Bleeding a Furnace – How It’s Done

Published on: October 9, 2013

An oil furnace requires a continuous flow of oil in the supply line and whenever that flow’s interrupted, bleeding a furnace may be required in order to resume operation. The most common cause of this is simply running out of oil. When the oil tank runs dry, air enters the supply line. When the tank is replenished with oil, the air in the supply line forms an air lock that prevents the oil from flowing freely to the furnace. Just like being familiar with how to light a furnace pilot light, knowing how to bleed a furnace safely may be standard procedure to avoid oil furnace service charges when the unit won’t light.   Follow these steps carefully:

  • Remember that heating oil is flammable. Keep a fire extinguisher nearby.
  • Cut a 1/4-inch diameter piece of plastic tubing to a length of 18 inches. Place a small amount of sand in the bottom of a large glass jar or can to weight it so it won’t tip over.
  • Turn off the furnace safety switch.
  • Locate the bleeder screw on the side of the fuel pump where the fuel line connects to the furnace. Attach one end of the length of plastic tubing to the bleed screw and the other end in the empty glass jar or metal can.
  • Loosen the bleed screw one-quarter turn with a crescent wrench or pliers.
  • Turn on the furnace safety switch and press the red reset button located on the fuel pump. Hold the plastic tubing securely in the jar when bleeding a furnace as heating oil mixed with air bubbles sputters out of the bleeding screw.
  • Observe the oil flowing from the bleed screw into the jar to determine when no more air bubbles are present and oil is flowing steadily from the bleed screw.
  • Re-tighten the bleed screw securely then remove the plastic tubing. Clean up any oil drips that may have occurred with a clean rag.

For more answers to questions about bleeding a furnace, contact us at AC Southeast® to find a contractor in your local area of the Southeast United States.