Why is My Oil Furnace Blowing Black Smoke? Possible Reasons and What to Do - My HVAC Workshop (2023)

An oil furnace blowing black smoke is one of the most serious HVAC issues you can run into as a homeowner. The black smoke, also known as soot, mainly contains particles of unburned sulfur and carbon. Your oil furnace can produce an excessive amount of soot due to a wide variety of reasons, but whatever the cause might be, the soot can compromise your indoor air quality and damage the furnishing and paint finishes in your house. A furnace that is blowing black smoke into your house might also increase the risk of carbon monoxide poisoning and fire accidents.

An oil furnace blowing black smoke is usually a result of incomplete combustion. This is usually caused by issues such as poor furnace maintenance, poor ventilation, low-quality fuel, after fire, and puff back damage, among others. Therefore, if you have noticed black smoke coming from your furnace, you could be staring at a severe HVAC failure, and you need to ensure the issue is addressed within the shortest time possible.

Read on to learn more about why an oil furnace may blow black smoke and the steps you can take to fix or avoid the problem.

Why Is My Oil Furnace Blowing Black Smoke?

Thick, black smoke coming from your oil furnace is often a sign of a serious HVAC emergency. Whether the black smoke is coming from the combustion box, the exhaust vent, or the chimney, there must be something seriously wrong with your furnace. But what exactly can cause an oil furnace to blow black smoke? Here are the most likely reasons your furnace is blowing black smoke.

Dirty Furnace Components

Your furnace needs to be cleaned and serviced regularly to continue working optimally. If you fail to seek regular furnace cleaning and maintenance, dirt and dust will build up on components such as the registers, the furnace filter, the blower compartment, the burner, and the supply ducts. Besides, soot might build up in the exhaust vent and block it partially or completely.

(Video) oil burner making black soot

The buildup of dirt, dust, and soot on these components will have the effect of making your furnace fail to burn oil as effectively as required. Unfortunately, incomplete combustion will cause your furnace to produce back smoke, some of which might leak into your living spaces, especially if the exhaust vent or chimney is blocked or damaged.

Puff Back

As a furnace gets older, its components wear out, causing it to malfunction. Unfortunately, this might cause a puff back, also known as delayed ignition.

A puff back occurs when atomized oil gets to the combustion chamber but fails to ignite immediately. When it eventually ignites, it produces a loud bang and a puff of black smoke. The smoke might exit through the chimney or fill the room in which the furnace is located, depending on the force of the explosion.

After Fire

After fire is a problem that usually occurs if your furnace has an oil leak or is burning oil improperly. These issues might cause a small amount of oil to remain in the combustion chamber after a heating cycle is complete. The remaining puddle of oil might continue burning even after the furnace has been turned off and the blower is no longer running.

The flame that continues burning even after your furnace has been turned off is what is commonly known as the after fire. Since the fan also goes off once your furnace has been turned off, the supply of oxygen to the combustion chamber will be limited, meaning that the after fire will produce a cloud of thick, black smoke.

(Video) black smoke coming out of chimney ,oil smell in basement

Dirty Oil Filter

The oil filter is an important component of an oil furnace. It essentially removes impurities from oil to ensure that the oil your furnace burns is as clean as possible. With time, your furnace’s oil filter might get clogged. Unfortunately, a clogged oil filter will not clean your oil as required, meaning that your furnace will have to burn oil with a lot of impurities, and consequently produce black smoke.

Why is My Oil Furnace Blowing Black Smoke? Possible Reasons and What to Do - My HVAC Workshop (1)

Low-Quality Oil

The quality of the oil your furnace is burning can also affect the color of the smoke it produces. If you use high-quality oil and your furnace doesn’t have any other problems, it should produce little smoke, and all you should see is wispy white smoke leaving the chimney.

However, even if your furnace is in perfect shape, burning low-quality oil contaminated with dirt, water, and other impurities will have the effect of causing your oil furnace to blow black smoke.

Cracked Heat Exchanger

The heat exchanger is a component of your furnace that ensures your indoor air gets heated without mixing with the fumes produced in the combustion process. With time, the heat exchanger might deteriorate and develop cracks and holes that might allow combustion fumes to leak out. Unfortunately, this means that the dark smoke from inside the heat exchanger might get blown through the ducts into the various room of your house.

