How to Bleed Radiators in a Hot Water Heating System

Bleeding radiators removes trapped air that prevents hot water from circulating properly. Turn off your heating system, locate the bleed valve at the top of each radiator, and use a radiator key to release air until water flows out.

1. Cool Down Before You Start. Switch off your boiler or heating system and wait for radiators to cool down completely. This prevents burns and allows you to work safely. Cold radiators also make it easier to identify which ones need bleeding since trapped air creates cold spots.
2. Assemble Your Bleed Kit. Get a radiator key (also called a bleed key), an old towel or cloth, and a small bowl or cup. The radiator key fits the square valve at the top of most radiators. If you don't have one, they're available at hardware stores for a few dollars.
3. Work From Source Outward. Work systematically through your house, moving from the radiator nearest your boiler to those furthest away. This follows the natural flow of your heating system and ensures you bleed them in the most effective order.
4. Find The Bleed Valve. Find the small valve at the top corner of the radiator. It looks like a small square socket, usually on the side opposite the pipes. Place your towel underneath to catch any drips and position your bowl below the valve.
5. Release The Trapped Air. Insert the radiator key into the bleed valve and turn it counterclockwise very slowly. You should hear air hissing out immediately. Turn just a quarter turn at first - you don't need to open it wide. Too much turning can damage the valve.
6. Wait For Clear Water. Keep the valve open until the hissing stops and steady water flows out. This means all the air has escaped. The water might be discolored initially, which is normal. Once clear water flows consistently for a few seconds, you're done with that radiator.
7. Seal It Back Up. Turn the radiator key clockwise to close the valve. Make it snug but don't overtighten - this can damage the valve threads. Wipe away any spilled water immediately to prevent staining or damage to floors.
8. Bleed Each Radiator Methodically. Move to the next radiator and repeat the process. Work through your entire house systematically. Some radiators may not need bleeding if they're heating evenly, but it's worth checking each one.
9. Verify System Pressure. After bleeding all radiators, check your boiler's pressure gauge. Bleeding releases water from the system, which can lower pressure. Most systems need pressure between 1 and 2 bar. If it's too low, you'll need to repressurise the system using the filling loop.
10. Restart And Verify Heat. Switch your heating system back on and let it run for 30 minutes. Check that all radiators are heating evenly from top to bottom. If any still have cold spots, they may need bleeding again or could have a different issue.