Black, gray or thick smoke coming from an oil-burning furnace is a sign of a serious problem and should be considered an emergency, whether the smoke is coming from the chimney, exhaust vent or combustion box. If the problem is not identified and resolved immediately, the furnace could start a structure fire or cause an explosion.
In many cases, the black smoke coming out of an oil furnace does not feel hot and does not irritate your eyes and nose. It is still dangerous, however, and contains carbon monoxide and carcinogens that can cause serious health problems. Avoid the front or door of the furnace, which could blow open if too much pressure builds up inside. Call the fire department immediately if you have a fire, explosion, white vapor cloud or other emergency.
Commonly called puff back, improper ignition occurs when the oil is pushed into the combustion chamber but is not immediately ignited. When the oil does ignite, it does so in a small burst or miniexplosion that produces a large puff of black smoke. A leaping flame is a trademark of puff back as the fire travels to the scattered oil droplets. Sometimes the flame will follow the oil out the furnace door, which can result in a house fire. If the small explosions become too strong, the burner and furnace can be damaged and the situation can become dangerous.
After fire is a result of improper burning or an oil leak. During combustion, a small amount of oil puddles in the bottom of the combustion chamber. This oil keeps burning even after the furnace has shut off. Because it is not controlled and the fan has turned off, it quickly consumes the oxygen in the combustion chamber and produces thick clouds of black smoke. If a flame remains after the furnace has cycled off, you have an after fire.
Immediately turn off the electricity and fuel supply to the furnace. In most cases, an emergency electric shutoff switch will be somewhere near the furnace. It’s important, however, to also turn off the supply of oil. Most furnaces have two shutoff valves — one at the burner and one at the tank. If it is safe to do so, turn off both. Do not turn the furnace back on until it has been checked by a licensed repairman.
Written by Carlye Jones; Updated December 19, 2018 | Source: https://homeguides.sfgate.com/troubleshoot-black-smoke-oil-furnace-102151.html