Why an Oil Furnace Blowing Black Smoke is a Serious Problem

If your furnace is blowing black smoke, there are several serious challenges it will pose. Some of the reasons we believe black smoke from an oil furnace is a serious cause for concern include:

(Video) beckett oil burner blowing black soot

Your Furnace Might Be Running Inefficiently

One of the main reasons your furnace might be producing black smoke has to do with improper combustion. For instance, if the supply of oxygen to the combustion chamber is inadequate, the oil will not get burned completely, and it might exit your furnace as a dark cloud of smoke.

Why is My Oil Furnace Blowing Black Smoke? Possible Reasons and What to Do - My HVAC Workshop (2)

Unfortunately, if your furnace is not burning oil properly, it will most likely consume more oil, meaning that your heating bills will increase exponentially. In other words, improper combustion will force you to dig deeper into your pockets to keep your house warm.

Black Smoke from Your Furnace Might Cause Property Damage

If the dark smoke coming from your furnace is a result of improper combustion, it will contain a lot of soot, which is essentially a black powder produced as a result of incomplete burning of organic matter. If the black smoke leaks into your house, soot will stick to your walls, fitting, and furnishes.

Unfortunately, soot can corrode some fittings and damage the paint on your walls. Restoring surfaces that have been damaged by soot can be very difficult and costly.

Black Smoke From your Furnace Might Pose a Serious Health and Safety Hazard

As mentioned earlier, the dark smoke coming from your furnace usually contains particulate air pollutants that might remain suspended in your indoor air. Unfortunately, this means that if some of the dark smoke is finding its way into your living spaces, it might negatively affect your indoor air quality. In addition to triggering and worsening the symptoms of asthma and other respiratory health issues, the impurities in the dark smoke could include carcinogens that could expose you and your family to the risk of getting cancer.

(Video) Setting the oil burner air adjustment

Besides, dark smoke from your furnace might increase the risk of fire accidents in your house. For instance, since soot is inflammable, the soot deposits created by the dark smoke could easily ignite and set your home on fire. Unfortunately, a fire accident might damage your house extensively and leave your family seriously wounded and terrified.

What to Do If Your Furnace is Blowing Black Smoke

Clearly, an oil furnace blowing black smoke is not an issue you can afford to ignore. Of course, there are some things you can do yourself to try to resolve the issue. For instance, if your furnace is too dirty, you can clean the components you can easily access using a dry piece of cloth or a vacuum cleaner. Besides, you can change the air filter and the furnace oil filter. However, before trying these DIY solutions, you should switch off your furnace completely as a safety precaution. Lastly, if you have been using low-quality oil, you can try a higher-quality oil.

If the above-mentioned DIY solutions do not fix the issue, you will need to seek the assistance of a professional HVAC contractor. These professionals will thoroughly inspect your furnace and ensure that any damaged parts are fixed or changed. They will also clean your furnace thoroughly and remove any blockages that might be affecting air circulation and oxygen supply to the combustion chamber. If it is not economically feasible to repair the underlying cause of the problem, you might be advised to invest in a new replacement furnace.

The Bottom Line

If your oil furnace is blowing dark smoke, I believe you now know what to check and the things you do on your own to fix the issue. However, unless you are a professional HVAC contractor, you should err on the side of caution and call a professional HVAC contractor as soon as you realize your furnace is producing a cloud of black or dark-grey smoke continuously or even momentarily.


How do I fix black smoke from my oil furnace? ›

If the smoke is black, the furnace is not burning the oil completely and fuel is being wasted. Call a professional service person for adjustments. Clean the blower at the beginning of the heating season and again about midway through the season. Clean soot from the stack control about midway through the heating season.

Why is my oil boiler blowing black smoke? ›

The boiler is producing smoke and/or soot

A smokey or sooty boiler may be due to a clogged flue pipe or cracked heat exchanger. We recommend that you inspect the flue pipe, especially where it connects to the chimney, and remove any dirt and debris that may have clogged the pipe.

What causes an oil furnace to smoke? ›

The oil filter can get clogged with impurities that come from the oil. When the filter doesn't filter the oil effectively, the dirty oil runs in the furnace. This can lead to smoke and a burning smell. You need to call a technician to check the furnace and replace the filter if needed.

What to do if your furnace starts smoking? ›

If You Smell Smoke
  1. Stay calm.
  2. Clean off your furnace.
  3. Clean your air vents.
  4. Change out furnace filters regularly.
  5. Clean out your ducts.
  6. Contact an HVAC professional.
Feb 18, 2019

What causes black soot to come out of furnace? ›

Partial combustion

As the furnace burns the fuel to produce heat, some remnants are left behind as the process continues. This phenomenon is referred to as partial or incomplete combustion. One of the byproducts of combustion is carbon, the main element in the black soot that you observe around your furnace.

