Oil Burner Puffbacks cause, cure and prevention:

This article explains the cause, cure, and prevention of potentially dangerous and sooty oil fired heating equipment puffbacks that can occur at an oil fired boiler, furnace, or water heater.

We explain why puffbacks can be dangerous, as well as expensive.

We describe the warning signs that a puffback problem may be developing and we give advice on what to do to prevent puffbacks.

What Causes Sooty Puffbacks at Oil-Fired Heating Boilers, Furnaces, Water Heaters

Here we explain oil burner or oil heating system puffbacks: what happens, and what causes them. Puff-backs are dangerous explosions of un-burned fuel that can cause building damage and personal injury. Puffbacks have been studied in the U.S. since at least as early as 1918 (Fellows 1918).

Watch out: avoid this most-common cause of puff-back explosions: if the reset button on your heating equipment has popped, turning the burner off, you might press it ONCE to see if the equipment will run and provide heat (or hot water) while you wait for repair service.

But do not keep pressing the button repeatedly as doing so, particularly if the burner does not run for 10 minutes or more, risks a dangerous puffback explosion when un-burned fuel in the heater finally ignites - causing a puffback explosion.

The reset button on aquastats is discussed in more detail

at AQUASTAT RESET BUTTON.

Article Topics

• OIL BURNER PUFFBACK DEFINITION - also similar: gas burner puffback explosions?
• OIL BURNER PUFFBACK CAUSES
• OIL BURNER PUFFBACK WARNING SIGNS
• OIL BURNER PUFFBACK EXPLOSION STRENGTH
• OIL BURNER PUFFBACK PREVENTION
• OIL BURNER SOOT DISTRIBUTION

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

See also WOOD STOVE, CATALYTIC PUFF-BACKS

Contact us to suggest corrections or additions and, if you wish, to receive online listing and credit for that contribution.

Definition of Oil Burner Puffback Explosion

Definition of heating system puffback: a puffback at oil fired heating equipment is literally an explosion of un-burned oil in the combustion chamber of an oil-fired boiler, furnace, or water heater.

The sudden ignition (at the start of a burner "on" cycle) of accumulated fuel results in a powerful explosion rather than the normal controlled-burning of fuel.

Depending on the quantity of oil that is ignited, the puffback can damage the boiler itself, may cause the flue vent connector (stackpipe) to become disconnected, and may blow soot throughout the building.

The cause of this puffback explosion is the ignition of un-burned oil lying on or in the bottom of the heating appliance combustion chamber.

Watch out: Puffbacks are dangerous, regardless of the fuel type. Puffbacks can occur as well in gas fired appliances but those are uncommon and in our OPINION should be discussed separately because the risk of an explosion of un-burned LP gas fuel or natural gas fuel is even more likely to be a catastrophic explosion that can cause a building fire as well as injury or death to building occupants.

The strength of a puffback explosion is discussed in more detail at

OIL BURNER PUFFBACK EXPLOSION STRENGTH

What Causes Sooty Puffbacks at Oil-Fired Heating Boilers, Furnaces, Water Heaters

The most common causes of the presence of this un-burned heating oil fuel are various sources of improper oil burner operation that leaves incompletely-burned heating oil at the end of one or more on-off cycles of the oil burner. These include:

• Leaks in the equipment oil supply piping, often visible as oil drips or leaks that occur when the equipment is not running, become air-leaks into the oil piping and oil burner when the burner is running.
  
  Our photo (above-left) shows heating oil drips on the floor near an oil burner. An alert home owner, home inspector, or heating service technician should recognize the meaning and importance of this clue.
  
  To atomize and burn heating oil, the oil burner's fuel unit compresses incoming heating oil to 100 psi or more.
  
  Because any extra air in the heating oil piping is also compressed to high pressures (often more than 100 psi) during oil burner operation, any air bubbles present in the system when the oil burner stops its run cycle is exposed to a pressure drop inside the system.
  
  The 100 psi air bubble expands, pushing additional un-burned heating oil out of the end of the oil burner nozzle and into the combustion chamber where it may accumulate until, in a dramatic "BANG!" the unburned oil is ignited at the beginning of a future oil burner on-cycle.
• Oil burner shutdown problems:
  
  Incomplete heating oil combustion may also occur if the "shut-down" phase of oil burner operation is not working properly. Inside of the fuel unit of most oil burners is a spring-loaded valve whose job is to quickly and securely stop oil flowing to the oil burner nozzle the moment that the oil burner's fuel unit RPM's begin to slow - when the oil burner is turning "off" at the end of a burn cycle.
  
  Even a small amount of dirt on the seat of this oil valve can cause it to leak oil into the combustion chamber instead of making a clean oil-flow shutoff.
• Poor oil burner maintenance:
  
  if an oil burner is running poorly, for example with a dirty oil spray nozzle that has become partly blocked, the result is a poor flame and possibly incomplete combustion of the heating oil being sprayed into the fire chamber.
  
  This, too, can lead to accumulation of un-burned heating oil and a subsequent ignition and puffback.

• Underlying chimney or heater installation problems: Keep in mind that the root problem may be more than just a need for an oil burner tune-up.
  
  For example a too-short chimney can be the cause of inadequate draft that leads to sooty burner operation and poor heating economy.
  
  See OIL BURNER NOISE SMOKE ODORS and
  
  see CHIMNEY TOO SHORT
  
  Our photo shows soot blowing out around the barometric damper - this system is having trouble and needs service.

Watch out: Sooty, smelly, noisy oil burner operation is a warning: you may be headed for a puffback: Sooty oil burner operation, blowing soot into the boiler room or other building areas, is not normal and it means that the system needs inspection, diagnosis, service, and repair.

