How to Bleed a Radiator: Step-by-Step UK Guide

The Short Answer

Get a radiator bleed key (£2–£4 from Amazon or any hardware shop). Turn off your heating. Hold a cloth under the bleed valve at the top corner of the radiator. Turn the key half a turn anti-clockwise. Air hisses out. When water starts dribbling, close the valve. Done. The whole thing takes about 2 minutes per radiator.

How to Know Your Radiators Need Bleeding

The classic sign: your radiator is hot at the bottom but cold at the top. That cold patch is trapped air sitting at the top of the radiator, preventing hot water from filling the whole thing. The radiator works harder to heat the room (because less surface area is actually hot), and you end up with an uncomfortable cold spot.

Other signs:

• Radiators taking longer than usual to warm up
• Some radiators barely getting warm while others are fine
• Gurgling or bubbling noises from radiators (that's the trapped air moving around)
• Cold spots anywhere on the radiator surface — top corners are the most common

Air gets into your heating system gradually over time. It's completely normal. Most homes should bleed their radiators once or twice a year — ideally at the start of the heating season (September/October) before you need them most.

What You Need

• A radiator bleed key — this is a small brass tool that fits into the square bleed valve on your radiator. They cost £2–£4 and you'll use it for years. Get a brass one, not the cheap zinc ones that round off after a few uses.
• A cloth or small towel — to catch the water that comes out after the air
• A small bowl or container — optional, but useful if you're doing several radiators

Step by Step

1. Turn Off Your Heating

Switch off your boiler or heat pump and wait 10–15 minutes for the radiators to cool down slightly. You can bleed radiators while the heating is on, but you'll get sprayed with hot water and the system pressure may drop too far. It's easier and safer with the heating off.

2. Find the Bleed Valve

The bleed valve is a small square-headed valve at the top corner of the radiator, usually the side opposite the main control valve. It looks like a small square peg recessed into a round fitting. Some modern radiators have bleed valves on the top edge rather than the side.

3. Bleed the Air Out

Hold your cloth under the valve. Insert the bleed key into the square fitting. Turn it anti-clockwise about half a turn — no more than that. You'll hear hissing as air escapes. Keep the key in position and let the air come out. This usually takes 10–30 seconds.

When the hissing stops and water starts coming out in a steady dribble, close the valve by turning the key back clockwise. Don't overtighten — just snug.

4. Check the Pressure

If you have a combi boiler or a sealed heating system, check the pressure gauge after bleeding. It should read between 1.0 and 1.5 bar when cold. If it's dropped below 1.0, you'll need to repressurise using the filling loop (usually a braided hose under the boiler with a valve). Your boiler manual will have instructions. If you have a heat pump, the process is the same — check the system pressure gauge and top up if needed.

5. Turn the Heating Back On

Switch the heating back on and check each radiator after 15–20 minutes. They should be warm across the whole surface. If there are still cold spots, the radiator may need bleeding again, or there could be sludge buildup rather than trapped air.

If Bleeding Doesn't Fix It

If a radiator is cold at the bottom but warm at the top, that's not an air problem — it's sludge. Iron oxide sludge builds up in old heating systems and settles at the bottom of radiators, blocking water flow. The fix is a chemical flush or a powerflush, which a heating engineer can do for £300–£600 for the whole system.

If you're installing a heat pump, a system flush is usually done as part of the installation. Adding a magnetic filter like the Adey MagnaClean after flushing keeps the sludge from coming back and protects the heat pump's heat exchanger.

For more on keeping your heating system efficient, see our energy efficiency guide. If you're considering a heat pump, our radiator guide covers what might need upgrading.