Smoke vs. Soot – What is the Difference?

Black Soot Deposition (BSD) and Furnace Puff Backs

The idiom stating “where there’s smoke there’s fire” is really describing that when a group of people who suspect someone has done something wrong, there is likely good reason to believe them. The same goes in fire damage restoration; if you see smoke, there is good reason to believe that a fire has occurred somewhere, but not always.

What is smoke?

Smoke is the visible suspension of particles (i.e., mostly carbon) and gaseous products produced by incomplete combustion of a fuel during an active fire. Soot is also a particle (mostly carbon) produced by incomplete combustion of a fuel. The distinction is that even though an open flame is involved, soot is not created and distributed by heat.

Smoke driven by heat during an active fire can deposit on ceilings and walls or on any surface it encounters in the path of the smoke plume as it moves throughout the home. As the heated smoke cools the particles carried by smoke simply fall out when they lose velocity making horizontal surfaces the source of the greatest accumulations of smoke particles.

What is soot

Soot can be produced inside a home by a variety of events (more on that in a moment) but due to the lack of heat, soot travels on indoor air currents sometimes aided by forced air heating and ventilation systems. These particles are often attracted to electrostatically charged molded plastics but can deposit on walls and ceilings similar to those of fire and heat-driven smoke.

Some soot particles are generally described as being very small, however, they may sometimes stick together to form larger more visible particles. The particles are most often found where restrictions to normal airflow occur in a home. For example, framed wall art creates restricted airflow up and down the wall in the area the frame meets the wall. Removing the art will often reveal dark streaks (accumulated soot) outlining where the frame was hung.

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In fact, anywhere airflow is restricted will reveal dark streaks on the surfaces where the restriction occurs; the carpet under skirted sofas and draperies, doors to carpeted rooms that are closed, etc. Temperature variations on walls and ceilings, particularly in cold climates, can also create an attraction of the warmer air carrying the soot particles to attach to areas of the wall that connect the interior to the exterior (wall framing, roof trusses for example) by a process called thermal bridging.

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What causes soot?

So back to an earlier point, without fire, what creates the soot? The technical name is Black Soot Deposition (BSD). BSD is typically the result of excessive or improper (not trimming the wick, placing the candle in areas of strong air currents) candle burning, particularly glass jar candles.

It can also be produced by malfunctioning gas appliances (e.g., gas fireplace logs and forced air gas furnaces). The malfunction is often due to lack of maintenance (e.g., cleaning and adjusting the burners) which may cause the flames to burn rich and produce more unburnt fuel (soot).

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What is a Furnace Puff Backs?

Deposits of soot are rarely found on horizontal surfaces, with one exception: furnace puff backs. Puff backs are caused when burners fail to ignite properly creating a buildup of fuel (typically fuel oil though gas furnaces can also puff back) in the combustion chamber. When the burners finally do ignite the excess fuel causes an explosion of accumulated fuel within the combustion chamber. This explosion has enough force to dislodge accumulated soot residues that are present on the interior of the furnace supply duct and exhaust system. The residues are then broadcast outside of the furnace itself by the force of the explosion.

In forced air heating systems, the ducts themselves become the conduit that carries the dislodged soot and discharges it into any room that has a duct supply or return register(s).

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In homes that have hot water baseboard heat, the bulk of the soot from the puff back comes from the boiler exhaust chimney. This results in the boiler room becoming pressurized with soot. The pressure created by the exhausting can push the dislodged soot through openings (penetrations) to the floors above often through the water pipes feeding the baseboard heaters.

Furnace Soot

Unlike BSD, furnace soot is larger in size due to the petroleum component of the fuel and much easier to identify. The larger, heavier, particles tend to fall out of the air stream when the velocity caused by the explosion slows causing large accumulations of soot on all horizontal surfaces. The smaller particles will behave similarly to those of BSD traveling on indoor air current, often aided by the fan in the forced air heating system. Another identification would be the often, unmistakable smell of the unburnt fuel oil present.

Do Puff Backs Happen All Over the Home?

In puff backs of any kind, it is not unusual for every room in the home to be affected by airborne soot. Homeowners should know that their homeowner’s insurance policy is subject to one-time (sudden) and accidental events. BSD may not fall under this description as the damage caused by BSD is difficult to recognize until it has built up in sufficient quantities to be visually observed. An argument can be made that the soot-creating event had to initially occur causing some level of soot damage. The buildup of soot caused by it happening on subsequent occasions is what made it noticeable. This is a very gray area when it comes to policy interpretation among property insurers.

If you notice dark streaks on your walls or carpet, you may have BSD occurring in your home. Call us and one of our experienced damage assessors can help to determine the cause. Rest assured that puff backs are 100% covered by your homeowners’ insurance policy. Homeowners should also keep in mind when reporting an event that insurance pays to fix the resulting effect of the event, not the cause of the damage.

Not all loss damage from smoke and soot are created equal! The soot damage caused by BSD can be light and can often be satisfactorily cleaned from walls, ceilings, and carpet. If you have had a furnace puff back, you will know it. Soot damage from puff backs can be extensive, usually requiring re-painting and replacing carpets, draperies, and upholstery.

Puff Back Cleanup in Virginia

Like any other form of smoke damage, effective results rely on the removal (cleaning) of the residues and particles. If this should happen to you call Restortech at (703) 204-0401 or request service here, we can help you on the road to recovery.