Watch out: A puffback that blows apart the heating flue vent connector risks a building fire or the release of combustion gases into building air.

See COMBUSTION GASES & PARTICLE HAZARDS

Complete List of Causes of Oil Burner Puffbacks

The various possible underlying cause of excess oil that is ignited at the start of a heating boiler on cycle and that fuels the puffback, are listed in the article starting with "leaks at equipment oil supply piping" and include a variety of problems.

But nearly all of the problems that are at the root cause of a puffback are maintenance or installation errors or omissions, including:

1. Failing to detect and fix an oil line leak. Air entering the line is a common cause of push-out of extra, un-burned fuel at the end of an oil burner on-cycle.
2. Failure to detect and correct a dirty oil burner nozzle or dirty clogged turbulator at the end of the nozzle assembly. Delavan also points out that the nozzle spray pattern can cause poor ignition:
   
   Above about 2.50 gph a hollow cone spray sometimes may cause delayed ignition. Changing to a solid cone sometimes helps that situation. - Delavan cited below
3. Failure to detect & repair damaged or improperly-adjusted oil burner electrodes or cracked, damaged electrode insulators, causing incomplete combustion and accumulation of un-burned oil
4. Oil burner transformer failure: a weak (inadequate spark) or an overheating transformer may contribute to or cause improper oil burner operation and a puffback (Montagna 2006)
5. An oil burner fuel unit (oil pump) whose internal check valve has gotten dirty or stuck, failing to stop the transmission of oil to the burner nozzle assembly quickly when the burner motor stops
6. Combustion chamber design, components, damage: a combustion chamber liner that absorbs and accumulates un-burned oil, or a damaged combustion chamber liner can contribute to a puff-back explosion by accumulating un-burned oil OR by failing to perform its role in providing a heated surface that assists the oil burner in achieving complete combustion of oil entering the chamber
7. An oil burner oil-delay-valve (or "quick stop solenoid valve") that has been installed to assure quick clean shutdown of oil to the burner when its motor stops is sometimes installed as a fix for the oil pump problem just above. But if that switch fails, or is removed or its wires disconnected, the problem recurs
8. A dirty squirrel cage fan in the oil burner blower assembly, leading to insufficient combustion air and a dirty sooty flame (but see our combustion air notes just below).
9. Failure to observe and correct an oil burner shutdown problem (caused by dirt in the system or any other operating problem
10. Ignoring oil burner trouble signs such as smells, sounds, and accumulation of soot and odors in the building
11. A bad chimney installation or design is a different puffback cause that is not a direct boiler maintenance issue, but even in that case, a bad chimney (too short for example) that was causing poor boiler operation ought to be diagnosed by the heating service tech and ought to result in a recommendation that it be corrected.
12. Inadequate combustion air can also lead to an oil burner puffback. An air-starved oil burner will run smoky and sooty, eventually soiling and clogging the igniters.
    
    If the unit has continual bad ignition and combustion there might be an accumulation of unburned oil that leads to a puffback. But in our OPINION this condition is more likely to lead to constant sooting and soot blow-out at the oil burner rather than a single event puffback.
13. Red dye in heating oil fuel? Some sources (Butcher 1997, Krajewski 1997, Laisy 1997, Santa 1997 & others) expressed a concern that red dye may affect oil burner combustion properties and thereby might contribute to burner malfunctions leading to a puffback
14. Oil Tank or Oil Piping Contribute to Oil Burner Puffbacks?

    At OIL TANK PIPING & PIPING DEFECTS we discuss oil line leaks (air in or oil out) as a common and important cause of oil burner poor operation that can ultimately lead to a puffback.

    A problem with the oil tank itself that might be related to a puffback might be a water leak into the tank that led to water in the fuel, picked up when the oil level was low (water waits on the bottom of the tank) or right after an oil delivery (stirs up water in the tank) and that subsequently led to poor oil burner operation.

15. Inherent Vices in Oil Burners that Contribute to Puffbacks?

    An inherent vice in an oil burner or oil fired heater that might contribute to a puffback might be a boiler or boiler control design that made the unit difficult to properly clean and service or adjust, but I'm not aware of such.

    Oil burner operation that leads to what I call "sloppy" oil burner shut down or start-up can lead to un-burnt heating oil in the combustion chamber, as we enumerated in the puffback article where I've included this discussion.

    About including the oil burner or the whole heater in the insurance claim, I pose that you'd need an onsite expert who could explain and document damage to the equipment that occurred as a result of the puffback, as opposed to simply causing the puffback.

16. Wear and Tear on Oil Burners Contribute to Puffbacks?

    "Wear and tear" contribution to an oil burner puffback sounds theoretically possible but is outside my experience. In my experience it's proper maintenance that is central to proper oil burner operation, combined with two additional factors:

    Occupants or owners need to notice when the oil fired heating equipment is not working normally (smells, soot, odors, loss of heat), and then to ask for service and repair.

    Service technicians need to notice conditions that are likely to presage or even explain a future puffback such as excessive sooting, chimney draft problems, even more subtle clues to draft problems such as a missing draft regulator, visible oil leaks, difficulty obtaining proper draft at the fire and in the breech, repeated service callbacks.

    There is a problem facing the service technician: too often the tech might notice a clue that indicates a problem that needs investigation or repair, but the tech has been given a large number of service calls to make and is thus in too much of a rush to even bring it up with the homeowner, or equally common, the technician mentions the concern and the homeowner says "Stop trying to increase your bill, you are already charging me too much, just clean and tune the boiler.".