Can bad oil filter cause black smoke? ›

If you see brown or black smoke exiting the pipe, your vehicle could be burning fuel or oil. The smell of burning oil is strong, so you might know right away that the exhaust is being caused by a clogged oil filter.

Does black smoke mean burning oil? ›

Too much burning oil will cause heavy black smoke clouds to emerge from your exhaust pipe.

Can valves cause black smoke? ›

Black Smoke

This means that the fuel injectors are either adding too much fuel or that the intake valves aren't letting enough air in. This could be caused by a leaking fuel injector, a faulty fuel pressure regulator, or a bad air filter.

What causes thick black smoke? ›

Thick, black smoke indicates heavy fuels that are not being fully consumed. At times, black smoke can be an indicator that a manmade material is burning such as tires, vehicles or a structure. As a general rule, the darker the smoke, the more volatile the fire is.

How do I know if my oil furnace is bad? ›

Oil Furnace Problems, Causes & Solutions:
  1. The Furnace Suddenly Goes Off.
  2. Excessive Smoke Is Coming From the Furnace.
  3. The Furnace Is Burning up Far More Fuel Than Usual.
  4. The Furnace Won't Come on or Start up.
  5. There's Not Much Heat Coming From the Registers.
  6. The Furnace Isn't Heating Correctly.
  7. The Blower Stays on Consistently.
Sep 15, 2017

Can an oil furnace cause carbon monoxide poisoning? ›

Every winter when the temperature drops, your furnace can become a silent killer. Gas- and oil-burning furnaces produce carbon monoxide (CO). CO is an invisible, odorless, poison gas that kills hundreds every year and makes thousands more sick. Follow these steps to keep your family safe this winter.

What does it mean when a furnace is smoking? ›

Your furnace's heat exchanger is CRACKED

When that happens, soot gets into the vents. Soot in your heat exchanger DOES put you at risk. Cracked heat exchangers can emit poisonous fumes or start a fire. CALL ServiceOne immediately if you see soot coming through your vents or smell the distinctive odor of smoke.

Can a dirty furnace filter cause smoke? ›

When this happens, air flow is compromised and you're at serious risk of a fire starting at the furnace level. And, bad news, the smoke from the fire will travel through your ventilation system quite easily.

Does smoke damage HVAC? ›

Regardless of where the fire is, internal and external HVAC components are at risk for damages from smoke, soot, and ash. These particles are pervasive because they're transported through the air.

Why is smoke coming out of my HVAC vents when the heat is on? ›

If the air temperature near the unit is below the dew point, this causes water vapor to form in the air and condense into water droplets, thus causing the fog or smoke. A dirty air filter or low fan speed may aggravate the condition.

What is the black soot coming from HVAC? ›

Black particles from the HVAC system are typically mold, rust or insulation. You should inspect air handling units, variable air volume (VAV) boxes, reheats, induction units and heat pumps, or secondary units serving the area of concern.

What is the black soot coming from HVAC vents? ›

The black dust coming out of your vents could be soot, which is the byproduct of burning a candle or using the fireplace. Because your AC is constantly sucking in air from your home to be cooled, it can pull in soot and blow it back into your home. One way to check if soot is the culprit is to check your air filter.

What is the solution to black soot? ›

Check out these ways to keep your home black soot free:
  • Use alternatives to lighting candles, such as candle warmers or LED candles.
  • Air the house out when lighting scented candles by opening windows.
  • Use high quality pleated air filters.

What are the symptoms of a clogged air filter? ›

Continue reading to find out some warning signs that your air filter may be dirty and need to be replaced.
  • Poor Acceleration. ...
  • Poor Performing Engine. ...
  • Different and Strange Noises. ...
  • Check Engine Light. ...
  • Tailpipe Smoke. ...
  • Poor Gas Mileage.

Can a clogged air filter cause oil burning? ›

Yes, it could possibly. A really dirty air filter will cause a greater restriction in the intake tract. This will cause greater vacuum levels. If your engine has weak/worn oil seals (such as valve seals), having higher vacuum can pull oil past these and cause the engine to burn oil.

