What Causes Black Smoke From an Oil Furnace?

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Smoke of any color — white, gray or black — coming from your oil furnace chimney frequently indicates a problem with your oil furnace. Although the problem may be as minor as a filthy system component, black smoke may point to a more serious problem that requires replacing failing system parts to avoid a possible safety hazard.


Unless you have a lot of experience, messing with your furnace isn't a good DIY project. Contact a furnace repair technician to inspect your oil furnace, diagnose the issue and make the necessary repairs. A professional will be able to diagnose and identify the issue quickly and safely.

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Black smoke from an oil furnace can be the result of something as simple as dirty furnace components or a clogged oil filter. Unfortunately, it can also mean bigger problems like puff back and after fires.

Dirty Furnace Components

When components, such as the register grill, supply ducts and blower fan compartments, build up with dust and dirt, smoke released from the chimney appears dark gray or black. These components must be dirt- and dust-free for the oil furnace to operate efficiently.


You can try to clean these components yourself, but exercise caution when you do. Always make sure that your furnace is off before you work on it. Clean your dirty components with a vacuum cleaner and soft, dry rag. It's generally necessary to clean your furnace at least once a year. Refer to the manual for instructions relevant to your furnace.

An oil filter is another component that can become clogged with impurities and need cleaning. The oil filter works to clean the oil supplied to the furnace. Think of it like the air filter in your car. When the filter is blocked, it cannot effectively remove dirt and water from the oil. The result is dirty oil that creates black smoke when the furnace runs.


The oil filter is often easy to replace, but every furnace is different. If you install the filter improperly, you won't get any heat. If you're in doubt at all, contact a repair technician to clean and most importantly replace the oil filter.

Understanding Puff Back

Oil furnaces and their components have limited life spans. When your oil furnace malfunctions, it can cause a puff back or delayed ignition. This happens when atomized oil travels to the combustion chamber but does not ignite right away. When it eventually does, you'll hear a loud bang and see a puff of black smoke. Depending on the force of the explosion, smoke either fills the room of the furnace or exits via the chimney.


If you experience this issue, contact the fire department as a precautionary measure and vacate the premises until the smoke clears out. The smoke contains toxins that are harmful to ingest. Phone your heater repair technician who can thoroughly inspect the oil furnace for damage and perform repairs.

The After Fire

Even after the burner cycles off, accumulated oil in the firebox continues to burn, causing what is known as an after fire. Once the burner is off, the fan stops running and oxygen is no longer delivered to the flame. An oxygen-deprived flame generates thick, black smoke that exits the chimney. Smoke continues to billow as long as the fire burns — until the fire is either put out or the oil is used up.


The firebox on your furnace is designed to withstand intense heat, so after fires aren't usually cause for immediate alarm. They still indicate a problem, however, and you should contact an HVAC repair technician to examine your oil furnace and fix it.




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