Generally these are not innate defects in the oil-fired heating boiler, furnace, or water heater itself. Rather they are defects in installation or maintenance. So I would not use the term "defective boiler" to explain a puffback. I'd use the terms "improper boiler operation" or "inadequate heating equipment maintenance".

I suspect that your insurance company either does not understand these facts, that their policy does not distinguish between improper or inadequate maintenance of the heating system and actual defective heating equipment, or that they are speaking a bit loosely (read carelessly) and are calling improper or inadequate maintenance (your responsibility) a "defective boiler".

My OPINION is that in general and quite often, a homeowners' insurance policy limits coverage where the root cause of a loss is improper or inadequate maintenance - those are exactly what I consider to be at the root of most oil burner puffback problems.

Oil Burner Furnace or Boiler Puffback Warning Signs

Before a catastrophic puffback occurs, most heating equipment will give plenty of warning in the form of heating oil or combustion gas odors and soot in the building.

See OIL BURNER NOISE SMOKE ODORS for details.

If you see black soot and debris on top of your boiler, furnace, or water heater,

or BLACK SOOT STAINS on CEILINGS or WALLS [photo]

in the boiler room, furnace room, or living area, the cause could be a poorly-operating heating system that needs prompt attention.

But also see THERMAL TRACKING & HEAT LOSS as other things can cause black stains indoors.

Our photo (left) shows an incompetent and potentially dangerous "fix" to an oil fired heater that was blowing soot all over itself and the furnace room.

You can see the remains of soot on top of the furnace (lower right), and the fix - someone sealed the opening from which soot was leaking - the draft regulator. This goofy repair fails to recognize and fix the underlying problem - a draft or blocked chimney and perhaps other troubles plague this heater.

If your oil burner continues to make a soft rumbling sound or observe pulsating oil burner flame  (or if you actually see combustion continuing at a reduced rate inside the combustion chamber) right after the oil burner has stopped "running" then there is an oil burner shutdown problem.

see details at OIL BURNER START-UP RUMBLES [PDF] Beckett Corporation, cited below.

If you hear noises when your oil burner starts each run cycle, perhaps a more modest "PUFF" or a small "bang" sound, unburned oil is probably being ignited.

In either case the system needs to be inspected, the cause of the noises diagnosed, and repairs made.

Oil burner inspection, diagnosis & repair are detailed

at OIL BURNER INSPECTION & REPAIR.

Diagnosing black stains on indoor surfaces in the living space, possibly caused by oil fired equipment sooty operation or puffbacks, is discussed

at THERMAL TRACKING & HEAT LOSS.

Reader question: Noise like a loud BANG! when oil burner starts

(Oct 23, 2012) Rab G said:

I have a 2 year old Trianco, external combi boiler which keeps cutting out. When I reset it, it makes a loud noise on igniting.

Reply:

Rab G:

A loud noise like a BANG when the oil burner ignites is usually a sign that un-burned oil in the combustion chamber is being ignited at burner start-up. This is DANGEROUS and can damage the heating equipment, blow off a flue vent connector and as a "puffback" can blow soot throughout the building.

Usually the problem is sloppy oil burner shut-down, fixed by changing out the fuel unit (expensive) or by installing an oil delay valve or stop valve (discussed at InspectApedia.com - just try the search function).

Reader Question: oil smell and smoke when heat goes on, I replaced some stuff, now what?

(Jan 25, 2014) Anonymous said:

mike,smoke and oil smell threw out my house every time my heat goes on everything seems to be running fine so today i replace both filters ,cleaned the nozzle, clean ducts above unit and it still happened. what's next?

Reply:

Anon you are describing a system not working safely and at risk of a messy or dangerous puff-back explosion. I can't say from just your note if the problem is a blocked flue, lack of combustion air, or a cleaning and adjustment problem.

But if it were my house and oil smell and smoke were being sent by my heater throughout my house I would TURN OFF THE SYSTEM IMMEDIATELY and would call my oil heat service company for emergency service.

The money you save by trying to fix the system yourself may be much less than the cost of re-painting the soot-covered house interior or repairing other damage.

Reader question on oil burner puffback

(Feb 1, 2014) Tom said:

I get puff-backs on very cold windy day/nights, I think Root Cause is from poor design chimney that allows down draft(short, boiler in basement). I have the same issue with when starting cold fireplace on 1st floor that is in same masonry chimney structure. I put a small portable heater next to boiler aluminum flue air inlet before entering masonry chimney and it seem to resolve problem after I hear the first explosion. Suggest anything different. Very helpful site on all aspects of boiler service

Reply:

Tom, I think it might be smart to get a certified chimney sweep to inspect and advise about the flues; puffbacks can get worse and serious, even dangerous; We don't know if it's a blocked flue, damaged flue, or bad design. Or you could be lucky and just need a different chimney cap; some diagnosis of the actual chimney problem is needed.

Force Exerted by an Oil Burner Puffback or Backfire Explosion

The strength of a puffback explosion and the extent of its effect on the building can vary widely depending first on the quantity of oil that is ignited, and second on a longer list of variables such as heater type, design, condition, materials, building construction, spaces, doorways, ductwork, etc.

Question:

During a "Back Puffing" event what would the draft measurement be above the flue collar? - JR, 2021/12/06

This Q&A were posted originally at

DRAFT MEASUREMENT, CHIMNEYS & FLUES

Moderator reply:

@JR,

Thank you for a helpful question;

A puffback explosion is a sudden, brief explosion: the ignition of collected un-burned fuel in an oil burner heated device like a water heater, furnace, or boiler.

