NISTIR 7506 

Impact of a Residential Sprinkler on the 

Heat Release Rate of a Christmas Tree Fire 

Daniel Madrzykowski 

Sponsored in part by 

Department of Homeland Security

NISTIR 7506 

Impact of a Residential Sprinkler on the 

Heat Release Rate of a Christmas Tree Fire 

Daniel Madrzykowski 

Fire Research Division 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-8661 

May 2008 

Department of Homeland Security 

Michael Chertoff, Secretary 

Federal Emergency Management Agency 

R. David Paulison, Administrator 

United States Fire Administration 

Gregory B. Cade, Assistant Administrator 

U.S. Department of Commerce 

Carlos M.Gutierrez, Secretary 

National Institute of Standards and Technology 

James M. Turner, Deputy Director 
 - ii - 
 - iii - 

ABSTRACT 

Although the number of Christmas tree fires is low, these fires carry a higher level of hazard than 

other fires that occur in a residential structure. This study, supported by the U. S. Fire 

Administration, has the following three objectives: 1) characterize the heat release rate of dry 

Fraser fir trees 2) demonstrate the ignition resistance of a tree with a high moisture content and 

3) examine the impact of a residential sprinkler on the heat release rate of a dry tree that is on fire 

in a compartment. 

The heat release rates of the trees which were allowed to dry ranged from 3.2 MW to 

4.3 MW. Trees that were kept in water, so that the needles maintained a moisture content in 

excess of 100 %, self-extinguished after being exposed to a flaming book of matches. 

The data from the furnished sprinklered room experiment demonstrated that even under 

conditions of extreme fire growth, a single sprinkler was able to prevent flashover and limit the 

spread of fire to other objects. The peak heat release rate, from the sprinklered room, was 

limited to approximately 1.8 MW. The furnished non-sprinklered room experiment generated a 

post-flashover heat release rate in excess of 6 MW. 

 

Keywords: Christmas trees, fire prevention, heat release rate, heat flux, mass loss rate, 

residential sprinklers 
 - iv - 

DISCLAIMER 

Certain companies and commercial products are identified in this paper in order to specify 

adequately the source of information or of equipment used. Such identification does not imply 

endorsement or recommendation by the National Institute of Standards and Technology, nor 

does it imply that this source or equipment is the best available for the purpose.
 - v - 

TABLE OF CONTENTS 

 Page

ABSTRACT..................................................................................................................................iii

INTRODUCTION......................................................................................................................... 7

EXPERIMENTAL APPROACH................................................................................................. 8

Tree Conditioning ..................................................................................................................... 8

Moisture Content Measurement.............................................................................................. 9

HEAT RELEASE RATE EXPERIMENTS.............................................................................. 10

Maintained Trees .................................................................................................................... 11

Dry Trees ................................................................................................................................. 11

Results ...................................................................................................................................... 12

ROOM EXPERIMENTS ........................................................................................................... 16

Results ...................................................................................................................................... 19

DISCUSSION .............................................................................................................................. 25

SUMMARY ................................................................................................................................. 27

ACKNOWLEDGEMENTS ....................................................................................................... 27

REFERENCES............................................................................................................................ 27
 - vi - 
 - 7 - 

INTRODUCTION 

The National Fire Protection Association (NFPA) reports that there is an estimated 

annual average of 210 home structure fires that begin with Christmas trees. Based on 

data from 2002 through 2005, these fires caused an average of 24 civilian deaths, 27 

civilian injuries, and $13.3 million in direct property damage per year [1]. 

The NFPA analysis also shows that although the number of Christmas tree fires is low, 

these fires represent a higher level of hazard. On average, 1 of every 9 Christmas tree 

fires resulted in a fatality compared to an average of one death per 75 non-confined home 

structure fires overall. Further, 49 % of Christmas tree fires spread beyond the room of 

origin. The fires that spread beyond the room of origin caused 94 % of the associated 

fatalities [1]. 

