How to flush a central heating system

Updated April 17, 2017

Flushing a central heating system can often be necessary task, particularly when the system is producing rust or sludge, a telltale sign that your system is operating inefficiently. Flushing may also be needed if a new system or radiator has been installed.

Turn off all the power that controls the heating system. Any gas or electricity connected to the boiler, as well as control switches on the boiler, should also be turned off. Locate the feed/expansion tank, also known as the header tank, in the loft. It will be the smaller of the two tanks. The tank's water supply must either be turned off, or the ball valve must be blocked. Make sure that there is no water in the ball. A leaky ball can make the heating system less efficient, and would need to be replaced. Stop water from entering the tank during draining by laying a piece of wood across the tank, lifting the valve onto it and tying it to the wood so it will remain shut off.

Connect the drain cock, usually located at the lowest part of the pipework, to a hose secured by a jubilee clip. The hose should be run outside the house, but avoid roads, sidewalks or other pavement that have the potential for accidents. Use a pair of pliers to open the drain valve.

Open up all the bleed valves on all radiators in order to speed up the flushing process. Open up the highest level valves first and work down to the lowest. Opening the lower valves too soon can cause water to push out from the valves instead of sucking in air.

Check the system to see if it is fully flushed once water has stopped running from the hose. Air in the system may have prevented some water from draining. If this is the case, loosen the ball cock in the header tank so about 6 inches of water can come in. This water should drain from the hose in a few seconds. If an air lock is stopping the water from draining, connect the hose's other end to a cold tap and shoot water into the blocked radiator.

Close all the bleed valves and the drain cock that have been opened. Release the ballcock in the feed/expansion tank to let it refill. Start bleeding the lower level radiators and work upward once the tank is full. Check all valves to make sure they are shut tightly.

Turn on or relight the boiler. You may hear a knocking noise as the result of air expanding in the system. Stabilise the system by bleeding the radiators again and checking the system for leaks.


Clear or dirty grey water is a sign that your central heating system has been properly flushed. Dirty black or rusty-coloured water indicates sludge or corrosion.


You can only follow these steps in flushing your central heating system if it is a standard venting system. If it is a sealed combination boiler system or if you are not sure which type you have, let a professional flush your system instead.

Things You'll Need

  • Hose pipe
  • Jubilee clip
  • Radiator key
  • Wooden rod (must be same length as header tank)
  • Stiff wire
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About the Author

Kevin Clarkston, a freelance writer, is a recent graduate of Northwestern State University, where he received a Bachelor of Arts. Since 2007, he has written freelance articles for bi-weekly papers "The Light" and "The Real Views," as well as newsletter articles for the National Center for Preservation Technology and Training.