How to Bleed a Radiator

When radiators are cooler at the top than the bottom, trapped air is usually the problem. So, what does bleeding a radiator do? Well, it simply lets that trapped air out, allowing hot water to circulate fully and heat your room efficiently.

To make things easy, we have put together a handy how-to video alongside this guide. You can watch the video to see the process in action, or read on for our guide on how to release trapped air from radiators.

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Before you bleed your radiators, turn off your central heating and let your radiators cool completely.
Open the release valve with a radiator key, wait for the hissing sound to stop, and close it as soon as water begins to leak out.
Check your boiler pressure once finished and re-pressurise your system if the gauge shows it has dropped below the normal level.

Why do you need to bleed a radiator?

Over time, air can seep into your central heating system. It will tend to bubble up to the top of your radiators, especially those furthest away from the boiler and those fed by downpipes. This air can stop your rooms from getting warm and make your system noisy.

What should you do before bleeding a radiator?

Before you start, always make sure your radiator is completely cooled. If you don’t wait, steam and boiling water can escape when the valve is released. You should also switch off your central heating and hot water at the boiler.

How do you bleed a radiator?

Radiator key
Adjustable wrench
Old towels
Bowl

If you’re wondering how to bleed a radiator without a key, you might be able to use a flathead screwdriver, but a dedicated radiator key is always the safest option to avoid damaging the valve.

To figure out how to check if a radiator needs bleeding, turn your central heating on. Run your hands over the surface carefully. If the radiator is warm at the bottom but cold at the top, air has built up inside.

Switch off your central heating at the boiler and wait until the radiators are fully cooled.

The bleed valve is usually a small, square pin at the top edge. A double radiator will have a valve on each panel, so check both. If you’re bleeding a bathroom radiator or trying to figure out how to bleed a towel radiator, the valve is often at the very top.

For more info on this, check out our guide on how to fit a towel radiator.

Place a towel on the floor and hold a small bowl right underneath the valve.

Place the radiator key into the release valve. Twist it carefully one or two times anticlockwise. You’ll hear a hiss as the air escapes. Usually, the hissing lasts just a few seconds.

As soon as you hear a gurgling sound or water starts to come out, turn the key clockwise to close the valve. Take care not to over-tighten it.

If the valve is tight and accidentally comes out completely, use an adjustable wrench to carefully replace it.

If any other radiators in your home need bleeding, complete the same process on these.

What to do after bleeding your radiator

Bleeding a radiator can reduce boiler pressure, so you may need to repressurise it afterwards. After bleeding your radiator, check the pressure gauge on your boiler. If it’s dropped, you’ll need to re-pressurise it by adjusting the filling loop. It’s a good idea to check your boiler manual for instructions.

Turn your heating back on and check for cold spots. If cold spots come back quickly, it might be a sign of a small air leak or sludge build-up. At this point, you might be looking at flushing the central heating system instead which is a much more involved job. If in doubt, ask a professional plumber for advice.

By releasing the trapped air in your radiators, your central heating system will run much more efficiently. The end of summer is a great time to tackle this task, so you can make sure your system is working perfectly and your home will be warm before the cold weather arrives.

If you’re currently tackling a few central heating jobs, you might also find our guide on how to change a TRV helpful.

FAQs

It’s a good idea to check your radiators for cold spots once or twice a year. The end of summer is a great time to do this, so your system is ready for the winter chill.

No. You should always switch off your heating before bleeding a radiator as if the radiator is hot during the process, boiling water will come out of the bleed valve as you’re working on it.

Most of the time, you’ll be bleeding a radiator with a key, but if you’re searching for how to bleed a radiator without a bleed valve, it’s likely your system is self-bleeding, or you might be looking at the wrong end of the radiator. If you genuinely cannot find a valve, or if you need to know how to change a radiator bleed valve, it’s best to speak to a qualified plumber.

The process is exactly the same for a combi boiler. Just remember to check the pressure gauge once you’ve finished. Combi boilers often need their pressure topped up after air is released from the system.

Bleeding a radiator only needs to take about fifteen minutes. Most of the time is taken up by prep like turning off the heating and laying down towels. The process of actually bleeding it shouldn’t take long, with the hissing only lasting a few seconds.