Evaluation of Method 28 Wood Heater Burn Rates 

Prepared for 

W. Allan Cagnoli 

Director, Government Affairs 

Hearth, Patio & Barbecue Association 

1901 North Moore Street, Suite 600 

Arlington, VA 22209 

Prepared by 

James E. Houck, Jeremy Clark, and Thomas Christensen 

OMNI Environmental Services, Inc. 

13327 NE Airport Way 

Portland, OR 97230 

September 21, 2009 
1

Introduction 

OMNI Environmental Services, Inc. (OMNI) was contracted by the Hearth, Patio & Barbecue 

Association (HPBA) to evaluate U.S. EPA Method 28 wood heater burn rates. Specifically, the 

veracity of the procedure to weigh burn rates and associated particulate emission rates used for 

the calculation of the overall certification emission rates was assessed. 

Burn rates are simply calculated by dividing the dry mass of wood burned by the duration of the 

fire. The duration of the fire has been defined a number of ways. For a given fire, methods for 

defining the fire duration that produce a larger numerical value produce a smaller calculated burn 

rate and visa versa because the burn rate is a simple ratio and the mass of wood in the numerator 

remains unchanged. A different technique was used to develop the probabilities of a given burn 

rate for use with the Method 28 test than is actually used in the Method 28 test itself resulting in 

the over-weighing of lower burn rates and their associated emission rates as measured following 

the Method 28 procedures. 

The burn rate weighted probabilities for calculating weighted average emission rates 

promulgated as part of the NSPS are based on in-home data collected from 43 homes in 

Vermont, New York, and Oregon with an automated emission sampler (AES) which was later 

referred to as the automated woodstove emission sampler (AWES)1-3 (see Figure 1). The 

AES/AWES was developed and deployed by OMNI. The AES/AWES system monitored 

temperatures in the connector chimney pipe at one foot above the wood heater. The endpoint of 

a “burn” was determined to be when the temperature dropped to 100 °F in the chimney connector 

pipe at the one-foot monitoring point. 

This approach for weighting burn rates was explicitly stated in the Proposed Rule and Notice of 

Public Hearing of the Federal Register “Standards of Performance for New Stationary Sources; 

Standards of Performance for New Sources; Residential Wood Heaters,” which states: 

“Burn rate data collected during in-home studies in the states of Vermont, New 

York, and Oregon were compiled to develop a frequency distribution of burn rate 

patterns and were used as the basis for developing a weighting scheme for use in 

this NSPS. The burn rate frequency distribution demonstrated that approximately 

50 percent of the measured burn rates were less than 1.2 kg/hr. 

“The committee agreed that the frequency distribution developed from the inhome studies reasonably represented the full range of actual consumer wood use. 

This distribution heavily weights the lower burn rates which are common when 

heaters are operated overnight and also spreads the weighting out over the a [sic] 

range that includes the higher burn rates that would be found in colder climates 

and on the coldest days. This distribution was felt to be more representative 

overall than other averaging schemes considered. For these reasons, the 

committee selected this as the appropriate weighting scheme.” 2
2

In contrast to the “one-foot, 100 °F” endpoint used to calculate burn rates from the AES/AWES 

data, the NSPS Method 28 test procedures use an endpoint defined as, “The test run is completed 

when the remaining weight of the test fuel charge is 0.00 Kg (0.0 lb).”4

 The temperature at one 

foot above the wood heater is generally well above 100 °F when the remaining weight of the test 

fuel charge is 0.00 Kg. This dichotomy will cause a burn rate measured and calculated by the 

Method 28 test procedure to be in actuality much smaller when defined by the “one-foot, 100 °F” 

endpoint used to develop the weighted probabilities in Table 28-1. 

It should be noted that the purpose of this research was to simply document the flaw in the logic 

using the in-home AES/AWES data to develop the weighted probability tables for Method 28 

due to the differences in calculated burn rates caused by their different endpoint definitions. Not 

evaluated were the representativeness of data from 43 homes in New York, Vermont, and 

Oregon to the Nation as a whole, and the effects on burn rates of: (1) in-home use of multiple 

tree species for fuel, including hardwoods, in contrast to Douglas fir, (2) the use of cordwood in 

contrast to dimensional lumber made into structured cribs, (3) the various moisture contents of 

in-home fuel, (4) the inclusion of both hot and cold starts, and (5) various in-home draft 

conditions influenced by differing chimney heights, chimney design and integrity, indoor and 

outdoor temperatures, as well as home elevation. 

Review of In-Situ AES/AWES 

The AES/AWES study was originally conducted to compare woodstove particulate emission 

sampling methods under in-situ and laboratory conditions and to compare the in-situ 

performance of different woodstove types. In-home evaluation of the woodstoves took place in 

34 homes in Glens Falls, New York (elevation: 344 ft) and Waterbury, Vermont (elev.: 614 ft) 

from 1985 to 1987, six homes in Portland, Oregon (elev.: 50 ft) in 1987, and 3 homes in Klamath 

Falls, Oregon (elev.: 4,099 ft) in 1990. All stoves were either 1986, 1988, or 1990 Oregon 

certified, or judged to be capable of certification. Test homes were selected that met local 

building and safety codes, could accommodate and properly operate the stoves, and could allow 

access to the AWES sampling equipment. Homeowners were given no specific training beyond 

being provided with, and advised to read, the instruction manual for their given stove model. 

