An oil furnace is bled by connecting a tube to the open bleeder nut, placing the end of the tube into a container to hold spent oil, and holding the furnace reset button. This will draw oil from the tank, pushing any air out of the line ahead of it.
If an oil tank runs completely dry, the furnace can begin to suck air into the oil line as it attempts to draw more fuel. These air bubbles can cause the furnace to sputter and go out, preventing normal operation even after the oil tank is refilled from the outside. Bleeding the furnace removes these bubbles and draws fresh oil into the system to restore normal operation.
When bleeding a furnace, a technician must watch the flow of oil from the bleeder nut carefully. As the air works its way out of the lines, it bubbles and splashes into the waste container. Only when the flow of oil is steady and dark are the lines clear, and the furnace ready for reactivation. The oil can come out with some force, so the container should be firmly held, or filled partially with sand to prevent the flow of oil from knocking it over and spilling the contents.