How to remove trapped air from a heating system

Written by daniel holloway
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How to remove trapped air from a heating system
Air can block hot water in large cast iron radiators. (white radiator image by Ekaterina Sidorenko from

Hydronic heating systems use hot water to heat a home by circulating water through radiators. The warm radiators then transfer heat to the air in the home. If air becomes trapped in the radiators, the water will be unable to circulate. This is called hydronic air lock. It is important to be able to remove trapped air from a heating system in order for the system to work properly.

Skill level:

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  1. 1

    Locate the pressure gauge on the boiler. Pressure should read between 12 and 15 PSI. If the pressure reads lower, add water to the system at the fill valve. The fill valve may be a gate valve or an automatic fill valve. You can manually add water to a system with an automatic fill valve by lifting the handle on top until the pressure in the boiler reaches at least 12 PSI. If the system is not properly pressurised, you may be unable to remove all the air from the system.

  2. 2

    Look for any automatic air bleeders in the system. Air bleeders look like a small can with a bicycle tire cap on top. They are located high on pipes to let out the air that naturally migrates to the highest point. Unscrew the cap a little. Air may come out but no water should. If water continually drips from the cap, the bleeder should be replaced.

  3. 3

    Locate the manual air bleeder. Each radiator should have a manual air bleeder. This will be located on one of the top corners of the radiator. Baseboard radiators may have covers over them at the ends, which can be removed by pulling up and out. The bleeder valves come in three types: knob, slot, and square.

    Knob: This type has a knob of handle to easily open the valve.

    Slot: This type has a slot to fit a flat head screwdriver.

    Square: This type is most common and requires a key with a square hole in it to open.

  4. 4

    Turn the thermostat down and make sure the system is not running when bleeding radiators.

  5. 5

    Open the bleeder valve on each radiator. If the radiator has a handle or knob on the bleeder, turn it counter-clockwise by hand. If it has a slot or square valve, use a screwdriver or radiator key, respectively. Open it slowly and a have a towel or cup handy to catch any water that may spurt out. If water spurts out immediately, check to see if the radiator is getting hot. If air comes out, keep the valve open until you get a steady stream of water. If, at any time, air stops coming out but no water follows it, you may to add water to the system. Be sure that you do not add cold water to a hot boiler and do not over fill the system.

Tips and warnings

  • Pressure in the system should be automatically maintained at 12 PSI. The pressure relief valve will release at 30 PSI.
  • Do not overfill a system, as this may cause leaks.
  • Do not add cold water to a hot boiler, as this will cause the boiler to crack open like a clam.

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