Can a dirty fuel filter cause black smoke? ›

Dirty air-filters that do not allow sufficient air (oxidant) into the combustion chamber for complete combustion of the fuel charge contribute to black smoke.

What happens when smoke turns black? ›

Black fire is a phrase used to describe smoke that is of high volume, turbulent velocity, ultra-dense and black. Black fire is a sure sign of impending auto-ignition and flashover.

What does black oil indicate? ›

Thick, very dark, or black engine oil usually indicates that your oil has been exposed to dirt or dust contaminants that lead to a soot build-up. Direct injection gasoline engines produce soot over time that causes standard motor oil to turn black and thick.

How do I know if I have burnt valves? ›

Common burnt valve symptoms include the appearance of blue-white smoke, excessive oil consumption, and poor engine performance. If the valve seals on your new or newly purchased vehicle are defective, you may be entitled to compensation under your state's lemon laws.

Will a bad valve cause smoke? ›

If a valve seal has lost its grip or piston rings get worn, they can cause oil to leak into the combustion chamber. This wayward fuel then mixes with other engine elements and burns, causing white or blue smoke to escape from the exhaust manifold.

Can dirty valves cause smoke? ›

It's common for some exhaust smoke to be present when you first start your vehicle, but if it begins to last longer than normal, your valve seal may be deteriorating. In addition, if you have a bad valve seal, the excessive smoke will tend to come in waves as an indicator of oil burning.

How do you treat black smoke? ›

There are several things you can do when you notice black smoke coming out from your exhaust:
  1. Always ask a specialist. The Motor Company's authorized service can help you with any problems with your diesel car.
  2. Clean the air system. ...
  3. Check your engine rings. ...
  4. Check the fuel supply. ...
  5. Use fuel additives.
Mar 20, 2020

How do you get rid of black smoke? ›

To remove soot and smoke from walls, furniture and floors, use a mild soap or detergent or mix together 4 to 6 tbsp. tri-sodium phosphate and 1 cup household cleaner or chlorine bleach to every gallon of warm water. Wear rubber gloves. Be sure to rinse surfaces with clear warm water and dry thoroughly.

How do you remove black smoke damage? ›

Use a Dry Soot Sponge

The absolute best and most effective tool for cleaning a soot stain off a painted wall is a soot sponge, also known as a dry cleaning sponge or chemical sponge. These are available in hardware stores and online, and they're made from a special type of rubber that grabs and absorbs soot particles.

What is the average life of an oil furnace? ›

Theoretically, the lifespan of an oil furnace is around 25 years. This estimate depends on a number of factors, in particular the amount of regular and preventive maintenance that was done on the equipment over the years. A well-maintained oil furnace has a much longer lifespan than one that was neglected.

Do I have to bleed my oil furnace? ›

Bleeding the fuel line isn't necessary unless the furnace doesn't restart once you've put more oil inside. Typically, the reason for bleeding the furnace has to do with fuel levels: If you let the oil completely run out, that's when the furnace may need extra help starting up again.

What are two warning signs of carbon monoxide poisoning? ›

The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. CO symptoms are often described as “flu-like.” If you breathe in a lot of CO it can make you pass out or kill you.

How do I know if my oil furnace is leaking carbon monoxide? ›

Warning Signs of a Furnace Carbon Monoxide Leak

Heavy condensation on the windows (found in the room with your furnace) Yellow or brown soot-like stains found around your furnace. A yellow pilot light (instead of a blue flame) Stuffy indoor air.

Do you need a carbon monoxide detector with an oil furnace? ›

Without a carbon monoxide detector, there's a risk that you could get carbon monoxide poisoning from an oil burner, if the leak wasn't identified quickly.

Can a dirty furnace cause a fire? ›

One of the biggest hazards caused by a dirty furnace filter is a fire in your furnace or ducts. In severe cases, if a furnace filter gets so clogged that air can no longer pass through, it can get sucked into the vent. This is a serious fire hazard that could damage your furnace system and your home.

How do I know if my furnace is backdrafting? ›

Combustion gases that back-draft into a house may leave a dark residue on the top of the water heater. The presence of soot is an indication of backdrafting, although its absence does not guarantee that backdrafting has not happened. A carbon monoxide analyzer can be used to test for backdrafting of that gas.

Why does my HVAC smell like smoke? ›

A component of your HVAC system may have overheated. The excess heat may burn insulation on wiring, resulting in a smoky smell. It could also burn any dust or debris on the fan. An overheated motor will also make an odor of smoke; the lubricant on the motor is responsible for the odor.

