Running out of oil for your furnace is certainly not wise, but with the high expense of oil, you might be enticed to attempt to squeeze every drop of oil from your tank before the next delivery. Should you try waiting as long as possible before your next delivery and accidentally let your oil tank to run dry, you may discover that air and sludge from the bottom of the empty tank have made it into your oil lines. Once you receive your next delivery, you will need to prime your oil-burning furnace so it will start.
1 Turn off the power to the furnace. There should be a toggle switch next to the furnace, but if you are unable to locate a switch, turn off the furnace breaker inside your home’s main circuit breaker box.
2 Check the furnace thermostat. Make sure the thermostat is on and set to a temperature higher than your home’s current ambient temperature. This will allow the thermostat to send the signal to the furnace that it needs to turn on to raise the temperature in your home.
3 Locate the bleeder valve on your fuel pump. It is similar to a bleeder valve on your car’s brake system. When you face the fuel pump, the bleeder valve commonly sits on the side of the pump at the four o’clock position. The bleeder valve can also be on the front of the fuel pump; it looks like a hex nut.
4 Place a container beneath the bleeder valve to catch the fuel. Use a 7/16- or 3/8-inch wrench to loosen the valve.
5 Turn on the power to the furnace. Be prepared for air and fuel to sputter from the valve. Press the red reset button to restart the furnace. The fuel will begin to pump through the line.
6 Leave the bleeder valve open until a steady stream of fuel is established. Close the valve and the furnace should turn on automatically. Repeat this process if the furnace does not turn on automatically. It could take five or six attempts before the furnace fires.
7 Pour the captured fuel into your oil furnace fuel tank.
Here is a video reference as well