The percentage of trees involved in structure fires represent an extremely small portion of 

the total number of natural Christmas trees sold, which is estimated at 30 million trees, in 

the United States each year [2]. The moisture content of each tree can play a dominant 

role in determining the fire hazard each tree represents. Research by Chastagner on 

evergreen twigs and by White et. al. on complete trees has shown that fresh cut branches 

or trees, that have been properly watered and maintained, i.e. trees (needles) with a high 

moisture content, are difficult to ignite with a small flame [2-4]. 

The moisture content of a fresh cut tree is typically in excess of 100 %. Once harvested, 

the tree’s rate of drying is dependent on the species of the tree, the initial condition of the 

tree, and the environmental conditions that the tree is exposed to after being cut. 

Environmental conditions include temperature, vapor pressure deficit and wind, during 

transport, retail display and use of the tree [2]. Guidance for caring for a cut tree is 

provided by the National Christmas Tree Association [5]. 

Previously, the National Institute of Standards and Technology (NIST) conducted a series 

of fire tests to characterize the potential hazard from ignition of dry Scotch Pine 

Christmas trees. Heat release rate was measured as a function of time from ignition using 

an oxygen consumption calorimeter. Seven trees were allowed to dry for approximately 

three weeks. These trees burned easily when ignited with a small open flame. Peak heat 

release rates ranged from about 1.6 MW to 5.2 MW [6]. These heat release rates for dry 

trees were significantly higher than heat release rates for “Christmas” trees that had been 

published prior to that time [4,7]. 

An eighth tree, kept in water until just before testing, could not be ignited. Prior to 

testing, the moisture content in each tree was measured at several points on the trunk 

using a moisture meter. The moisture content of the trunk ranged from 25 % to 35 %. 

While the moisture content of the tree trunks varied by only 10 %, the first seven trees 

differed significantly in apparent dryness of the needles and branches from the eighth 

tree. 
 - 8 - 

The 1999 NIST Scotch Pine Christmas tree report also offered some qualitative guidance 

on determining if the tree is dry and prone to an easy ignition based on the condition of 

the needles. The needles on the first seven trees were dry to the touch and slightly brown 

in color. The needles broke easily when bent slightly and fell from the tree when the 

branches were shaken. The eighth tree had a much greener color and the needles were 

very pliable. When a needle was pulled, the tree branch would bend slightly before the 

needle would separate [6]. 

EXPERIMENTAL APPROACH 

This series of Fraser fir experiments were conducted with the following three objectives. 

The first two objectives extend from the 1999 NIST study: 1) characterize the heat 

release rate of dry Fraser fir trees and 2} examine the ignition resistance of a tree with a 

high moisture content. The third objective was to examine the impact of a residential 

sprinkler on a dry tree that is ignited within a furnished compartment. 

Tree Conditioning 

Twelve Fraser fir trees were purchased from a commercial tree lot in Mt. Airy, MD on 

November 27, 2006. The trees had been harvested from a tree farm near Gettysburg, PA 

on November 17, 2006. Half of the trees were placed in a tree stand with no water. The 

other six trees were prepared for display as recommended by the National Christmas Tree 

Association [5]. The preparation includes cutting approximately a 13 mm (0.5 in) disk 

of wood from the base of the tree trunk. The cut is made perpendicular to the axis of tree 

trunk, as recommended. After the tree trunks were cut, the trees were placed in reservoir 

type tree stands. The reservoir has a capacity of 4.42 L (1.17 gal). The trees were kept in 

the NIST Large Fire Laboratory in Gaithersburg, MD. Water was added daily as needed 

to keep the reservoirs full. 

The weather in the Gaithersburg area was mild during the time the cut trees were being 

conditioned. While the trees were kept inside the fire laboratory, the laboratory has 

limited heat and humidity control. During hours of operation, the laboratory environment 

tends to follow the outside conditions as significant amounts of make-up air are required 

when the exhaust hoods are operating. Weather data from November 2006 through 

January 2007 is provided in Table 1 to provide a sense of the conditions. 

