How to bleed a radiator

When to bleed a radiator

Let’s start by looking at the signs of a radiator that needs bleeding. You may need to bleed your radiator if:

• your radiator has cold spots when the heating is on;
• there is a cold spot at the top of a radiator;
• the entire radiator is cold;
• you notice grubby, damp patches or mould sneaking in on the walls, particularly in rooms that aren’t used very often;
• there are rattling or gurgling noises coming from your radiator.

If you notice any of these signs then there’s likely to be air trapped inside your radiator. This is preventing hot water from circulating and heating up.

Whilst some of these symptoms could indicate a number of problems, your first port of call should be to try bleeding the radiator.

Is your radiator cold at the top?

A cold spot at the top of a radiator is the most common sign that your radiator needs bleeding. It essentially means air has collected in the radiator, preventing hot water from circulating and heating.

This air must be released in order for your radiator to heat up effectively. Patchy warmth in your radiator is definitely a sign it needs to be checked out – before it stops heating up altogether.

How to bleed a radiator in 7 simple steps

Now you know how to spot when your radiator might need bleeding, follow our simple step-by-step guide to check and bleed your radiators. Or watch our video, where our Home Expert, Chris shows you how it’s done.

Bleeding a radiator is simple, and doesn’t require a whole bunch of specialist tools. All you need to bleed your radiator are the following items:

• A radiator key
• Bucket or mug to catch any water
• A cloth to clean up any spills
• Safety gloves to protect yourself against any scolding hot water

Warning: The water inside radiators can be very hot, so take care not to scold yourself. Wear protective gloves and clothing, and be sure to also protect your floors!

1. Turn off your heating and wait for the radiators to cool so the radiators aren’t too hot to touch.
2. Place a cloth below the radiator bleed valve.
3. Locate the radiator bleed valve and ensure that a cloth or a bucket is placed below it to catch any water that may be released. Bleed valves are typically located at the top corner of a radiator and look like a round hole with a square inside.
4. Insert the radiator key and turn it anti clockwise. You should hear a hissing sound as the air escapes.
5. Turn the radiator key clockwise to close the valve when the hissing stops and water starts to leak out.
6. Repeat this process for any of the radiators that need bleeding in your home.
7. Check the boiler pressure. Bleeding radiators can sometimes lower your boiler pressure so be sure to check this after you’ve finished bleeding your radiators! If you have low boiler pressure you will need to repressurise your boiler.

How to bleed a radiator: Your FAQs answered

Our Home Experts and plumbers get questions about radiators—and how to bleed them—all the time. So we put it to them:

“What questions are you most often asked about bleeding radiators? And how do you answer them?”

Like the pros they are, they delivered:

What do you do when bleeding radiators but no air or water is coming out?

If no water or air comes out when you bleed the radiator, then the valve could be blocked with paint. Close the inlet and outlet valve at each end of the radiator, then remove the screw from the centre of the bleed valve. Insert the radiator key into the bleed valve and slowly turn it anti-clockwise (a quarter of a turn should be enough).

How often should you bleed radiators?

You should do a full check of the radiators in your home every couple of months. We recommend checking your radiators before the cold weather arrives, ensuring your heating system is running efficiently when you need it most.

Can you bleed a radiator when the heating is on?

You must not bleed a radiator when the heating is turned on as it may be too hot to touch and hot water could spray out of it, potentially causing burns to the skin. Ensure the heating is turned off before you begin bleeding a radiator. Additionally, letting out air when the pump is running will only draw more air into the system from elsewhere.

Can you use automatic radiator valves to bleed a radiator?

Depending on what type of radiators you have in your home, it may be possible to bleed them automatically using an auto vent. These handy devices are attached to your radiator valves and let out air gradually over time, helping to keep your radiators working efficiently. With an auto vent you won’t need to bleed them manually.

How do I know if bleeding my radiators has worked?

Once you’ve closed your radiator valves, it’s safe to turn your central heating and hot water back on. If you’ve been successful, your radiators should start to heat up within a few minutes, and the heat will be distributed evenly across the whole radiator. You can also check the pressure gauge on your boiler and check that it reads between 1 and 2 bars.

How long does it take to bleed a radiator?

The average radiator should take no longer than 20-30 seconds to bleed. This can sometimes vary depending on the size of your radiator and the amount of air that is trapped inside.

Are your radiators still cold after bleeding them?

Sometimes, you just can’t fix things by yourself. And nobody likes a cold radiator that refuses to heat up.

For help diagnosing and fixing the issue, contact HomeServe and we’ll put you in touch with a fully qualified, Gas Safe registered heating engineer. 

Summary

• If your radiator is cold at the top when the heating is on it means there is air trapped in your radiator
• Bleeding the radiator removes the air so it can fill with hot water, making your heating more efficient
• Most radiators can be bled by turning the bleed valve anti-clockwise using a radiator key or a flat head screwdriver
• Before winter is the best time to check if your radiators need bleeding, but it's a good idea to check every couple of months if your heating comes on more regularly across the year