How do you know if your furnace is clogged up? ›

When you power up your furnace, check the color of the flame that it produces. Normally, the flame is blue, and a change in the color from blue to yellow indicates a clogged furnace flue. The furnace flue vents out carbon monoxide, but when it has a blockage, carbon monoxide remains inside the furnace.

What does a clogged furnace filter look like? ›

What Does a Dirty Furnace Filter Look Like? A dirty air filter is gross – often coated in gray or tan filth that may flake or fall off when disturbed. If it looks like this, it's time to replace your filter.

What would a dirty air filter cause HVAC? ›

A dirty filter restricts the air flow into your HVAC systems air handler. This restricted air flow places additional strain on the air handler fan motor and could, over time, burn out the motor and cause your system to overheat and ultimately fail.

Can a HVAC unit cause fire? ›

Your HVAC system is vital to maintaining a livable home environment. However, it also poses a fire hazard, particularly if it isn't maintained properly.

How to do a smoke test on HVAC system? ›

With the HVAC system on, carry an incense stick or smoke pencil steadily along the ducts and watch for movement in the ducts – this shows you air is escaping the duct system.

Can HVAC units catch fire? ›

(WRGB) — There is nothing more refreshing than air conditioning on a hot summer day but that cool air can turn hot real fast if you're not properly maintaining your unit. Experts warn that rising temperatures can strain air conditioning units, turning them into fire hazards if they are not properly cared for.

Why is my oil furnace smoking? ›

The oil filter can get clogged with impurities that come from the oil. When the filter doesn't filter the oil effectively, the dirty oil runs in the furnace. This can lead to smoke and a burning smell. You need to call a technician to check the furnace and replace the filter if needed.

How do you clean smoke out of air ducts? ›

Vinegar is an excellent odor neutralizer, even for a strong smoke smell. Cleaning the surfaces of your home with vinegar can be helpful in removing the scent of smoke from these areas.. Your best bet is to leave bowls of vinegar out in the affected areas for several days to neutralize the smell.

How do I stop my oil burner from smoking? ›

Clean Out the Burner Nozzle – If this nozzle is even just slightly blocked, some oil will fail to combust. It will end up in the combustion chamber where it will explode, creating the noise and “puffback.” Having a pro clean out this nozzle during maintenance can prevent this from happening as well.

What causes smoke to turn black? ›

The type of fuel and how hot it's burning. In general, a hotter fire will convert more fuel into elemental carbon, which forms into tiny particles that absorb light and appear in the sky as black smoke.

What burning is black smoke indicative of? ›

Thick, black smoke indicates heavy fuels that are not being fully consumed. At times, black smoke can be an indicator that a manmade material is burning such as tires, vehicles or a structure. As a general rule, the darker the smoke, the more volatile the fire is.

What does black smoke usually mean? ›

Billowing black smoke is generally a sign that the fuel-air ratio in your engine is too rich. This means that the fuel injectors are either adding too much fuel or that the intake valves aren't letting enough air in. This could be caused by a leaking fuel injector, a faulty fuel pressure regulator, or a bad air filter.

What color is the smoke from the oil furnace? ›

A properly functioning oil burner generates little smoke. You will see wispy white smoke exiting the chimney. The color of the smoke resulting from a puff back, however, ranges from dark gray to black, depending on how much soot has been shaken loose from the burner and is entrained in the smoke.

Can you get carbon monoxide from an oil burner? ›

Gas- and oil-burning furnaces produce carbon monoxide (CO). CO is an invisible, odorless, poison gas that kills hundreds every year and makes thousands more sick. Follow these steps to keep your family safe this winter.

Why is my oil smoking? ›

Smoke with a blue hue indicates burning engine oil. Engine oil can burn when it is far overdue for a routine oil change—or if you have replaced this fluid with the wrong oil type. Oil may also burn if you have a failing part somewhere in your engine, including a broken valve, leaking seal, or faulty piston ring.

How long does it take to bleed an oil furnace? ›

If this 'lockout' occurs, it is recommended that that oil supply to the burner is bled. This process, which is very simple and takes less than five minutes to complete, can be carried out by either the user of the boiler or a heating engineer.

Can I clean my oil furnace? ›

Well-tuned oil furnaces should not require annual heat exchanger cleaning. If necessary, the interior of an oil furnace heat exchanger can be cleaned using compressed air and a commercial-grade vacuum.


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