Bleeding your radiators is a good habit to get into. This very simple operation is essential to evacuate air bubbles and sludge that can prevent your heating system working properly.

When to bleed your radiators?

• before restarting your boiler.
• when the whole surface of a radiator isn't the same temperature.

How to bleed the air from your radiator?

1. Turn off the heating. Before you begin, turn off your heating system’s circulation pump. The easiest way is to set your boiler to the "summer" position. Wait until the radiators are cold.

2. Always start with the radiators at the lowest level. If you need to bleed several radiators, gradually move upstairs to the last radiator in the system.
   
   

   Warning: if you only bleed the radiators on the top floor (where the air is most likely to accumulate), they may re-fill with air as soon as you turn the circulation pump back on, drawing the air up from the lower floors

3. Fully open the thermostatic valve.
    
4. Start the bleed
   • Locate the air vent, also referred to as the "bleed screw". This may be a brass knob or hex-head screw, depending on the model. Whichever it is, it is always at the top of the radiator, on the opposite side to the thermostatic valve.
      
   • Have a radiator key to hand. If the bleed screw has a hex head, you will need to use a special butterfly key. Otherwise, simple needle-nose pliers or a socket set will be enough.
      
   • Place a container under the bleed screw. Unscrew it gently, until you hear air whistling. After a while, the valve will spit water. Leave the valve open for a few seconds until the trickle of water is regular, then close it. Note that the escaping water may be brownish. This is normal: it contains some of the sludge from the radiator.
5. Tighten the screw without forcing it when the heating water is clear again, as if you were closing an ordinary tap. A small drop may form just after closing. However, if the flow is continuous, it is better to replace the bleed screw.
    
6. Set the thermostatic valve, which you had fully opened, to the desired temperature.

After bleeding your radiators: restore the pressure

When bleeding your radiator(s), you may have lowered the pressure within the circuit. You can build it back up again by adding water via the filling valve.This is generally underneath or very near the boiler.

• Open it carefully and watch the pressure gauge (a meter with a needle) on the boiler dashboard.
• Stop filling and close the tap as soon as the needle reaches the recommended pressure. This differs depending on the model, but can be identified by looking at the green area of the pressure dial.

If in doubt, contact your heating engineer.

Is there really sludge in my radiators?

The fact that air bubbles can get trapped in your heating system is not surprising. However, you may be surprised to learn that sludge can also clog the pipes.

In fact, it is not really what is normally understood as sludge. Because it circulates around a closed circuit, the heating water oxidises the metal of the pipes. As it passes through, it carries away the metal oxides, which become detached from the pipes. This gives the water the brownish colour, thick texture and unpleasant odour that give it the appearance of sludge.