As demonstrated by previous studies, the fire safety of a tree is directly related to 

moisture content. [2-4,6] . Chastagner has shown that Fraser fir branches will ignite and 

burn when exposed to an open flame at a moisture content of approximately 30 % or less 

[3]. The trees that were not maintained with water were showing signs of moisture loss 

in mid- January, after being relocated to an area in the building adjacent to a gas-fired 

space heater for a week. The signs of moisture loss included needles that would easily 

fall off of the tree, some discolored needles, and ends of branches that were becoming 

brittle and would snap instead of bend. 
 - 9 - 

Table 1. Weather Conditions for Gaithersburg, MD, November 2006 – January 2007 [8] 

 

 November 2006 December 2006 January 2007 

 

High Temperature 

(oC/oF) 

23.4 / 74.1 23.4 / 74.1 20.3 / 68.5 

Low Temperature 

(oC/oF) 

-3.3 / 26.1 -7.5 / 18.5 -10.0 / 14.0 

Average 

Temperature 

(oC/oF) 

9.4 / 48.8 5.6 / 42.0 3.8 / 38.8 

High 

Pressure 

(hPa / in) 

1035.8 / 30.59 1035.8 / 30.59 1040.5 / 30.73 

Low Pressure 

(hPa / in) 

995.8 / 29.41 997.9 / 29.47 1002.6 / 29.61 

Average Relative 

Humidity (%) 

67 57 56 

Moisture Content Measurement 

On the day the trees were fire tested, the moisture content (MC) was measured using an 

Arizona Instruments Computrac Moisture Analyzer, Model MAX 200XL. The apparatus 

measures the mass of needles removed from the tree. Then the needles are heated to a 

temperature just above 100 oC until a steady mass reading is obtained indicating that the 

moisture has been removed. 

The MC is determined by the difference of the initial (wet) mass minus the dry mass. 

The difference is then divided by the dry mass. The resulting value is multiplied by 100 

to yield the percent moisture content as shown in equation 1 below. 

×100 − =

dry

wet dry

Mass

Mass Mass

MC [1] 

Using the moisture analyzer apparatus, the dry trees had needle MCs in the range of 6 % 

to 9 %. The trees that had been kept in a water filled tree stands had MCs of 120 % to 

132 % on the day that they were fire tested. The trees that had been maintained with 

water still exhibited good needle color and retention, and flexible branch tips. The 

uncertainty of the moisture content of the needles measurement is estimated to be ± 2% 

[9]. 

Another measure of MC was made on each tree by using an electrical resistance moisture 

meter on the trunk sections of the tree. The moisture content of the tree trunks were in 
 - 10 - 

the range of 20 % to 35 %. There was no significant difference between the dry trees and 

the trees that had been supplied with water. The resolution of the meter is 0.1 % MC. 

The estimated expanded uncertainty is ± 2 % [10]. 

HEAT RELEASE RATE EXPERIMENTS 

The heat release rate experiments were conducted in the NIST Large Fire Laboratory 

utilizing the 6 m by 6 m oxygen depletion calorimeter. The estimated uncertainty is ± 

11% on the measured heat release rate with an expanded uncertainty of 2. Details on the 

operation and uncertainty in measurements associated with the oxygen depletion 

calorimeter can be found in [11]. The data was recorded at intervals of 1 second on a 

computer based data acquisition system. 

A single Schmidt-Boelter total heat flux gauge was installed 1 m above the base of the 

load cell and 1 m away from the edge of the widest portion of the tree. Results from an 

international study on total heat flux gauge calibration and response demonstrate that the 

uncertainty of a Schmidt-Boelter gauge is ± 8 % [12]. 

The tree in a stand was positioned on a load cell with a resolution of a 0.001 kg. The 

estimated total expanded uncertainty is ± 0.005 kg [13]. The experimental arrangement 

for the heat release rate experiments is shown in schematic form Figure 1 and in a 

photograph in Figure 2. 

Figure 1. Heat release rate experimental arrangement 
 - 11 - 

Maintained Trees 

Ignition experiments were conducted on three of the trees that had been kept in water. 