During that brief interval (seconds) there will be extreme back pressure in the combustion chamber. It seems certain that at that moment there will be no negative draft in the chimney but rather an extremely positive one in the chimney - as the forces of the puff-back explosion not only blow soot out all over the building (typically a lot of soot exits at the barometric damper and more at other boiler or furnace openings such as combustion air inlets).

Puffbacks can cause damage ranging from a burst of local soot around the oil fired heater to more-significant damage that actually blows parts of a water heater, furnace or boiler apart (often at least the flue vent connector parts) and soot damage can extend widely through the building.

At OIL BURNER PUFFBACK SOOT EFFECTS

a reader described a puffback explosion at his home:

The cause of the explosion and concussion which was felt throughout the house was what my heating contractor calls late ignition.

The explosive force lifted and threw the entire flue pipe off he furnace and ripped it out of the wall where it connected to the chimney and landed in two two pieces to the right and left of the furnace

Acrid grayish smoke filled the house and the furnace kept operating for another minute or two despite after first turning down the thermostat. I then turned off the emergency switch at the head of the stairs when the smoke started to clear.

 

Guide to Preventing Oil Burner Puffbacks in buildings

The best ways to avoid an ugly and dangerous oil burner puffback

• Have all of your oil fired heating appliances serviced regularly, normally once a year. Encourage the service technician to do more than a superficial cleaning (oil filter and nozzle change) - pay extra if necessary to have the equipment opened, cleaned, and inspected thoroughly.
  
  The best time to have oil fired equipment cleaned is in the spring at the end of the heating season. You will get a better cleaning job if you schedule this work in the spring when the oil company is less swamped with service calls and the technician may be allowed more time to do a proper job.
• Stay alert for odors, noises, soot in the boiler / furnace room or elsewhere in the building, or rumbling at the oil burner, and call your heating service company for inspection, diagnosis, and repair if needed.
• Watch for oil leaks or air leaks  anywhere on the oil piping system or at the oil burner - since oil leaks are also air leaks in many cases. Our photo (above-left) shows a dripping flare connector at oil piping entering the fuel unit on the oil burner.
  
  Signs of air leaks in (or oil leaks out) of the oil piping system include: 
  • Visible drips of oil at oil piping fittings and connections
  • Accumulated dirt and crud at oil piping fittings and connections anywhere in the system (this is why meticulous cleaning of everything is a sign of good oil burner service by your heating technician - it makes it easy to watch for leaks
  • Oil burner rumbles and burbles on at shut down - the burner does not stop sharply and quickly
  • Noisy operation of the fuel unit (heating oil pump) may be due to air in the lines (or other causes like a bad coupling)
• If the problem is a dirty oil shutoff valve (check valve) inside the oil burner's fuel unit, an easy "fix" is to install an oil-delay valve (quick stop valve) on the oil burner - this extra valve is designed to shut off cleanly the moment oil pressure begins to drop at the end of an oil burner run cycle. Details about oil delay valves or stop-quick valves are
  
  at FUEL UNIT, HEATING OIL PUMPS

Debris visible in the flue vent connector (stack pipe) visible through the barometric damper draft control opening. If you see soot, rust flakes, and debris in the flue vent connector this means that the heating system needs to be cleaned and serviced.

If the heating system has "just been serviced" this debris means that service was incomplete. If the heating system has not been serviced, then this clue means service is needed.

Lack of regular inspection and service of oil fired heating equipment risks loss of heat and related building damage, or sometimes, a messy and dangerous puffback.

A discussion of this particular photo and those rusty sooty fragments in view is found

at HOW to INSPECT a BAROMETRIC DAMPER

Soot Effects from Oil Burner Puffbacks

The extent to which oil burner soot will be blown throughout a building when a puffback explsion occurs varies from minor and local to the oil burner to extensive and throughout the building.

Variables affecting the distribution of oil burner soot after an oil burner backfire or puffback include at least

• the force of the puffback explosion that in turn depends on the amount of un-burned fuel that was ignited,and the availability of combustion air
• the confining effects of oil burner, water heater, boiler, or furnace components, sides, jackets, flue vent connector, chimney as well as the presence or absence of a door closing the utility room where the equipment operates
• the location of the oil burner puffback equipment in the building
• the building design, materials, and layout that permit soot transmission throughout the structure
• the presence of soot distributing features such as furnace ductwork

Reader Question: Where does the Soot Go during a Puffback? Other puffback or heating system "explosion" assessment questions.

I am a retired 71 year old former civil servant who just read your article “Responding to Oil Burner Emergencies.

I am enclosing my original message to [a] FDNY chief which sets forth the facts of my recent oil furnace explosion. Can you help me in answering the questions I originally raised in my e-mail to the chief [below]?

Can you point me to some references with respect to the following facts resulting from furnace explosion in my basement in Long Island which seems of similar origin to some to the secondary effects you raised in your article.

The cause of the explosion and concussion which was felt throughout the house was what my heating contractor calls late ignition.

The explosive force lifted and threw the entire flue pipe off he furnace and ripped it out of the wall where it connected to the chimney and landed in two two pieces to the right and left of the furnace

Acrid grayish smoke filled the house and the furnace kept operating for another minute or two despite after first turning down the thermostat. I then turned off the emergency switch at the head of the stairs when the smoke started to clear.

There was no fire thankfully. Contractor says that the fan in this forced air system continued to operate to reduce the heat in the furnace. I also note that the faceplate outside of the fire box was loosened and the insulation around the plate protruded from the plate and was charred at the very ends or rims of the plate.

