If your radiators are warm at the bottom but cold at the top, or if your heating system is making gurgling noises, there is a good chance air has become trapped inside. Bleeding your radiators releases that air and lets hot water circulate freely again — it is one of the simplest maintenance tasks a homeowner can do, and it costs nothing.

This guide walks you through the process step by step.

• A radiator bleed key (available from any hardware shop for around £1–2)
• A cloth or small towel
• A bowl or jug (optional, for catching drips)

Most modern radiators use a standard square bleed valve, but some older ones require a flat-head screwdriver instead of a key.

Switch your central heating on and let it run until all the radiators are fully warm — usually 10 to 15 minutes. This builds up pressure in the system and makes it easier to identify which radiators need bleeding.

Walk around the house and feel each radiator. If any are cold at the top but warm at the bottom, make a note of them. These are the ones that need attention.

Once you have identified the problem radiators, turn the heating off and wait 20 to 30 minutes for the system to cool down. You do not want to bleed a radiator while the water is hot — you risk scalding yourself, and the pressure in the system will be higher than necessary.

The bleed valve is a small square-headed valve, usually found at the top of the radiator on one side. It looks like a small metal square set into a larger round fitting.

Place your cloth or towel underneath the valve to catch any water that drips out.

Insert your bleed key into the square fitting and turn it slowly anti-clockwise — about a quarter to half a turn is usually enough. Do not remove the valve completely.

You should hear a hissing sound as the trapped air escapes. Hold the cloth close to catch any water that follows. Once the hissing stops and a steady stream of water appears, the air has been fully released.

Close the valve by turning the key clockwise until it is snug — do not overtighten, as this can damage the valve.

Bleeding radiators releases air from the system, which can cause the boiler pressure to drop. Check the pressure gauge on your boiler — it should read between 1 and 1.5 bar when the system is cold.

If the pressure has dropped below 1 bar, you will need to top it up using the filling loop (usually a flexible braided hose connecting two valves beneath the boiler). Open both valves slowly until the pressure reaches 1.2 bar, then close them again.

If you are not sure how to do this, check your boiler's manual or give us a call.

Work through each radiator that had cold spots, starting with the ones on the ground floor and working upwards. This helps push air towards the top of the system where it can be released more easily.

If your radiators are still cold after bleeding, or if the same radiators need bleeding repeatedly, there may be a deeper issue:

• Sludge build-up — over time, rust and debris can accumulate in the system, restricting flow. A power flush can clear this.
• Pump failure — if the circulation pump is not working properly, hot water will not reach all the radiators.
• Boiler fault — a boiler that is not reaching temperature will result in lukewarm radiators throughout the house.
• System imbalance — if some radiators are always cold and others are very hot, the system may need balancing (adjusting the lockshield valves on each radiator to distribute heat evenly).

These issues are best diagnosed by a qualified heating engineer.

If you have bled your radiators and are still having problems with your heating, PROTEC Plumbing can help. We cover Ipswich and the surrounding Suffolk area and are Gas Safe registered.

Call us on 01473 561159 or contact us online to book a heating inspection.