Air can get caught in many different systems, from cooling compartments used in engines to hot water heaters in the house. If air is trapped in a central heating system used to heat water, it is probably trapped in the radiator or boiler, which can seriously affect the efficiency of your water heating system. Bleeding refers to the process of allowing the trapped air to escape, restoring your system to its original, water-only design.

Detecting Air

Air can enter into the hot water system in many different ways, from tiny air bubbles trapped in the water itself to small air leaks that may enter through the pumping system. In any type of liquid system, air will always rise to the top, since it will always be lighter than the liquid. This means that the air will collect at the top of the radiator and in the top-most pipes that the system uses. This provides you with an easier way to tell if air has entered the system. With the central heating turned on, you can feel the sides of the radiator. If the top and upper sides feel cooler than the rest of the radiator, this is a good sign that air has become trapped in the system. If all parts of the radiator feel warm, then air in the system probably isn't an issue. If the entire radiator is cool, then there is probably another type of problem at work affecting the entire system.

Bleeding Air

Before bleeding, you should turn the central heating off to avoid leaking any more air into the system while you work on it. Bleeding is a nearly unavoidable problem, so radiators are equipped with a bleeding valve located on the top of the radiator system. These bleed valves are often designed to be opened by a specific bleed key. If you have lost your bleed key (or never had it) then you can contact the manufacturer of the radiator to find a new one. Sometimes the valves can be opened by normal means, like a wrench. When opening the valve, turn it only halfway, counterclockwise. You should be able to hear the air escape in a hiss. This will be followed by escaping water, so have a towel ready to catch the leak. When water begins to come out, close the valve in the same manner, but don't let it shut too tight. You may need to adjust the pressure settings of the system afterward, but be sure to consult your owner's manual for the precise settings.

Source:

DIYdata: Air in a water Central Heating Systems

Online Tips: How to Bleed a Boiler

Allpar: Bleeding the Chrysler 2.2/2.5 liter [and similar] cooling system