The ignition source for the trees consisted of a cardboard book of 20 paper matches that 

was ignited by an electrically heated wire. The match book was positioned in each tree 

approximately 0.61 m (2 ft) above the floor and approximately 0.30 m (1 ft) into the tree. 

In each test, the flame from the burning matchbook ignited needles in close proximity 

(within 50 mm or 2 in) of the matchbook, typically a branch with needles that was 

exposed to direct flame. The fire, however, did not spread and the flames selfextinguished within 90 seconds or less. Each of these trees had MC measures of greater 

than 100 %. No measurable changes in heat release rate, heat flux, or mass loss were 

observed during the ignition trials on the high moisture content trees. 

Figure 2. Tree in stand on load cell under oxygen consumption calorimetry hood 

Dry Trees 

Similar ignitions were used on five dry trees. In each case, the matchbook ignition was 

of sufficient energy to ignite the tree, additional branches became involved, and the tree 

continued to burn. The dimensions and initial mass of the trees are given in Table 2. The 

masses indicated for trees 1 through 4 include the mass of the plastic tree stand, 1.20 kg. 
 - 12 - 

Table 2. Dry Tree Measurements prior to testing 

Tree No. Height (m / ft) Width (m / ft) Mass (kg / lbs) 

1 2.05 / 6.75 1.52 / 5.00 12.37 / 28.31 

2 2.54 / 8.33 1.55 / 5.10 13.81 / 30.38 

3 2.39 / 7.83 1.52 / 5.00 13.48 / 29.66 

4 2.23 / 7.30 1.63 / 5.35 10.97 / 24.13 

5 2.06 / 6.77 1.45 / 4.75 14.83 / 32.63 

 

The first four trees were tested in a vertical orientation, as shown in figure 2. The last 

tree, tree 5, was removed from the tree stand and positioned on the load cell as shown in 

figure 3. 

Figure 3. Tree 5 prior to ignition 

Results 

Figure 4a through 4d are a series of photographs that show the stages of fire growth from 

just after ignition to just before total burn out. The span of time from Figure 4a to 4d is 

approximately 60 seconds. Note the amount of vertical flame spread versus the 

horizontal flame spread in photograph 4b. In the last photograph of the series, the 

majority of the tree trunk and many of the larger branches are intact after the needles 

have burned away. 
 - 13 - 

 a. 1 second after ignition b. 10 seconds after ignition 

 c. 35 seconds after ignition d. 61 seconds after ignition 

Figure 4 a – d. Photographs of Tree No. 3 

Figure 5 displays the heat release rate time history of each of the five trees. The peak 

heat release rates ranged from 3.2 MW to 4.3 MW for the trees 1 through 4. The 

majority of the needles were burned in less than 60 seconds. Tree 5 which was 

positioned horizontally had a peak heat release rate of 1.6 MW and a longer burn time. 
 - 14 - 

0

5

10

15

20

25

0 40 80 120 160

Tree 1

Tree 2

Tree 3

Tree 4

Tree 5 side

Heat Flux (kW/m

2

)

Time (s)

0

1000

2000

3000

4000

5000

0 40 80 120 160

Tree 1

Tree 2

Tree 3

Tree 4

Tree 5 (side)

Heat Release Rate (kW)

Time (s)

Figure 5. Heat Release Rate vs Time for dry Fraser firs 

Figure 6. Heat flux vs time for dry Fraser firs 
 - 15 - 

The total heat flux measured from each of the burning trees is shown in Figure 6. The 

peak heat fluxes for trees 1 through 4 show a variation of a factor of two between trees 1 

and 2 and trees 3 and 4. Note that the heat flux peaks occurred after the HRR has peaked. 

The trees were ignited on the side opposite the heat flux sensor. As a result, the unburned 

tree branches blocked some of the radiant energy as the fire was spreading up through the 

tree and the peak heat fluxes occurred as the branches closest to the heat flux sensor were 

burning. Hence the variation in heat flux is a factor of position and view factor for the 

given position of the heat flux sensor. The variation is not representative of overall heat 

flux emission of the trees. In future experiments, additional data can be collected by 

positioning several sensors around the tree and comparing the averages for each fire. 

