Black stains on an indoor ceiling (C) Daniel FriedmanOil Burner Puffbacks
Cause, Cure, Prevention of Puffbacks on Boilers, Furnaces, Water Heaters

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Black stains on an indoor ceiling (C) Daniel Friedman

Here we explain oil burner or oil heating system puffbacks: what happens, and what causes them.

Watch out: if the reset button on your heating equipment has popped, 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, particluarly 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 where we also list the location of all of the various reset buttons and controls found on heating equipment.

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.

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

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.

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.

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.

Leaky oil piping (C) Daniel Friedman

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:

Soot blowout at draft regulator (C) Daniel Friedman

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.


Oil Burner Furnace or Boiler Puffback Warning Signs

Blocked barometric damper (C) Daniel Friedman

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.


If you see black soot and debris on top of your boiler, furnace, or water heater, or black soot stains on ceiling or wall surfaces [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 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.

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.


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 - 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 ducs above unit and it still happened.whats next?


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


Tom, I think it might be smart to get a certified chimnmey 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.

Guide to Preventing Oil Burner Puffbacks in buildings

Leaky oil line (C) Daniel Friedman

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

Debris in a stack pipe (C) Daniel Friedman

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

Role of Heating Boiler Operation in Oil Burner Puffbacks & Soot Explosions

Reader Question: Do "faulty boilers" cause puffbacks? Where do I look for puffback damage to the boiler itself?

Oil burner soot on ceiling © D Friedman at

Thank you for your informative website. As a homeowner trying to deal with a puffback it is a sight for *dirty* eyes!! Under the heading
What Causes Sooty Puffbacks at Oil-Fired Heating Boilers, Furnaces, Water Heaters [article above] there is a statement in paragraph one that states "Depending on the quantity of oil that is ignited, the puffback can damage the boiler itself...".

This seems to be the opposite of what my insurance carrier is telling me. They maintain that a faulty boiler causes a puffback and thus is not covered under the homeowners policy because it is the culprit, only the damage it causes is.

Where would I look to determine what, if any, amount of unburned oil could have actually damaged my boiler? Thanks, D.F.

Our InspectAPedia photo (above left) shows soot on the ceiling of a garage just outside a boiler room where an oil-fired heating boiler was operating improperly and blowing soot into the building - a puffback from this system was imminent. This is not the building discussed by D.F. above. As you can see by the exposed wall studs in the right side of the photo, we had already begun demolishing the sooty drywall in this home.


Thank you so much for your answer. I have since learned a bit more about boilers and would like to correct some confusion with my wording now. I do apologize as boiler jargon is really not my thing. My boiler has been maintained but it is aged. I had shut down the system for the season, or so I thought. It was an unusual damp, wet May. I suspect one of the kids hit the on switch at the entry to the basement.

Regardless, since the boiler has to be replaced I am trying to learn. I am trying to understand the term *inherent vices* of a boiler that exclude replacement coverage under the homeowners policy.

I am coming to understand that there are some condition that are known to cause the boiler to "damage itself". In other words, wear and tear, causing a puffback.

However, the wording in your article led me to question whether the oil/puffback could be an "external" element that caused the damage to the boiler. Its sorta like what came first, the chicken or the egg. There were no visible leaks so I am wondering how the "excess oil" could have gotten there.

Are there any possibilities that tie in the oil tank (outside underground)? Either way, I am learning it is an uphill battle to actually get the boiler replacement cost covered which; is a real expensive bummer. After all, protecting against a big expense like this is why you buy insurance in the first place.

Reply: Do faulty boilers cause puffbacks? No, oil burner problems, oil piping leaks, or chimney maintenance troubles are the root cause of puffbacks.

Soot from oil burner © D Friedman at

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with or cause of malfunction in a heating boiler. That said, here are some things to consider:

Your quotation from your insurance company stating that the puffback was caused by a "defective heating boiler" is confusing because it is not a precisely correct description of heating equipment puffback problems.

The explanation we offer of the mechanism of a puffback in oil fired heating equipment is more detailed than a typical insurance adjuster would offer even if s/he understood the cause of puffbacks, but our explanation is also the correct one.

Our photo (above left) shows soot fragments on top of a water heater that was installed close to an oil-fired heating boiler that was not operating properly - the same boiler whose soot was deposited on the garage ceiling and walls in our earlier photo just above. Failure to notice and do something about this sooty boiler operation is a failure to notice and correct a boiler operation or maintenance problem.

List of Common 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:

are examples of conditions that lead to an oil burner puffback. Here are two more possible causes of oil burner puffbacks:

None of these are innate defects in the heating boiler 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.

Where do I look for puffback damage to the boiler 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.

Follow-Up remarks: 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.

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.".

Oil Tank or Oil Piping Contribute to Oil Burner Puffbacks?

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.

Soot Effects from Oil Burner Puffbacks

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:

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.

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!



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).


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