How to Bleed Air From a Hotwater Heating System

Updated February 21, 2017

A hot water heating system is a quiet way to heat your house. Since there is no need for a blower to distribute hot air from the furnace, there is no background noise to interfere with conversation. Heat is distributed throughout the house by circulating water through pipes connected to the boiler. When air is introduced to the closed system, it can become trapped in baseboard radiators. Trapped air will prevent proper heating by reducing the amount of water allowed to enter the radiator. When this happens, you need to remove the air to restore heat to the room.

Place a pan or tray underneath the radiator bleed-off valve.

Open the valve with a radiator key or screwdriver. You will hear a hissing sound as the air escapes.

Close the valve as soon as water begins to escape.


The system will need to be operating to have sufficient pressure to bleed the radiators. Do not remove air from the expansion tank. Air in the expansion tank provides cushioning for the system's pipes and fittings.


The water in your system will be hot. Open the bleeder valves slowly and only far enough to allow air to escape.

Things You'll Need

  • Radiator key or screwdriver
  • Pan or tray
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About the Author

Finn McCuhil is a freelance writer based in Northern Michigan. He worked as a reporter and columnist in South Florida before becoming fascinated with computers. After studying programming at University of South Florida, he spent more than 20 years heading up IT departments at three tier-one automotive suppliers. He now builds wooden boats in the north woods.