How to Bleed Radiators — Step-by-Step UK Guide

What You'll Need

Before You Start

• Turn on your heating and let all radiators warm up fully — this makes it easier to identify which ones have trapped air (cold at the top, hot at the bottom).
• Walk around the house and feel each radiator. Make a note of any that are cold at the top, completely cold, or making gurgling sounds.
• Turn the heating off and allow the radiators to cool for 15-20 minutes before bleeding. This reduces the risk of scalding from hot water and reduces pressure, making it easier to bleed.
• Use our Radiator Sizing Calculator to check whether your radiators are correctly sized, and our Heating Balancing Calculator to balance your system after bleeding.

Step-by-Step Instructions

Step 1: Identify Radiators That Need Bleeding

With the heating on, check each radiator by running your hand across the surface (carefully — they may be hot). A radiator that is hot at the bottom but cool or cold at the top has trapped air. Also listen for gurgling or ticking sounds, which indicate air in the system.

Step 2: Turn Off the Heating

Switch off the central heating and wait 15-20 minutes for the radiators to cool down. This makes the process safer and more effective — with the pump off, air settles at the highest points rather than being pushed around the system.

Step 3: Start with the Lowest Radiator

Begin with the radiator closest to the boiler on the ground floor. Place your towel and container under the bleed valve, which is usually at the top corner of the radiator (the small square-headed valve).

Step 4: Open the Bleed Valve

Insert the bleed key and turn it anti-clockwise approximately a quarter to half turn. You should hear a hissing sound as air escapes. Hold the key steady and keep the towel in place — water will follow once all the air has escaped.

Step 5: Close When Water Flows

As soon as a steady stream of water (not sputtering) comes out of the valve, close it by turning the key clockwise. Do not over-tighten — just snug. Wipe up any water that has dripped.

Step 6: Work Through All Radiators

Repeat the process for each radiator that needs bleeding, working from the ground floor up to the top floor. This ensures air is pushed upwards through the system in an orderly way.

Step 7: Check the Boiler Pressure

After bleeding all radiators, check the boiler pressure gauge. It will likely have dropped. Most combi boilers need to be between 1.0 and 1.5 bar when cold. If the pressure is low, use the filling loop to top up — open it slowly until the gauge reaches the correct level, then close it.

Step 8: Test the System

Turn the heating back on and wait for all radiators to warm up. Check each one again — they should now be hot across the entire surface. If any are still cold at the top, repeat the bleeding process. If a radiator is still cold after bleeding twice, there may be a circulation problem or sludge build-up.

Common Mistakes to Avoid

• Bleeding with the heating on: The pump circulates air around the system, making it harder to bleed effectively. Always turn the heating off first.
• Opening the valve too far: A quarter turn is enough. Opening it fully can cause water to spray out and makes it harder to close.
• Forgetting to repressurise: If you bleed several radiators, the boiler pressure can drop significantly. A low-pressure boiler will lock out or fail to heat properly.
• Not bleeding in the right order: Start from the bottom of the house and work up. Bleeding top-floor radiators first can push air into lower ones.
• Ignoring recurring air: If you need to bleed radiators frequently (monthly or more), there is likely a fault introducing air — a leaking valve, corroded pipework or a failing pump seal. Call a Gas Safe engineer.

When to Call a Professional

• If radiators are cold at the bottom but hot at the top — this indicates sludge, not air, and requires a power flush.
• If the boiler pressure keeps dropping after repressurising — there may be a leak in the system.
• If you hear loud banging (kettling) from the boiler — this suggests limescale build-up in the heat exchanger.
• Any work on gas appliances must be carried out by a Gas Safe registered engineer.

Use our Radiator Sizing Calculator and Heating Balancing Calculator to optimise your heating system.