I believe explosion raised the subfloor in my Bedroom which was not nailed and covered with wall to wall carpet. My Bedroom which was down and narrow corridor on the first floor about 40 feet from the boiler and under the basement windows, which did not shatter.

The floor is not slanted and raised at the threshold. What is interesting is the batts of insulation do not appear dulled or have soot adhered to them in the eyes of the insurance inspector. Question then is if the flue was off the furnace and chimney wall where did the soot and gases go. I suggested to them that it went into the floor past the bates which are held up with wire .

They also question the extent of the explosion since there is no evident chinks in the plaster board directly above the furnace.(The bates to the right and left of the furnace) were loose and torn and others fell to the floor below the bedroom area.The walls in the upstairs are gray with light slimy soot and a lot of dust.

The Remediation Company suggested by the insurer says that the soot on the wall is slight and random and mostly in the corners occupied by spider webs.

I wonder whether the concussion reaching down the corridor is demonstrable and whether the intensity of the explosion could have bounced off the chimney wall and headed down the length of the basement to the bedroom area above.

Further is soot stain always accompanied with a delayed ignition explosion or are they somehow consumed or dispersed by the concussion or rush of air.

I hope I can have the favor of a response before the insurance adjuster works his smoke and mirror arguments on me (pun intended). - K.S.

Comment from FDNY

It sounds like you experienced a puffback AKA delayed ignition. Your oil burner went into the ignition cycle and oil was atomized and pumped into the firebox. It did not ignite right away and when it did, there was too much atomized oil and the result was a small explosion instead of just an ignition.

As far as your questions about the direction and extend of the force of the explosion, the amount of soot I am afraid that I do not have any expertise in that aspect of oil burner emergencies. I do know that puff backs can be of varying severity. I have seen a lot of smoke and a little as a result of them.

I have read that the cleanup can be extensive. You might get better answers if you look for a website frequented by oil burner mechanics and clean up people. - F.M.

Reply: Notes on Investigating a heating system puffback or oil fired furnace "explosion"

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem such as the exact cause of the heating system event as well as its effects on the building. In my experience it is a virtual certainty that an expert on-site inspection will discover information that is not apparent to a homeowner who is not experienced in heating system or building problems.

Therefore, while we can make a brief attempt to help answer your questions, without more site details, it would be a mistake to try to conclude much about the effects on your building of the puffback you describe. That said, here are some things to consider:

Watch out: The first priority is to make sure that the heating system and chimney are safe to operate. Do not run the equipment before it has been evaluated and repaired. One should call their insurance company very promptly following a puffback if one is considering making a claim, as the insurer may want to see conditions at the building before the equipment has been removed or changed.

F.M. (above) provided a concise and accurate explanation of how a puffback explosion occurs - the ignition of un-burned heating oil in an oil fired heater that leads to an "explosion" or puffback . The severity of the event depends on quite a few variables including particulars about the heating device where the event occurred, the quantity of oil being ignited, building layout, room sizes, and more.

The direction of forces of an explosion of any sort as well as the movement of soot from what is usually a very messy event when a puffback occurs, depend on building structure, room sizes, open spaces, open or shut doors, routing of HVAC ducts and even wiring and plumbing or other components that may provide passages for soot movement.

Regarding your observation that building insulation (presumably fiberglass batts) looked "clean" - a little investigation should make it easy to assess both the material and the passage of soot from the event.

Certainly one would expect the surface of insulation exposed to a puffback to contain more soot (and look dirtier) than the interior of the insulation or the underside of the floor above. You can confirm that by visual inspection. We could test an insulation sample at various depths for soot levels, but frankly it is most likely not cost justified to do so.

It should be apparent that you would not expect soot from a puffback to be invisible near the heater but more severe in a hidden cavity such as within layers of subflooring. Air currents indeed can move in unanticipated directions and pathways in a building, as can soot and debris, but from a puffback I'd expect soot to leave a trail from point of origin to various destinations.

Regarding your note that the furnace did not stop running after the puffback event, that's quite plausible and I agree with your contractor's explanation that the furnace tried to run in response to the unit's temperature control - not a device that senses that a flue has come disconnected nor that a burner is operating poorly.

(In contrast, an oil burner that is running poorly often will result in a shut-down by the flame sensing device if a cad-cell sensor was in use at the primary safety control. Other older safety controls such as a bimetallic spring-operated stack relay won't respond in the same way.)

Your observations that

faceplate outside of the fire box was loosened and the insulation around the plate protruded from the plate and was charred at the very ends or rims of the plate

are consistent with a puffback and (prior to a puffback) could have been present as a clue of improper operation of the equipment.

Your observation that

I believe explosion raised the subfloor in my Bedroom

is not one on which someone could or should offer an assessment or explanation without having inspected the building.

Your question

if the flue was off the furnace and chimney wall where did the soot and gases go. I suggested to them that it went into the floor past the batts which are held up with wire

if I understand it to mean that there was a difference of opinion about how much soot went where following the puffback, should be answerable by at least two approaches:

• Follow the soot trail of depositions in the building. Most of the soot would have been produced and blown about by the puffback event itself, not by the following operation of the oil burner for another few moments. So the time period of operation with the flue vent connector disconnected may be less of a soot-source than the puffback itself.
• If there is difference of opinion about how much cleaning is needed because of the puffback vs. a question of what dirt or debris was pre-existing, my own field approach and recommendations often include an assessment of building conditions, deposition patterns and locations, and identification of various sources of sooting and staining (such as thermal tracking -
  
  see STAINS & THERMAL TRACKING

Regarding your comment that

Further is soot stain always accompanied with a delayed ignition explosion or are they somehow consumed or dispersed by the concussion or rush of air

a poorly-operating oil heating appliance will often send soot into building air for quite some time before a puffback event. In such cases soot stains may accumulate over quite a long time before there is a sufficiently horrible catastrophe to provoke repair or replacement of the equipment.

