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Chimney Doctor's Homeowner's Guide

Internal v. External—or Escaped—Hostile Chimney Fires

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By William Hussel

JUNE 24, 2009

There are two distinctive types of hostile chimney fires that can cause thermal shock inside a flue system: (I.)internal and (II.) external; usually, both are covered by homeowner’s insurance coverage. 

Internal Hostile Chimney Fire:

Chimney Fire

Click photo to enlarge

This type of chimney fire is the most well-known to the public.  It is essentially a fire that is generated from built up combustible creosote inside a flue system.  The fire can be started with a spark from the burning fireplace below; this type of chimney fire is therefore considered to be an “internal hostile chimney fire”.  The combustible creosote burns at an extremely high temperature (2000 °F) causing thermal shock to the flue tiles and often cracking them or warping a metal chimney.  This type of chimney fire may leave pyrolized creosote residue inside the flue system (there are situations when this residue  may not be left-as noted below).

An internal hostile chimney fire can be an aggressive fire in which the homeowner recognizes a loud gushing air sound through the fireplace caused by the chimney fire burning out of control with large amounts of fuel and oxygen.  The other type of internal hostile chimney fire is one that is limited by the amount of either combustible fuel or oxygen.  These chimney fires are slow burning as are approximately 70% - 80% of chimney fires and are typically not recognized by the homeowner.  They are usually found by a chimney technician performing a routine video inspection during the chimney cleaning process. 

Regarding the residue in flue systems following internal hostile chimney fires, the following paragraph is contained in the “Chimney Fires: Causes & Effects” publication (attached):

“Actually, field experience confirms that a small minority of fires do remove most of the deposits, leaving a nearly clean flue.  These fires appear to usually involve intense free-burning combustion with a relatively small amount of deposit and very high draft.  The creosote is not actually consumed by the fire but is torn off the walls and carried out the flue by the high draft or falls to the bottom of the flue.  The expelled residue can be found all over the roof and yard unless it has been carried away by wind or rain.”

External (Escaped) Hostile Chimney Fire:

The second type of fire can also cause significant flue tile cracking or metal warping due to thermal shock is an “escaped” fire.  This is classified as an external hostile chimney fire because the combustion source is external to the flue system.  Per the publication “Chimney Fires: Causes & Effects” this type of fire is one where the flames escape to an area of the fireplace or chimney system that is not intended to contain neither high temperature flames nor a fire.  Generally speaking, this type of thermal shock fire is one where the fire extends out of the firebox (intended to contain the fire) and extends through the damper into the flue system where thermal shock damage ensues.  This second type of chimney fire damage leaves no pyrolized creosote because there is no combustion within the flue system.

External hostile chimney fire damage can be compared to smoke damage in a residential house fire in which the combustion is in one area of the structure and the smoke escapes and causes damage in a different area (where there is no combustion).

Another example is a grease fire in a skillet left unattended on a hot stove burner that catches on fire;  the heat from the fire damages the stove, window, cabinets, surrounding drywall, etc.  Here again, only the heat damages the surrounding areas; there might not be any combustion except in the skillet.  These two above examples would normally be covered by homeowner’s insurance.

In the 28 years that I have owned our chimney company, I have witnessed a substantial number of external hostile chimney fires where flames have caused damage in unintended areas of chimney systems.  The most dramatic example I have witnessed was a large, new home in which an external hostile chimney fire had cracked every flue tile in the entire chimney flue system with the first fire that the homeowners had started in the fireplace.  In this case, the homeowner made the first fire with a large amount of kindling wood; the flames were drawn up into the flue tile system and subsequently cracked all the tiles.  Obviously, there was no pyrolized creosote residues after the damage was done, since this was the first fire in the flue system and the only combustion was in the firebox below. 

In a second instance, I inspected a chimney where literally every single flue tile had been cracked, and they had completely collapsed into the smoke chamber area below.  There was no evidence of any pyrolized combustible creosote inside any of the flue tiles. 

A third example is where a Class A (metalbestos) chimney was severely warped from an intense chimney fire, but had a shiny interior surface after the fire.  In this example, deposits were likely drawn up the chimney due to a very high draft in the insulated Class “A” metal chimney.

A fourth example is documented by a technical advisor of the National Chimney Sweep Guild who also serves at his local fire department.  He was called upon to contain a known chimney fire.  Upon returning to the site a few days later, he found the interior surfaces of the flue tiles “looking like new”, with absolutely no black residue.

A fifth, very recent, example occurred at a well known high end hotel near Milwaukee after our company had cleaned the chimney in their main lobby only two days prior to their starting their next fire.  The flames were generated below the flue system and were pulled up through the flue system.  The flames were extending 5’ above the chimney when a police officer discovered the fire and called the fire department.  After further analysis of this fire, it was determined that there was absolutely no combustible material or residue (i.e. pyrolised creosote) in the flue tile system because it had just been cleaned and that the combustion and high temperatures originated entirely from the smoke chamber below.  We used a video camera/monitoring system (witnessed by hotel management) to identify the specific source (creosote in the smoke chamber) of these flames.  This chimney fire is also an example of a documented “slow burning chimney fire”; the owner had no knowledge of the fire, except for the police officer seeing flames above the chimney.

Per the "Chimney Fires: Causes & Effects" Publication (Attachment I): "Courts are increasingly willing to construe an excessive fire as hostile even if the combustion process never actually leaves the place where it was kindled”.  The publication also states that "even objects that are intended to be in contact with a normal fire or the heat given off by the fire are covered by the fire peril if they (the flue tiles) [words added] become damaged by an unusually hot or otherwise abnormal fire."  These statements make it very clear as to how insurance companies should handle claims for damage of external (escaped) hostile chimney fires.

Other types of external hostile chimney fires from a fireplace or woodstove can occur for the many reasons listed below.  These are examples of how flames from the fireplace or woodstove can escape, possibly damaging flue tiles or warping metal chimneys with no evidence of residues in the flue system.

  1. Burning a Christmas tree (or boughs) in a fireplace or woodstove.
  2. Burning Christmas wrappings or other paper, trash, and newspapers in a fireplace or woodstove.
  3. Burning an extra large kindling fire in a fireplace or woodstove especially if the pieces are small and are stacked to allow unlimited air to reach the burning surfaces.
  4. Using flammable liquids (charcoal lighter fluid, kerosene, etc) in a fireplace or woodstove.
  5. Having excessive combustible creosote in the black metal vent pipes of a woodstove.  If this lower level creosote is ignited, flames can be drawn up through the flue system where there may be no combustible creosote.

It should also be noted that flue damage is more likely to occur with conditions of:

  • High winds that create a venturi effect and create a high vacuum in the flue system, causing the flames to be drawn up the flue.
  • Colder outside temperatures where the flue system is at a low ambient temperature prior to contact with the high temperature flames, obviously, thermal shock is more likely to occur in this instance.  Also, this situation explains why thermal shock damage to flue systems is more common in colder regions such as Wisconsin.

Lastly, it is important to note that pyrolized creosote can be removed from a flue system over time.  Examples include rain or precipitation washing this residue down the flue system, or a chimney sweep cleaning the flue system and not reporting any damage to that flue system.

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