How to Bleed a Radiator

Radiators often develop cold spots, usually at the top, when air gets trapped inside the system and prevents hot water from circulating effectively. This phenomenon forces your boiler to work harder than necessary, driving up your energy bills while your living space stays chilly. A simple bleed clears this trapped air and restores the radiator's full heating capacity. Done well, this process is quick, clean, and requires no specialized plumbing knowledge. You are essentially replacing pockets of air with hot water from your heating loop. The goal is to reach a steady, air-free flow of water, which signals that your radiator is once again operating at peak efficiency.

1. Cool Down First. Switch off your central heating system and allow the radiators to cool down completely. Working on a hot system risks steam burns and makes it harder to identify when the air has fully escaped.
2. Find Your Target. Find the square bleed valve usually located at the top corner of the radiator. You may need to look behind the radiator or check the side opposite the thermostatic valve.
3. Protect Your Space. Place a small container or a thick rag directly underneath the valve. This catches the small amount of water that will inevitably spray out once the air is purged.
4. Release the Air. Insert the radiator key into the valve and turn it slowly counter-clockwise. You will hear a distinct hissing sound as the trapped air escapes; stop turning once you feel resistance.
5. Confirm Full Flow. Keep the valve open until the hissing stops and a steady stream of water begins to trickle out. This confirms that all the air has been displaced by water.
6. Secure and Verify. Turn the key clockwise to tighten the valve, ensuring it is snug but not over-tightened. Wipe away any moisture and check the pressure gauge on your boiler to ensure it remains within the recommended range.