You expressed concern about the insurance company's response to this problem. In my experience the event is usually supported by onsite evidence - certainly your description of what you saw and heard is compelling. And I agree with F.M. that cleanup requirements may be extensive and ... expensive.

But I hesitate to suggest points of disagreement before you have heard the results of an investigation and recommendations by and from your insurance company and also from your heating system maintenance and repair company. After those experts have been on-site, if you have specific concerns that remain, we may be able to help by suggesting some questions to ask and some points for further investigation.

Some photographs of the system and building conditions could also be helpful in allowing further comment or suggestion.

Where do I look for puffback damage to the oil-fired heating appliance itself?

Unless a visual inspection of the exterior of your heating boiler (or water heater) shows obvious bulges or cracks or missing parts (for example a blown-off inspection port cover) I don't think a homeowner can safely and accurately assess the condition of the heating system after a puffback.

Your heating service technician would be expected to examine the combustion chamber and the accessible/visible boiler internal surfaces and components for physical damage as well as to accurately diagnose and fix maintenance problems that can cause puffbacks.

And you won't be able to see the "amount of unburned oil" that damaged the boiler or that caused a puffback because that oil is gone - consumed in the burning and explosion of the puffback.

Furthermore, because of the physical shock to boiler components during a puffback explosion, I wouldn't assume that the post-puffback boiler would run exactly as it was running before that event. For example a puffback could loosen an oil line connection, resulting in a greater air or oil leak than was present before the event.

The closest we can come to guesstimating the history of a puffback would be to note reports of the history of boiler operation complaints (odors, noises, sooting), the frequency and extensiveness of prior heating equipment maintenance and/or service calls, and the date of the last service call before the puffback.

Finally, although it is much less likely, a boiler that had just been serviced, and serviced properly, could have still have a puffback if, for example, a contaminant in the fuel tank or fuel delivery led to a clogged burner nozzle.

 

Research on Oil Burner Puff-Back Explosions

• AUDELS OIL BURNER GUIDE, INSTALLING, SERVICING, REPAIRING - an older version of Audel's Oil Burner Guide
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
• Beckett Corporation, OIL BURNER START-UP RUMBLES [PDF] Beckett Corporation, retrieved 2021/12/06 original source: https://www.beckettcorp.com/support/tech-bulletins/solving-rumbles-and-pulsations/
  Excerpt:
  Combustion noise can occur in three major areas: at start-up, during the run cycle, and at shutdown. The following information is provided to help you effectively troubleshoot these areas.
• Brumbaugh, James E. Audel HVAC Fundamentals, Volume 2: Heating System Components, Gas and Oil Burners, and Automatic Controls. Vol. 18. John Wiley & Sons, 2004.
• Butcher, T. A. Computational fluid dynamics in oil burner design. No. BNL-52537; CONF-9704179-. Brookhaven National Lab., Upton, NY (United States), 1997.
• Butcher, T. A., Y. Celebi, and Wai Lin Litzke. Low excess air operations of oil boilers. No. BNL-52537; CONF-9704179-. Brookhaven National Lab., Upton, NY (United States), 1997.
• Delavan
• Delavan, A Total Look at Oil Burner Nozzles, available from Delavan Fuel Metering Products, a reference guide for oil burner service technicians, Delavan Spray Technologies, Fuel Metering Products Division, PO Box 969, Bamberg SC 29003, Tel: 800-982-6943 or 803-245-4347, ( Delavan is a division of BF Goodrich); web search 12/30/2010, original source: http://www.delavaninc.com/pdf/total_look.pdf
  
  Note & disclosure: when we (DF) were servicing oil burner equipment, Delavan nozzles gave us such excellent results, especially using hollow cone spray patterns in solving combustion problems, that this brand was our standard stock-item on our truck. Other brands of oil burner nozzles are of course also of excellent quality. It is the proper matching of nozzle size and type and spray pattern to the equipment that is critical.
  
  Excerpt, p. 30
  
  If the burner sometimes fails to ignite smoothly and starts with a puff, what can be done to improve it?
  
  1. Check the electrode points for proper spacing. The points should be 1/8” to 3/16” apart, 9/16” above the centerline of the nozzle and then spaced correctly forward from the face of the nozzle for each different spray angle.
  
  2. Clean carbon and dirt from the points and from the insulators.
  
  3. The transformer may be weak and not delivering full voltage or current. If you don’t have another transformer immediately available, file the electrodes to a sharp point. This will give a better spark.
  
  4. Check for cracked insulators. Sometimes an insulator may be cracked under the electrode bracket and it is difficult to find. This could cause a high voltage leak, thus reducing the voltage at the points.
  
  5. A partially plugged nozzle causing off-center spray can cause delayed ignition.
  
  6. Above about 2.50 gph a hollow cone spray sometimes may cause delayed ignition. Changing to a solid cone sometimes helps that situation.
  
