Flushing a heating system is a chore people tend to put off until the last minute. Instead, make it part of your end of summer routine. Performing a heating system flush while the weather is still nice outside will spare you from trying to do it in the middle of a snowstorm. Flushing out the heating system keeps your system operating at its peak and helps prevent problems in the future.
Turn off the power to the boiler. Depending on your installation there may be a switch where the boiler is installed to shut off the power, or you may have to turn off the power for the entire circuit at the main switch for your house.
Turn off the water supply for the boiler. This will prevent it filling up while you're trying to flush the system. Find the water supply valve, which is usually located on the water supply pipe leading into the boiler. Turn this knob clockwise to turn off the water supply.
Go inside the house. Locate the radiator that is furthest away from the boiler and also at the highest point in your house. For example, if the boiler was located on the side of a two-story house, go to the opposite side of the house on the second story and find the radiator located there. Use the radiator key or wrench to open the radiator bleed screw.
Allow the water inside the boiler two hours to cool off. Otherwise you could scald yourself in the next step.
Go back to the boiler. Open the relief valve, if present, on the side of the boiler. Attach a garden hose to the boiler drain valve. Run the other end to a nearby sink or drain. Open the valve and allow the water inside the boiler to drain out.
Close the relief valve and drain valve.
Go back inside and close the bleed screw on the radiator that you opened in Step 3.
Turn on the water supply for the boiler. Allow it to fill.
Restore power to the boiler. Allow the boiler to run in a normal heating cycle. Check for leaks. Tighten any bleed screws or valves if necessary.
You may hear the boiler and radiator pipes knocking when you start it up after flushing. This is normal.
If in doubt about any of the above steps, consult a professional.