Bleeding a hot water boiler system

Updated July 20, 2017

A boiler system is a heating unit that utilises the heating ability of boilers. The boiler is a sealed container that heats the water running through it until it turns into steam and then forces the steam throughout your home's heating system. As the steams cools, it turns back into water and settles into the boiler system and the individual heating radiators in each room. Air can get trapped in these components, and bleeding can correct inadequate performance of the unit.

Turn off the boiler power and wait for the system to cool down.

Connect a length of garden hose to the threaded spigot on the return line of the boiler system and make sure the water supply is connected and running. Put a bucket at the end of the hose to catch the water.

Turn off all of the zone valves from the boiler system set-up and open all of the valves located on each radiator. Open one of the closed zones in order to bleed it first.

Slowly move the feeding valve where the water enters the system and at the same time release the handle of the spigot where the hose is attached. Turn the valve slightly anytime you see the water pressure rise any higher than 25 on the gauge.

Turn off the spigot as soon as no more bubbles are coming through the water from the hose. Close the zone valve you just bled and open the next and bleed all of the zones in the same way.

Disconnect the hose and move to each radiator. Locate the small outlet along the top of the radiator on the front or sides.

Insert the radiator key into the outlet and turn counterclockwise to release any remaining air in the unit. Hold a towel under the key to catch any drips of water that escape.

Turn the key clockwise to tighten the outlet once you see a few drips of water. Turn the boiler system back on.


You can have a plumber install a self-bleeding system into your boiler to check constantly if there is any built-up air.


Do not touch the water coming out of the hose as it might be dangerously hot.

Things You'll Need

  • Garden hose
  • Bucket
  • Radiator key
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About the Author

Based in New Jersey, Erica Porter has been writing fashion related articles since 2001. Her work has appeared on the Breakthrough and eHow websites. Porter holds a Bachelor of Arts in English from the University of Rochester.