How to drain a radiator boiler system

The boiler in a hot water heating system heats water and moves it through the home's radiators by way of a series of pipes. The boiler's efficiency is inhibited by sediment and rust that can build up in the system over time. Draining, then refilling the boiler once a year can extend the life of a water heating system substantially. Draining the boiler is not difficult, though the process can take several hours.

Turn off the boiler's power by shutting off the circuit at the main service panel. Allow the boiler to cool for at least two hours. Turn off the boiler's water supply.

Attach a garden hose to the drain located at the bottom of the boiler. Put the hose's other end into a utility sink or floor drain. Open both the boiler drain valve and the bleed valve of the radiator at the highest location in the house. To open the radiator's bleed valve, use a screwdriver, a valve key, or, on some radiators, the provided knob. Water will begin to drain out of the boiler.

Open the bleed valve of a radiator located closer to the boiler when water stops draining. Allow the water to drain until the flow again stops. Locate the valve or gauge at the top of the boiler and remove it with a wrench.

Close the boiler drain valve. Place an elbow fitting into the open gauge or valve opening. Place a funnel into the elbow fitting and pour in rust inhibitor. Rust inhibitor can be purchased at stores that carry heating supply materials. Consult the instructions on the rust inhibitor container for how much to pour into the boiler.

Remove the elbow fitting and the funnel and reattach the gauge or valve. Close all of the open radiator bleed valves. You are now ready to refill the boiler.


Draining a boiler can be smelly work: it's best to do this job during the summertime, with the windows open.


Do not attempt to drain a boiler without first shutting off the boiler's power at the main service panel.

Things You'll Need

  • Garden hose
  • Screwdriver or valve key (available at hardware stores)
  • Wrench
  • Elbow fitting
  • Funnel
  • Rust inhibitor (available at stores that carry heating supply materials)
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About the Author

Michelle Kerns writes for a variety of print and online publications and specializes in literature and science topics. She has served as a book columnist since 2008 and is a member of the National Book Critics Circle. Kerns studied English literature and neurology at UC Davis.