Forensic Engineering Analysis of a Pre-fabricated Direct-Vent Fireplace Fire

F.E. Analysis of a Pre-fabricated Direct Vent Fireplace Fire

www.warrenforensics.com

Basic Case Facts• New home• New fireplace installation• Owner’s in the home only

one night• Next day operated F.P. for

the first time• 2 to 3 hours after turning it

on a fire occurred in the cavity surrounding the F.P.


Basic Case Facts• Fire Department responded

quickly and extinguished the fire

• Flame damage limited to the chase cavity

• Home and contents sustained substantial smoke damage


Fire Place & Chimney System

• DESA – Prefab F.P.• Model T32NB• Natural Gas• Direct Vent• Sealed/Glass Front


Fire Place & Chimney System• Double walled horizontal

chimney system• Combustion gases

discharged out through the inner pipe

• Make-up air for combustion is pulled in through the annulus space


Fireplace & Chimney System• TV / Media shelf installed

over the top of the fireplace & Chimney

• Fireplace installed adjacent to an exterior wall in a wood-framed chase

• Short horizontal chimney run from the fireplace through the exterior wall


Fireplace & Chimney System• Installers used wrong

chimney kit• Wrong length of horizontal

chimney pipe (6” fixed length used; 10” needed)

• Resulted in a 2-inch gap / opening in the chimney pipe inside the chase

• Combustion gases would therefore discharge into the chase

• Error not corrected before drywall installed and forgotten about


45 Elbow

6” Horz. Run

Chase Cavity• Box-framed wood 2 x 4

construction sheathed with drywall (interior 3 walls)

• Plywood media shelf over top of fireplace and chimney

• Exterior wall cavity above media shelf was insulated with spray foam insulation

• Exterior wall 2 x 4s with combustible fiber board sheathing and brick veneer (exterior 3 walls)


Fire Investigation: Plaintiff Investigator’s Conclusion

• Based on the physical dimensions of the space, and position of the fireplace and interior wall surfaces there was a 2-inch gap/opening in the pipe

• The 2-inch gap allowed some of the combustion gases to discharge directly into the chase cavity and ignite the combustible construction


Defense Investigator’s Conclusions• There is “no probability “ that the

fireplace/chimney system could operate with a 2-inch gap long enough to ignite the combustible construction of the cavity

• Discharging combustion gases would be pulled immediately back into the make-up air annulus and subsequently into the combustion chamber

• The fire in the combustion chamber would be extinguished (choked out)

• With a 2-inch gap the fireplace would self-extinguish in less than 70 seconds


Defense Investigator’s Conclusion Bases• “First Principles in fire dynamics” –

deposition testimony• “the direct-vent fireplace will not

function if there’s a gap like that. It will go out” – deposition testimony

• “there is no probability that the fireplace would operate for the duration necessary to competently ignite the solid wood framing members of the fireplace’s enclosure” – written report


Did the defense Expert Have Any Need for Testing to Validate His Opinion?

• Q. Do you believe that you should conduct a demonstration or test of that situation with a gap before you can give opinions to a reasonable degree of certainty that comport with the scientific method?

• A. No. I can use cognitive test to – based on first principles to make that opinion.


Engineering AssignmentPerform an engineering investigation and analysis of the fire and evaluate the opinions put forth by both fire investigators.


Engineering Investigation & Analysis• Review of all the:• Available scene data including photos,

measurements and interviews.• Litigation discovery including depos.• Performed additional interviews and data

collection as necessary.• Research and review of product and associated

investigative literature.• An exemplar fireplace, chimney system and chase

was constructed, instrumented and operational testing completed.


Testing Set Up• An exemplar assembly was set up

to substantially represent the incident set up (fireplace, chimney with gap and surrounding cavity)

• A protocol was drafted and the defense invited to participate (they declined)

• Three operational test were performed with slight variations in the chimney pipe alignment across the gap


Testing Set Up – General Images


Test One – Parallel Alignment• The two pipe sections

were aligned parallel and horizontally with a 2-inch gap in between.

• Note: Using the scene dimensions of the chase and the same model fireplace and chimney kit a 2-inch gap was in fact generated.


Test Two – Y Plane Offset• Keeping the same set

up except:• The two pipe sections

were offset vertically relative to one another using the normal available play in the 6-inch pipe section


Test Three – X, Y Plane Offset• Same as test two but with a

horizontal offset added• The justification in the slight

variation in tests two and three is that it is unlikely the installer took the time to use a level as I did in the first test to align the pipe perfectly across the gap.


Test One Results• Duration: 7 hours +• Fireplace operated

continuously with no changes to flame

• Flue gas temperatures: 620 F

• Plywood shelf max temperature 220 F

• Max wall temperature 210 F


Test Two Results

• Duration: 3 hours• Fireplace operated

continuously with no changes to flame

• Flue gas temperatures: 670 F

• Plywood shelf max temperature 299 F

• Max wall temperature 307 F


Test Three Results• Duration: 6.5 hours• Smoldering ignition of the wall

after approximately 90 minutes• Max wall temperature before

loss of t-couple contact 660 F• Fireplace operated continuously

with no significant changes to flame

• Flue gas temperatures: 670 F +


Test Three Results – Temperature Time Curve

Test Number Three

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350 400 450

Time (min)

Te

mp

era

ture

(F

)

(T1) 1" Above Fire Stop Plate (T2) 3:00PM 1" From Flue btw Board & Firestop Plate



Smoke!

Analysis – “First Principles”

• Flowing gases have momentum• Hot gases do rise (buoyancy)• Rising hot gases impacting the ceiling of a

compartment will form a descending hot gas layer (NFPA 921)

• Once a descending hot gas layer reaches an opening the gases will flow out of the opening (NFPA 921)


Analysis – “First Principles”

• Cool air will be drawn into a compartment around the lower portion of a ventilation opening when hot gases are flowing out around the top portion of the opening


Analysis – Ignition of Wood

• The “short-term” ignition temperature of wood is around 482 F. (NFPA 921)

• Smoldering ignition of wood can be initiated at lower temperatures of around 356 F (Kirk’s p. 109)

• Once sufficient heat, oxygen and flammable pyrolysis gases are available smoldering combustion can transition to flaming combustion


The time required for smoldering combustion to transition to flaming combustion is not predictable (NFPA 921 5.7.4.1.7.2)


Conclusions

The plaintiff’s investigator’s conclusion as to the probable cause of the fire was consistent with

my testing and analysis.


Conclusions• Contrary to the defense fire investigator’s

conclusions:

• In every test the fire in the combustion chamber continued to burn without any abnormalities (and it never went out)

• Furthermore the fireplace operated without any “trouble” with sufficient time to ignite the combustible materials of the chase cavity


Conclusions

• In Fact the fireplace continued to operate long after the combustibles in the chase had ignited

• The flue gases being discharged out of the gap do provide a competent ignition source for the combustible construction materials of the chase cavity.

• Sufficient make-up air is drawn back into the fireplace to sustain normal combustion within the firebox.


Conclusions

• It’s O.K. to have a hypothesis

• Just properly evaluate it and test it before you draw your gun (report or testify)


Technical Question

What is the time period required for a smoldering fire to transition to a flaming fire?


Jerry TindallM.S.M.E., P.E., [email protected]


Forensic Engineering Analysis of a Pre-fabricated Direct-Vent Fireplace Fire

Design

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