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 40 80 120 160

Tree 1

Tree 2

Tree 3

Tree 4

Tree 5

Normalized Mass Loss 

Time (s)

Figure 7. Normalized mass loss vs time for dry Fraser fir trees 

Figure 7 shows the normalized mass loss of each tree. The normalized mass loss is the 

mass of each tree as it is burning divided by the its original mass. The first four trees lost 

between 55 % and 60 % of their initial mass within 60 seconds after ignition. Tree 5 did 

not burn as fast or as thoroughly due to its horizontal positioning. During the experiment, 

branches that were near the base end of the horizontally positioned tree did not burn. This 

resulted in less mass loss compared to the trees that were in a vertical position. 
 - 16 - 

ROOM EXPERIMENTS 

In order to examine the impact of a residential sprinkler on a dry tree, two full-scale 

experiments were conducted. The first experiment was conducted in a non-sprinklered 

room. The second experiment was conducted in a room with a residential sprinkler. The 

sprinkler was installed in accordance with the manufacturer’s listing instructions [17]. 

Both rooms had similar furnishings (size, fabric, etc.) and interior finish. The ceiling and 

walls were made from two layers of 12.7 mm (0.5 in) thick gypsum board. The exposed 

surface of the inner layer of gypsum board was painted with two coats of latex paint. The 

floor of each compartment was composed of 19 mm (0.75 in) thick plywood. The 

plywood was covered with 12.7 mm (0.5 in) thick polyurethane foam padding. Nylon cut 

pile carpeting with a polyolefin backing was installed on top of the padding. General 

material descriptions, overall dimensions and the mass of each item is provided in Table 

3. Size and mass of the dry trees used in the experiments is given in Table 4. 

Table 3. Room Furnishings 

Item Materials Dimensions (m / ft) Mass (kg/lbs) 

Padding Polyurethane Foam 3.63 x 3.45 x 0.012 / 

11.9 x 11.3 x 0.04 

20.4 / 44.9 

Carpeting Nylon 

w/Polypropylene 

backing 

3.63 x 3.45 x 0.012 / 

11.9 x 11.3 x 0.04 

22.4 / 49.3 

Sofa Polyester Fabric, 

Polyurethane foam 

seat cushions, 

Polyester batting 

filled back cushions 

2.09 x 0.86 x 0.75 H / 

6.86 x 2.82 x 2.46 H 

47.7 / 105.0 

Chair Polyester Fabric, 

Polyurethane foam 

seat cushions, 

Polyester batting 

filled back cushions 

0.81 x 0.86 x 0.75 H / 

2.66 x 2.82 x 2.46 H 

29.1 / 64.0 

Table Wood 0.61 x 0.71 x 0.60 H / 

2.00 x 2.33 x 1.97 H 

9.6 / 21.1 

Lamp Shade 

(top dia x bottom dia x H) 

Polystyrene 0.18 x 0.48 x 0.31 H/ 

0.58 x 1.58 x 1.00 H 

0.38/ 0.84 

Lamp Ceramic w/ metal 0.15 dia x 0.55 H / 

0.5 dia x 1.8 H 

1.13 / 2.5 

Framed picture Wood frame, glass 0.85 x 0.70 x 0.024 

2.8 x 2.3 x 0.08 

5.0 / 11.0 

Total 135.7 / 298.6 

 
 - 17 - 

Table 4. Dry Tree Measurements prior to room experiments 

 

Room Height (m / ft) Width (m / ft) Mass (kg / lbs) 

Non-sprinklered 2.40 / 7.87 1.50 / 5.00 14.84 / 32.65 

Sprinklered 2.40 / 7.87 1.50 / 5.00 14.64 / 32.21 

Figure 8. Plan view of furnishing and instrumentation arrangement 
 - 18 - 

Figure 8 shows the dimensions of the test compartment, the arrangement of the tree and 

furnishings, and the location of the instrumentation. Each compartment experiment was 

instrumented with thermocouples, a heat flux gage, a radiometer, and video cameras. 

