How to drain & refill a closed hot water boiler

Updated February 21, 2017

As part of your hot water boiler's routine maintenance, you must occasionally drain and refill the boiler. This flushing of the boiler helps to get rid of accumulations of sediment that reduces the efficiency and total water capacity in your unit, as well as preventing rust build-up in the system. Homeowners should flush their boilers at least once yearly, or whenever the water begins to change colour, turning brownish from build-up of materials within the boiler. It's a quick procedure that you can complete in a matter of hours.

Shut the power to the boiler off from the circuit breaker panel in your home. The shutoff switch should be clearly marked on the panel diagram. If the boiler is a gas unit, then set the gas valve to Pilot. Wait three hours in order to give the water contained in the boiler time to cool enough for safe drainage.

Locate the boiler drain faucet somewhere along the bottom of the unit. Connect the end of a garden hose to the boiler drain, screwing it firmly onto the faucet threads. Take the other end of the hose and place it in a nearby utility sink, or locate a nearby floor drain to empty the boiler water into.

Shut the cold-water inlet valve on the boiler to cut off incoming water to the boiler. Turn the valve clockwise to stop the flow of water completely.

Lift the relief valve lever on the boiler in order to vent the unit, allowing air into the boiler during the draining process to speed up drainage.

Open the drain valve on the boiler, turning it slowly so that the flow of water begins gradually. Wait for the flow of water draining from the boiler to stop. The time required depends largely on the size of the tank and the amount of water present in the system at the beginning of the draining process.

Locate the highest placed radiator in your home, if radiators are present. Open the bleed valve on the radiator using a screwdriver in order to allow hot water in the pipes of the home to drain back down through the boiler. When the flow of water stops, open the bleed valve on the nearest radiator to the boiler to complete the drainage process. Wait for the flow of water to stop completely from the boiler.

Close the boiler drain valve and then disconnect the garden hose. Close the bleed valves on both radiators. Close the relief valve on the side of the boiler as well. Make sure that the valve can close fully without interference from debris.

Open the cold-water inlet valve, and then open a nearby hot water faucet. Allow the faucet to run as the boiler fills with water. Wait until no more air bubbles escape from the faucet, indicating that water fills the boiler completely. Turn the faucet off.

Restore power to your boiler, or return the gas back to the heat setting. Use the newly flushed boiler as you normally would.


Flushing the boiler can produce an unpleasant odour, open the windows in the area to provide ventilation. Wear safety gloves during the process to prevent scalding from hot water.

Things You'll Need

  • Garden hose
  • Screwdriver
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About the Author

Larry Simmons is a freelance writer and expert in the fusion of computer technology and business. He has a B.S. in economics, an M.S. in information systems, an M.S. in communications technology, as well as significant work towards an M.B.A. in finance. He's published several hundred articles with Demand Studios.