  7. The air setting on the burner may be wide open, thus tending to blow the fire out before it is established. Adjust the air correctly.
• Fellows, Julian R., Alonzo P. Kratz, and Seichi Konzo. THE ILLINOIS SMOKELESS FURNACE [PDF] (1918) University of Illinois at Urbana Champaign, College of Engineering. Engineering Experiment Station., 1947.
  Excerpts:
  
  7. Puff-Back
  
  Puff-backs consist of periodic puffs resulting from the ignition of gas accumulated in the coking chamber, and have been troublesome to overcome in many designs of stoves and furnaces. A thorough investigation was made in the laboratory of some of the factors promoting puff-backs. These factors included coal sizing, smokepipe draft, size of coking-air orifice, and design of the cokingair deflector passage. The following conclusions were drawn from the results of the study:
  
  (a) Puff-back occurs only when a critical amount of coking air enters the coking chamber coincident with the presence of flames over the surface of the charge of fresh coal. An increase or a decrease in the amount of coking air will prevent the puffs.
  
  (b) Puff-backs can be eliminated by the use of proper control of the coking air. The use of a damper which completely closes the cokingair inlet during the off-periods of operation is believed to be essential. The firing door should be well fitted, since excessive leakage around a poorly fitted door may cause the furnace to puff even when the cokingair damper is closed.
  
  (c) Smokepipe draft has no effect on puff-back except as it may affect the amount of air that enters the coking chamber.
  
  (d) All other conditions being equal, the manner in which the coking air was delivered into the coking chamber was found to be important. A properly designed deflector plate, as described in Appendix B, Section 12, was helpful in reducing the possibility of puff-backs.
• Krajewski, Richard F., and T. A. Butcher. Research, development, and testing of a prototype two-stage low-input rate oil burner for variable output heating system applications. No. BNL-52537; CONF-9704179-. Brookhaven National Lab., Upton, NY (United States), 1997.
• Laisy, J., and V. Turk. Fuel oil quality task force. No. BNL-52537; CONF-9704179-. Brookhaven National Lab., Upton, NY (United States), 1997.
• Miller, John P., Randy s. Stier, DETECTING LOSS OF FLAME IN OIL REFINERY FIRED HEATERS USING ADVANCED PRESSURE DIAGNOSTICS [PDF] (2013) retrieved 2021/12/11 original source: https://www.emerson.com/
  
  Note: Fired heaters as discussed in this paper are a class of indirect-fired equipment used to heat process fluids in oil refineries and other process industries. However there are quite similar concerns with the smaller oil fired heating equipment used in residential and small commercial installatins - Ed.
  
  Abstract:
  Maintaining stable combustion is critical to the safe operation of fired heaters. When the ratio of air flow to fuel flow to a burner transitions from a point inside the burner’s operating envelope to a point outside the burner’s operating envelope, the combustion process becomes unstable and then stops. If this loss of flame is not recognized and acted upon properly, an explosion may occur.
  
  One possible way to detect loss of flame may be to monitor pulsations in the difference between the flue gas pressure inside a fired heater and atmospheric pressure at a given elevation (commonly referred to as “draft”). During stable combustion, the draft will pulsate slightly (see Figure 1). [in the PDF, not shown here - Ed.]
  
  This is commonly referred to as combustion rumble. As a burner’s air/fuel ratio transitions from inside to outside the burner’s operating envelope, the draft pulsations may exhibit a sharp increase in amplitude and frequency, caused by instability in the burner flames.
  
  Flame instability is often a precursor to flame out. Draft transmitters with Advanced Diagnostics Statistical Process Monitoring (SPM) capability monitor the pulsations in the draft measurement and detect the increase in amplitude or frequency associated with flame instability. SPM data may be integrated with heater operations. An alarm limit on the SPM standard deviation may be configured to alert a heater operator to potential combustion problems or flame out.
  
  Valero has worked with Rosemount to evaluate the use of SPM technology for detection of flame instability or flame out. Recent testing was performed on five different burners at three different burner vendors. The technology was also evaluated on a heater at the Valero Texas City Refinery. Experience gained from these tests may be used to determine where to set the alarm limit for detecting flame instability. There is a correlation between the volumetric firing rate of a burner/heater system and the draft standard deviation.
  
  This correlation can be exploited for selecting an appropriate alarm limit in an operating fired heater. Burner vendor tests indicate that a draft standard deviation alarm limit of 30 thousandths inches of water column (m-inWC) may be appropriate. Additional work is required to validate these observations but initial results indicate the use of advanced pressure diagnostics may be an effective way to detect loss of flame.
• Montagna, Frank. Responding to" routine" Emergencies Workbook. PennWell Books, 2006.
• Nelson, D. NAOHSM and the OIL HEAT INDUSTRY on the INTERNET [PDF] (1997) Brookhaven National Lab., Upton, NY (United States), 1997. No. BNL-2537; CONF-9704179
• Santa, T. Residential fuel quality. No. BNL-52537; CONF-9704179-. Brookhaven National Lab., Upton, NY (United States), 1997.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-20 by (mod) - oil burner puffback after oil tank over-filled

Ali

I'm sure sorry that you had a puffback explosion and hope the the first priority - making the heating system safe and functional - has been addressed.

The causes and underlying explanation of puffback explosion are given on the page above.

I'm unclear about exactly what your oil delivery person did nor how excess oil was removed.

There are certainly possible mistakes, such as leaving an air leak in the oil line, that could result in a later puffback explosion.

But when a puffback explosion occurs the first time a boiler is turned back on immediately after service and cleaning, then the problem is most-likely due to the service and cleaning procedure that were just performed, less directly than something that was done days earlier and considering that the heater ran through multiple heat-on cycles after that event.

By "less directly" I mean that if the oil delivery guy left the system not working properly that could have led to incomplete combustion of oil, accumulation of sludge and crud on the oil burner nozzle, loss of heat, and a subsequent need for a no-heat service call.