Each thermocouple array was composed of Type K, 0.25 mm bare bead thermocouples. 

One thermocouple array (TC Array 1) was located 0.61 m (2.0 ft) out of the front left 

corner of the compartment. The vertical array had thermocouples located 0.025, 0.3, 

0.61, 0.91, 1.22, 1.52, 1.83, 2.13 m below the ceiling (BC). Another vertical array of 

thermocouples (TC Array 2) was located in the open doorway. This array had 

thermocouples located at 0.025, 0.3, 0.61, 0.91, 1.22, 1.52, and 1.83 m below the door 

lintel (BL). 

Single thermocouples were installed 25 mm below the ceiling and adjacent to the 

ionization smoke alarm and automatic fire sprinkler. 

The standard uncertainty in temperature of the thermocouple wire itself is ± 2.2 °C at 

277 °C and increases to ± 9.5 °C at 871 °C as determined by the wire manufacturer [14]. 

The uncertainty of the temperature in the environment surrounding the thermocouple is 

known to be much greater than that of the wire [15, 16]. Small diameter thermocouples 

were used to limit the impact of radiative heating and cooling. 

A Schmidt Boelter total heat flux gauge was installed near the center of the compartment 

and 0.15 m above the floor. The sensing surface was facing the ceiling. In addition, a 

Schmidt Boelter type radiometer with a sapphire window and 150 o view angle was 

installed 2 m back from the open doorway and 1 m above the floor of the compartment 

on the outside of the compartment. The sensing surface was facing the open doorway. 

As noted before, the uncertainty of a Schmidt-Boelter gauge is ± 8% [12]. 

The experimental compartments were arranged so that the open doorway was under the 

6 m by 6 m oxygen consumption calorimeter. Given that the combustion products collect 

and mix in the compartment prior to flowing into the oxygen consumption calorimetry 

hood for measurement, the heat release rate will not provide a precise time history of the 

fire growth rate. However, the heat release rate data does provide a comparison between 

the two compartment fire experiments. 

Ignition was conducted in a similar manner to the heat release rate experiments. 

The moisture content of the needles was between 6 % and 9 % for each of the trees used 

in these experiments. The moisture content of the trunk of each tree was between 25 % 

and 30 %. These conditions were similar to those of the dry trees used in the heat release 

rate measurements. 

 
 - 19 - 

Results 

The heat release rate, temperatures from the thermocouple arrays and the total heat flux 

near the floor of the room for the non-sprinklered experiment are presented in figures 9 

through 12 respectively. In the non- sprinklered experiment, the smoke alarm activated 

11 seconds after ignition, the sofa was burning approximately 45 seconds after ignition 

and the compartment transitioned to flashover at approximately 60 seconds after ignition. 

Manual suppression using a small hose line from outside the compartment was initiated 

at 90 seconds after ignition. 

0

1000

2000

3000

4000

5000

6000

7000

0 40 80 120 160

Heat Release Rate (kW)

Time (s)

Figure 9. Heat release rate vs. time for the non-sprinklered compartment 

The radiant heat flux, measured 2 m from the doorway and 1 m above the compartment 

floor, outside of the compartment is shown in Figure 13. The peak heat flux of 27 kW/m2

occurs at approximately 70 seconds after ignition. 

Manual 

suppression 

begins 
 - 20 - 

0

200

400

600

800

1000

0 40 80 120 160

25 mm BC

0.30 m BC

0.61 m BC

0.91 m BC

1.22 m BC

1.52 m BC

1.83 m BC

2.13 m BC

Temperature (

oC)

Time (s)

Figure 10. Temperature vs Time, TC Array 1 in the non-sprinklered compartment 

0

200

400

600

800

1000

0 40 80 120 160

25 mm BS

0.30 m BS

0.61 m BS

0.91 m BS

1.22 m BS

1.52 m BS

1.83 m BS

Temperature (

oC)

Time (s)

Figure 11. Temperature vs. Time, TC Array 2 in the non-sprinklered compartment 
 - 21 - 

