How to Bleed Air From a Furnace Fuel Line

Air gets trapped in fuel lines when you run out of oil, replace a filter, or have a leak that pulls atmosphere into the system. Your furnace won't fire—the burner clicks, the motor runs, but no flame appears. The fuel pump is working, but it's trying to push air instead of oil, and the combustion chamber can't ignite nothing. Bleeding the line is straightforward work that takes 20 minutes and costs nothing but a little fuel and your attention.

1. Kill the Power First. Turn off the furnace at the thermostat and the electrical switch. Wait five minutes for the system to cool and depressurize. Find the fuel pump—it's the component bolted to the side of the burner assembly, typically a cylindrical metal or composite unit about the size of a soda can. The bleeder screw is on top, usually a small brass fitting with a slot or hex head.
2. Gear Up for Visibility. Get a clear plastic tube that fits snugly over the bleeder screw (usually 1/4-inch diameter), and place the other end into a small container or jar. The tube lets you see the fuel and air bubbles clearly without spraying fuel everywhere. Position the container so spilled oil drips into it, not onto the furnace or floor.
3. Crack the Screw Gently. Using a wrench or screwdriver (depending on the fitting style), turn the bleeder screw counterclockwise about one-quarter turn. Don't yank it open—just crack it enough that fuel can seep out. You should see nothing at first, or maybe a few drops, because air is still occupying that space.
4. Press, Wait, Repeat. Locate the red reset button on the fuel pump (usually recessed slightly into the top). Press it firmly once, hold it for one second, and release. This mechanical action forces fuel from the tank toward the pump and out through the bleeder screw. You'll see air bubbles emerge into the tube first—that's the air leaving the system. Keep the button pressed for no more than five seconds at a time.
5. Watch for Steady Flow. Continue pressing the reset button in one-second intervals, watching the tube. You'll see a foamy, bubbly stream at first. Keep going. After 10–15 presses, the bubbles will slow, the stream will become darker and more uniform, and finally you'll see steady liquid fuel with no air pockets. The fuel should flow easily without sputtering or hesitation.
6. Seal and Wipe Down. Turn the bleeder screw clockwise until it's snug. Don't over-tighten—these fittings are soft brass and strip easily. Wipe away any spilled fuel with a rag. Let the pump sit for 30 seconds, then look for any seeping around the screw. A tiny bead of fuel is normal; a stream means you need to tighten slightly more.
7. Confirm the Ignition. Switch the furnace back on at the electrical panel and adjust the thermostat to call for heat. Listen for the burner to fire. You should hear the ignition sequence—a spark, a whoosh of combustion, and the steady roar of the burner. If it fires immediately, you're done. If the burner cycles on and off without catching, repeat the bleeding process once more.
8. Handle Waste Oil Right. The fuel in your collection container is waste oil mixed with any debris in the tank. Never pour it down a drain or into the ground. Take it to a waste oil disposal facility, recycling center, or hazardous waste collection event. Many municipalities accept waste oil for free. Store it in a clearly labeled container away from children and pets until disposal day.