How To Bleed A Radiator


A radiator key

If your radiators are cold at the top and warm at the bottom, you need to 'bleed’ them to get rid of the trapped air that's causing the cold spots.

It’s a job that should be done once a year to keep your central heating in tip-top condition - if they need bleeding more often than this, air is getting into the system and this should be sorted out by a professional.


Radiator keys cost around £2 from any DIY store


Five minutes per radiator



Open The Radiator Valves

Step One: Open The Radiator Valves

Fully open any thermostatic radiator valves, including the valves on any heated towel rails, and run your central heating for 10 minutes. Turn off the system.

If your radiators are cooler in one area of the house, the radiators aren't properly balanced. The nearest radiators to the boiler are taking more than their share of the hot water from the system. Call in a professional to fix it.

Release The Air

Step Two: Release The Air

Put the radiator key into the valve in one of the top corners of the first radiator and slowly turn anti-clockwise. Have a cloth handy to catch any drips. If air is trapped in the radiator, it will escape through the valve and there will be a hissing sound as the air comes out. As soon as water begins to drip out, close the valve again and wipe away any water.

Some systems have an automatic air release valve fitted. This usually has a small red top which should be slack to enable the air to escape.

Check The Rest Of Your Radiators

Step Three: Check The Rest Of Your Radiators

Work your way around your system, repeating the process for each radiator. Take care not to open the valves more than necessary. Often air pockets will only occur in one or two of the radiators. Turn the heating on and check there are no dripping valves. Tighten the valves if necessary.

  • Cold spots at the bottom of radiator are a sign of rust and sludge build-up that's sitting in the bottom of the radiator. Often this chemical reaction will produce a gas that will cause larger cold spots. The radiators will need to be flushed by a heating engineer.
  • Don't leave leaking exterior overflow pipes - the water may freeze and block the pipe, causing a flood.
  • If you're leaving the house empty and unheated for long periods in the winter, drain the central heating system.


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