0

50

100

150

200

0 40 80 120 160

Heat Flux (kW/m2

)

Time (s)

Figure 12. Total heat flux vs time in the non-sprinklered compartment 

0

5

10

15

20

25

30

0 40 80 120 160

Radiative Heat Flux (kW/m2

)

Time (s)

Figure 13. Radiant heat flux outside of the non-sprinklered compartment 
 - 22 - 

The sprinklered compartment experiment had a similar fuel load, dry tree and 

instrumentation as the non-sprinklered compartment. The significant difference was the 

installation of a pendent, residential sprinkler. The sprinkler had an activation 

temperature of 68 oC (155 oF). For a maximum coverage area of 3.7 m (12 ft) by 3.7 m 

(12 ft), the listed minimum water requirement is 34.1 L/min @ 58.1 kPa (9 gpm @ 

8.4 psi) [17]. The water supply to the sprinkler was set to 34.1 L/min (9 gpm). 

The smoke alarm and the sprinkler both activated 10 seconds after ignition. The fire 

developed so rapidly that the temperature of the thermocouple next to the sprinkler 

measured approximately 500 oC (932 oF) just prior to activation. The thermocouple 

adjacent to the smoke alarm measured approximately 350 oC (662 oF) just prior to alarm. 

The heat release rate and temperatures are shown in figures 14 through 16. The rate of 

heat release was reduced by the activation of the sprinkler, however, the heat release rate 

continued to increase as the tree continued to burn. That being said, the water from the 

sprinkler kept portions of the tree below the point of ignition from burning and limited 

the flame spread on the furnishings. As a result, the compartment did not flashover and 

the peak heat release rate was kept to less than one third of the non-sprinklered 

compartment experiment. 

The peak temperatures in the upper half of the compartment ranged from approximately 

200 oC (400 oF) to 700 oC (1300 oF). The temperatures, figure 15, in the lower half of the 

compartment range from 30 oC (86 oF) near the floor to 67 oC (152 oC) at 1.52 m (5 ft) 

below the ceiling. The temperatures in the doorway are significantly lower than in the 

compartment as shown in figure 16. Measured heat flux increased less than 0.5 kW/m2

during the sprinklered experiment. 

Photographs in figure 17 show fire development in the doorways of the sprinklered room 

and the non-sprinklered room. The photographs were taken near the peak heat release 

rate of each experiment, at approximately 60 seconds after ignition. 

 

Post experiment photographs are presented in Figure 18. In the photograph from the 

sprinklered room, thermal damage is limited to the tree and wall and ceiling areas 

adjacent to the tree. Most of the furnishings in the room did not ignite. In fact the lamp 

was still operating after the fire experiment. A small portion of the sofa in contact with 

the tree ignited. The fire spread to the sofa was limited to that initial area and was 

extinguished by the sprinkler within 3 minutes and 30 seconds of ignition. The 

photograph from the non-sprinklered case shows damage that is consistent with postflashover conditions, burn damage throughout the room, floor to ceiling. All of the 

furnishings in the non-sprinklered room ignited. 
 - 23 - 

0

500

1000

1500

2000

0 40 80 120 160

HRR (kW)

Time (s)

Figure 14. Heat release rate vs time for the sprinklered compartment 

0

100

200

300

400

500

600

700

800

0 40 80 120 160

25 mm BC

0.30 m BC

 0.61 m BC

0.91 m BC

1.22 m BC

1.52 m BC

1.83 m BC

2.13 m BC

Temperature (

oC)

Time (s)

Figure 15. Temperature vs time, TC Array 1 in the sprinklered compartment 
 - 24 - 

0

100

200

300

400

500

0 40 80 120 160

25 mm BS

0.30 m BS

0.61 m BS

0.91 m BS

1.22 m BS

1.52 m BS

1.83 m BS

Temperature (

oC)

Time (s)

Figure 16. Temperature vs time, TC Array 2 in the sprinklered compartment 

 

Figure 17. Comparison of the sprinklered room, on the left, with the unsprinklered room 

(Both photos were taken at approximately 1 minute after ignition.) 
 - 25 - 

 

Figure 18. Post fire photographs (Sprinklered room on the left and non-sprinklered room 

on the right.) 

