How to Bleed Your Radiator: A Step-By-Step Guide

Finding cold spots on your (running) radiators is a fairly common household issue.

When you do, it’s usually a sign you have air trapped inside your heating system.

This means your boiler is working harder to heat your home, which increases your energy bills (and your carbon footprint).

Fortunately, the fix is easy: Bleeding your radiators is a straightforward process. You don’t need to be an expert to do it.

Signs your radiator may need bleeding

The most common sign that a radiator needs bleeding is cold spots at the top of it. Because air rises, bubbles gather here, leaving it cold whilst the bottom is warm.

You might also hear noises in your pipework, such as creaking, gurgling, or whistling sounds when the heating is on. It’s a sign of air pressure irregularities causing vibrations.

Finally, a key sign is your heating performing badly, e.g., it’s slow to heat up and/or doesn’t feel as warm as before.

How often you should bleed your radiator

Even when there are no cold spots on radiators, they should generally be bled about once a year. 

You should ideally do it at the start of autumn, before the period when you will be using your heating the most.

Should you turn radiators off in rooms you don’t use?

If you’re not using a room, it seems logical to turn its radiator off and save money. But with modern condensing boilers (the most common type of UK boiler) this can actually cost you more.

When you switch off radiators in some rooms, the remaining ones have to work harder to heat your home.

This is because your boiler’s efficiency drops. It now has less surface area to distribute heat across, so it has to send water out at a higher temperature to compensate. 

The better approach is to turn unused radiators down via their thermostatic radiator valve (TRV), not off.

Before you begin: Critical safety checks

Before you start bleeding your radiators, do the following to check for underlying problems: 

• Inspect the floor underneath the radiator for water
• Look around the valves for rust.

Both of these are signs of a leak or a boiler issue, in which case bleeding the radiator will not solve the problem. Instead, you should call a heating engineer.

However, if neither of these signs is present (your radiators are dry and rust-free), you are ready to start.

To begin with, you will need to gather the following: 

• A dry cloth
• A dust sheet 
• A large bowl
• A radiator key. 

If you don’t have a radiator key, you can buy one at most DIY/hardware stores. But if you can’t do this, read our guide on how to bleed a radiator without a key.

Step-by-step: How to bleed your radiator

Step 1: Prep your heating system

Turn off your central heating at the boiler and let the radiators cool down completely. 

Doing this prevents you from burning yourself. And it stops hot water from spraying and soaking your floors.

Step 2: Locate the bleed valve

Look at the top side of your radiator for a small metal valve. This valve has a square centre that matches a standard radiator key. 

If you have modern radiators, you may not need a key because the valve has a small slot designed for a flathead screwdriver.

Step 3: Release the trapped air

Place your dust sheet or large bowl underneath the valve and hold a dry cloth over it. Insert your radiator key or flathead screwdriver and slowly turn it anti-clockwise by a quarter to a half turn. 

You will hear a hissing or whistling sound as trapped air escapes.

Step 4: Close the valve tightly

The moment the hissing stops and water begins to trickle out, turn the key clockwise to close the valve. Do not over-tighten the screw, as this can damage the valve threads.

Step 5: Clean up and test

Wipe any excess moisture off the radiator immediately with your dry cloth to prevent rust from forming.

If water continues to drip from the valve, check that you have closed it fully.

Potential problems

If when you first open the valve (step 3) there’s no hissing and nothing comes out, the valve may be stuck or seized. In this case, call an engineer.

And if a lot of water comes out quickly, close the valve immediately. The system may be overpressurised. 

Check your boiler pressure

Bleeding your radiators releases water and air, which causes the pressure inside your central heating system to drop.

This is a closed system and does not top itself up automatically. If the pressure drops below one bar, many modern boilers will refuse to turn on.

After you have finished bleeding your radiators, check the pressure gauge on your boiler. The needle should sit between 1.0 and 1.5 bars when the system is cold.

If it doesn’t, repressurise your boiler.

What if the radiator is still cold at the bottom?

If you have bled your radiator but the cold spots are at the bottom, you have a different problem. 

This indicates a build-up of metallic sludge.This collects inside your radiator as air, water, and rust mix together and blocks hot water from circulating.

Bleeding the top of the radiator will not solve this issue. Instead, you will need a professional heating engineer to perform a chemical flush or a power flush to clear the sludge.

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