonly run under solid floors or inside stud walls, and cutting into the wrong spot to “have a look” causes more damage than the leak itself.
Honestly, most of the time when someone calls us about a heating system that keeps losing pressure, it turns out to be one small pinhole in a pipe run behind a wall, not the boiler itself. A proper survey finds that pinhole in hours rather than after weeks of guesswork and patch repairs that don’t hold.
Frequently Asked Questions
Most homes need radiators bled once or twice a year, typically when the heating first goes back on for autumn. If you’re bleeding the same radiator more than every couple of months, air is entering the system faster than normal and it’s worth investigating why.
Weekly bleeding usually means water and air are both escaping the sealed system somewhere, whether through a pinhole leak, a weeping joint, or corrosion producing hydrogen gas. Constant re-bleeding alongside falling pressure is a sign worth getting checked rather than a maintenance routine.
Yes. Bleeding releases a small amount of water along with the trapped air, so a small pressure drop is expected. Top the system back up using the filling loop to the middle of the normal range, typically around 1 to 1.5 bar when cold.
Cold at the bottom usually points to sludge or debris that has settled inside the radiator, not trapped air. Bleeding won’t fix it. A power flush and correctly dosed inhibitor usually resolve it, and it’s worth checking the rest of the system for related corrosion.
Air pockets themselves don’t usually damage a boiler directly, but the low pressure that often comes with them can trigger fault codes or shut the boiler down. Persistent air can also point to a leak that, left unchecked, causes damp and structural damage over time.
A standard radiator key, a small square-socket key sold at any hardware shop for a couple of pounds. Some modern radiators use a flat-blade screwdriver instead, so check your valve type before you start.
It’s safer to turn the heating off first and let radiators cool slightly. Bleeding a hot radiator risks scalding from the water and steam that escape, and the pressure reading is easier to judge once the system has settled.
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