DISCUSSION 

The results from the Fraser fir tree fires were consistent with the previous NIST study on 

Scotch pine trees. In both cases, trees with dry needles were readily ignited with a 

flaming source and generated high heat release rates. The Scotch pines, which ranged 

from 9.5 kg to 20 kg, generated peak heat release rates from 1.6 MW to 5.2 MW. The 

Fraser firs, which ranged from 9.7 kg to 12.6 kg, generated peak heat release rates from 

3.2 MW to 4.3 MW. 

Changing the orientation of the tree from vertical to horizontal, slowed the flame spread 

rate and reduced the peak heat release rate by more than 50%. 

The ignition resistance of the trees with needles that had a percent moisture content in 

excess of 100% was demonstrated. No flames were spread beyond the area of ignition 

and the area of ignition self-extinguished. This was also consistent with the findings of 

the Scotch pine study. 

The last objective of this study was to examine the impact of a residential sprinkler on the 

heat release rate of a dry burning tree in a compartment. A residential sprinkler with one 

of the lowest listed flow rates was chosen for use in this study. 
 - 26 - 

0

1000

2000

3000

4000

5000

6000

7000

0 20 40 60 80 100 120

HRR (kW)

Time (s)

Sprinkler Activation

Manual Suppression

Unsprinklered

Sprinklered

Figure 19 shows a comparison of the heat release rates measured from the two room 

experiments. The sprinkler controlled the fire, prevented flashover and limited the heat 

release rate to less than 1.8 MW. Other fuels in the room were prevented from igniting. 

The non-sprinklered experiment had a peak heat release rate of more than 6 MW, which 

was reached in approximately 70 seconds after ignition. All of the fuels in the room were 

involved in the fire. 

It should be noted that only a single sprinkler was used in this experiment. In a typical 

installation, more than one sprinkler would be exposed to the ceiling jet created by the 

rapidly developing dry tree fire. Hence it would be likely that more than one sprinkler 

would activate. However, in an actual installation of an NFPA 13D system, the flow rate 

of the system is designed to supply the listed flow rate for at least two sprinklers. Hence 

the first sprinkler to activate would have a larger flow rate than subsequent activations, 

potentially having a greater initial impact. 

In 1982, dry tree experiments were conducted in two single story, ranch style model 

homes in Scottsdale, AZ [18]. In each case, a dry Christmas tree was ignited in the living 

room. In one of the experiments, two sprinklers activated and controlled the fire within 

45 seconds. In the other experiment, 6 sprinklers were activated before the fire was 

controlled. Even though more than two sprinklers were activated, the fire was limited to 

the tree itself, similar to the results of the other Scottsdale test and the one room 

experiment presented in this report. 

Figure 19. Sprinklered and non-sprinklered heat release rate comparison 
 - 27 - 

SUMMARY 

Properly maintaining a cut Christmas tree is important to retaining a high moisture 

content in the needles of the tree to limit accidental ignition and prevent rapid flame 

spread. A tree which has dry needles can readily ignite with a flaming source and 

generate heat release rates that are capable of causing flashover in residential scale 

rooms. 

The data from the sprinklered room experiment demonstrated that even under conditions 

of extreme fire growth, approximately 1 MW within 10 seconds after ignition, a single 

sprinkler was able to prevent flashover, control the tree fire and limit the spread of fire to 

other objects. 

ACKNOWLEDGEMENTS 

The author would like to thank Matt Bundy, Steve Kerber, Roy McLane, Jay McElroy, 

Laurean DeLauter, and Jack Lee for assisting with the conduct of the full scale 

experiments. 

The support of U.S. Fire Administration, especially project officer Meredith Lawler is 

greatly appreciated. The cooperation of Ellis Schmidt from the National Christmas Tree 

Association and Thomas Deegan from Viking Sprinkler Corporation proved useful in the 

conduct of these experiments. 

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