An experienced service tech might remember (or might forget) to check the combustion chamber for an accumulation of un-burned oil before turning the system back on.

On 2020-12-20 by Ali

I had my oil furnace cleaned three weeks ago and about a week latter the oil deliver man came to deliver oil and overflow the oil tanks causing a leak
on the floor. He went to the furnace and opened it to removed some of the oil to prevent the continuous oil leak from the oil tank cap. While he was removing the oil he kept on pressing on the ignition starter to force the furnace to run in order for him to remove the oil faster.
Couple days latter the temperature in the house dropped from 70 to 58 degrees. I called the maintenance company that cleaned my system and when he turned on the furnace a puffback explosion occurred. What caused the puffback explosion in this case?

On 2020-09-17 - by (mod) -

Priscilla

When you get smoky flame at shut down on an oil burner then the most-likely cause is the oil burner's fuel unit our pump's shut down valve is dirty, clogging, not working properly. That's an internal valve in the pump that's designed to stop the oil flow quickly when the motor stops turning. If it doesn't work then low pressure oil is sent through the nozzle where it burns incompletely, hence the smoke.

The risk is a dangerous puffback explosion.

That shutoff valve is not usually serviced in the field but rather the service tech will replace the fuel unit itself.

Don't try it yourself.

On 2020-09-17 by Priscilla Riddle

Montgomery ward 700 oil furnace.
Started smoking up vents after it shuts off.
Chimney is clean.
How to clean it. Model mp98

On 2020-04-20 - by (mod) -

Stay safe and do ket me know what happens.

You shoul know that while you can get the burner to run, is not possible to properly set up and tune a modern oil burner without measuring smoke, co2, temperature, draft.

On 2020-04-20 by Anonymous

danjoe, thanks for setting me straight on this, while I thought that was the next logical step, I appreciate your explanation-
I'll do this tomorrow after work and report back in. Thanks

On 2020-04-20 - by (mod) -

Ian

Lest you feel bad about the problem that you were describing, i admit I've certainly done the same thing and had the same problem.

You did the right thing to turn off the heater. The real risk is a dangerous and terribly messy puff back explosion into the building rather than just black smoke going up the chimney.

You need to be confident. The problem is just some temporary spillage and not something worse like a leak in the oil line connections or nozzle adapter that is continually spraying unburnt oil into the combustion chamber.

If you're confident that there's no actual leak and that there was a single event then what a service tech would do would be probably to pull the burner and wipe out any pooled oil to get things as clean and dry as possible.

She might then wait some hours before turning the system back on. At that point there will be some smokey exhaust but it should clear up in a minute or two. If it continues Beyond a couple of minutes I would be concerned particularly if the smoke was not diminishing and at that point I'm afraid I would have to turn the system off again for more cleaning and inspection for leaks.

On 2020-04-19 by Ian

My boiler had a few false starts, and I guess it sprayed fuel into the combustion chamber which is sitting in a puddle. I replaced the electrodes and the nozzle and fuel filter, and it started right up and sounds smooth and quiet- but it is spewing out black smoke, so I shut it off. My question is, will the fuel oil evaporate or should I do something else?

On 2019-04-15 - by (mod) -

Kelly

You may be hearing the sudden ignition or 'explosion' of un-burned oil that collects in the combustion chamber if an oil burner is not working properly.

Watch out: such explosions can be dangerous as well as making a sooty mess of a building. Ask your heating service company to inspect and repair or adjust the system.

On 2018-01-22 by Kelly Mingus

What causes water sound in my vent and black smoke came out of the vent once. There is a boiler system here.

Reader Question: where does oil burner soot go and does it cause a puffback?

21 January 2015 Lori said:

We had a puffback recently in our home and the mess was unbelieveable. We have our furnace serviced twice a year, but obviously something wasn't done correctly.

I have a general question - for an oil furnace, where does the residual soot go - up the chimney? If the chimney is not maintained, then I am guessing the soot will build up in the chimney and then eventually back down into the furnace. Can this cause a puffback as well? I am new at this stuff, so your help is much appreciated. Your site has been very informative!

Reply:

Lori

Soot particles produced by fossil fuel combustion end up either deposited in the heating appliance or in the chimney except for those that manage to escape into the air along with flue gases discharged to the outdoors. Part of proper oil fired heating appliance service includes cleaning and adjustment to keep the soot production low without setting combustion temperatures so high that while there is still less soot too much of your oil fuel dollar goes up the chimney.

Annual cleaning and service are very important for all heating appliances but particularly so for oil fired heaters since some soot deposition during the heating season is normal.

A puffback itself may be indirectly caused by soot accumulation if that accumulation prevents proper combustion or prevents proper operation of the oil burner. But the immediate cause is the ignition of incompletely-burned or un-burned oil in the combustion chamber - an event that certainly has other causes (such as air leaks in the oil piping or any oil burner operating problem that prevents complete combusion).

...

Continue reading at OIL BURNER INSPECTION & REPAIR - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see OIL BURNER PUFFBACK FAQs - questions & answers posted originally at this article

Or see these

Recommended Articles

• CEILING STAIN DIAGNOSIS
• FIRE & SMOKE ODOR REMOVAL
• GHOSTING DARK STREAKS or LINES: CAUSES to diagnose lines or rectangular areas of stains on walls or ceilings
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• SOOT HAZARDS in BUILDINGS
• SOOT on OIL FIRED HEATING EQUIPMENT
• STAINS CANDLES FIREPLACE WOODSTOVE
• THERMAL TRACKING BRIDGING GHOSTING
• WOOD STOVE, CATALYTIC PUFF-BACKS

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