How to bleed your radiators

By: Ryan Jay

26 Jun 2019

As winter looms, the chances are you’ve started to use your central heating. But if your home isn’t feeling as toasty as it should, it’s probably time to bleed those radiators.

When radiators get air trapped inside of them, your boiler has to work much harder than it should to warm up your home. The more radiators in your home that are affected, the bigger an issue this becomes. Bleeding your radiators removes the air and makes your system much more energy efficient and cost effective, so it’s well worth the hassle of doing it. 

But here's the best part - bleeding your radiators is actually rather quick and easy.

Why do you need to bleed radiators?

A natural side effect of the heating and cooling of water is the production of air bubbles. Within your central heating system, there’s nowhere for this trapped air to escape. The bubbles rise to form larger pockets of trapped air which displace the water in your radiators.

Because air doesn’t conduct heat as well as water, you might notice these air pockets as cold patches on your radiators. At this point, the central heating system will have become wasteful and your home won’t reach the desired temperature without turning the thermostat up.

If you don’t bleed your radiators, the problem will worsen. Eventually, your central heating won’t get to the correct temperature, even with it turned right up.

When should you bleed your radiators?

With the production of air being a natural process of the central heating system, bleeding your radiators is one household task you’ll have to do on a regular basis. As a guide, bleeding them twice a year should keep them kicking out plenty of heat. If you’re not sure, feel for cold patches on your radiators, particularly near the top of each one. If you find any, it’s time to bleed your radiators.

Maintaining your central heating system by bleeding radiators will keep your home snuggly warm and your energy bills as low as possible. You can bleed your radiators at any time of year, but right at the beginning of winter is the best time as, with the heating on, you’ll be able to identify problem radiators more easily, and then enjoy them working efficiently for the rest of the season.

Should you bleed radiators if you have a combi boiler?

You can bleed radiators yourself if you have a combi boiler, but be aware that sealed central heating systems like those with a combi boiler need to be re-pressurised after bleeding radiators. This is done with a lever or valve located near the water supply to the boiler. The process will be detailed in the boiler instruction manual. If there aren’t any instructions, or you’re not familiar with topping up the pressure in your boiler, you should call in a professional to bleed your radiators for you.

Being more energy efficient

Bleeding your radiators can help you save energy, but don’t forget that it’s just as important to get into energy saving habits and choose more efficient products for your home. You can read more about this in the energy saving advice section of our website.

What do you need for bleeding radiators?

A radiator key or flathead screwdriver

Depending on the type of bleed valves on your radiators, you’ll either need a radiator key or a flathead screwdriver. Most radiators are bled using a radiator key but if you’re not sure, find a bleed valve. They’re located on the left or right side of the radiator at the top. A bleed valve on an older radiator looks like a hole with a square bit in the middle. On some newer radiators, the bleed valve has a straight groove for a flathead screwdriver.

Old cloths

When you open the bleed valve, the pressure inside the central heating system will force out the trapped air. When all the air has escaped, water will start to come out. You’ll need to press an old cloth against the bleed valve as you open it to minimise mess. It’s also a good idea to place more rags on the floor underneath to catch any stray drips.

How do you bleed radiators?

So you’ve got your radiator key and rags at the ready – what now? Let’s find out how to bleed a radiator:

Step 1: Turn the heating on

Illustration of hand turning radiator on

The heating must not be on when you actually bleed radiators, but it needs to be on for a while before you start. When it’s been on long enough that the radiators should have warmed up, check each one for cold patches. Make a note of any with cold spots as these are the ones you’ll need to bleed.

Step 2: Switch the heating off

Illustration of hand turning radiator off

The system needs to have warm water in it for bleeding radiators, so switch the heating fully off and wait for the radiators to cool a little. About 20 minutes should be perfect – the time it takes to sit down with a cuppa and a biscuit.

Step 3: Prepare the area with old cloths

Illustration of cloths to underneath the bleed valve

Place some old cloths or rags on the floor underneath the bleed valve to catch any spills.

Step 4: Open the bleed valve

Illustration of hand opening the bleed valve

Put the radiator key in the bleed valve and hold a rag around the valve with your other hand. Turn the key anticlockwise about half a turn until air hisses from the valve.

Step 5: Wait for water to come out

Illustration of water coming out of the bleed valve

When water starts to trickle out of the valve, all the air is out. Turn the radiator key about half a turn clockwise to close the bleed valve. Be careful not to overtighten it as this can damage the valve.

Step 6: Wipe up any drips

Illustration of hand wiping radiator dry with cloth

If any water remains around the valve area or has dripped down the radiator, wipe it with a dry cloth to prevent rust.

Repeat steps 3 to 6 for all radiators with cold patches.

Important information regarding our advice and tips
We try to make sure that the information we include in our blog is correct. Unfortunately information can become outdated, and we can’t guarantee that we won’t ever make a mistake. With that in mind, we accept no responsibility (including loss, damage or injury) for your use of the advice on our blog, or the wider website. Please always consult a professional if you intend to carry out DIY and you’re not fully confident in doing it yourself.

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