All test fuel was provided by the homeowners and was region-specific; both hardwood and 

softwood were used. The AWES system incorporated a wood scale which was attached to a 

data logger system. Fuel weight consumed during testing was determined by the homeowner 

placing wood on the scale and noting current coalbed conditions prior to loading the stove. 

AWES sampling was conducted only during the stove operation time, which was defined as 

stack temperature greater than 100 °F at one foot above the flue collar. Fuel loading could occur 

at any point in the test cycle, therefore one test cycle may consist of multiple fuel loads. It 

should furthermore be noted that the raw data from all three studies is no longer available. 
3

Methodology 

To document the difference in the two burn rate (endpoint) approaches, two tasks were 

conducted by OMNI: 

(1) From OMNI’s archived records, endpoint temperatures at eight feet (above floor/scale) were 

obtained for ten different certified heater models at each burn rate that was part of their Method 

28 certification tests. (Model names have been kept confidential.) The mean, median, and 

standard deviation of endpoint temperatures were calculated and for illustrative purposes a plot 

of endpoint temperatures vs. burn rates was constructed. To provide comparability among runs 

all eight-foot temperatures were normalized to an average 68 °F room temperature. 

(2) Because Method 28 requires temperatures to be monitored at eight feet (above floor/scale) 

not at one foot (above heater) as used by the AES/AWES system, four tests were run with two 

certified non-catalytic heaters following Method 28 procedures with thermocouples at both the 

one-foot and eight-foot monitoring points in order to develop a correlation between the one-foot 

and eight-foot temperatures. In addition to these four tests, one run for each wood heater was 

previously conducted by OMNI and these data are also reviewed here. In total there are data for 

Method 28 categories 2, 3, and 4 burn rates for each wood heater. The category 2 burn rates 

provided here were the lowest burn rates that could reasonably be achieved with the heaters. 

Category 1 burn rates were originally targeted but could not be reached with the heaters. 

Average one-foot and eight-foot temperatures over the course of the Method 28 burn and 

instantaneous one-foot and eight-foot temperatures at the endpoint of the Method 28 test were 

tabulated. To provide comparability among runs, all one-foot and eight-foot temperatures were 

normalized to a 68 °F room temperature. While not in the original scope of work, the total time 

it took the stack to cool to 100 °F at the one-foot height was also measured for the four new runs 

that were conducted. 

Results 

The eight-foot temperatures obtained for ten certified wood heaters along with burn rates and 

times to the end of the tests are provided in Table 1. The means, medians and standard 

deviations for the overall data set, as well as for each burn rate category, are summarized in 

Table 2. A plot of end-of-test eight-foot temperatures versus burn rates are shown as Figure 1. 

The average and instantaneous end-of-test one-foot and eight-foot temperatures for the two 

certified wood heaters that underwent the six tests at category 2, 3 and 4 burn rates are provided 

in Table 3. Plots of one-foot, eight-foot, and room temperatures versus time for all six tests are 

provided as Figures 2-7. 
4

Summary 

The mean and median end-of-test eight-foot stack temperatures for the ten certified wood heaters 

across all burns (n = 46) were 225 °F and 199 °F, respectively. The average difference between 

one-foot and eight-foot temperatures as determined by temperatures averaged until the end of the 

Method 28 test was 173 °F and the difference as determined by the instantaneous endpoint 

temperatures was 122 °F. These data suggest that typical one-foot temperatures at the end of the 

Method 28 test are in the range of 320 °F to 400 °F, well above the 100 °F temperature used to 

determine burn rate weighted probabilities for Method 28 calculation of emission rates. Not 

surprisingly, higher endpoint temperatures corresponded to higher burn rates. A hypothetical 

calculation was run as though the Method 28 burn rates were to be taken at face value and the 

AES/AWES and Method 28 conditions were directly comparable. The average hypothetical 

burn rate (had the Method 28 endpoint been a one-foot stack temperature of less than 100 °F) is 

more than three times lower than the burn rate calculated from the actual Method 28 endpoint. 

References 

1. Federal Register, Table 28-1, Vol. 53, No.38, Page 5910, February 26, 1988. 

2. Federal Register, Vol. 52, No. 32, Page 5002, February 18, 1987. 

3. Federal Register, Vol. 53, No. 38, Page 5905, Section 9, Reference 3, February 26, 1988. 

4. U.S. EPA Technology Transfer Network, Emission Measurement Center, CFR 

Promulgated Test Methods, Method 28, Certification and Auditing – Wood Heaters, 

page 1426, www.epa.gov/ttn/emc/promgate.html, February 2000. 

5. Radian Corporation, OMNI Environmental Services, Inc., Cumulative Probability for a 

Given Burn Rate Based on Data Generated in the CONEG and BPA Studies. Package of 

materials submitted to the Fifth Session of the Regulatory Negotiation Committee. July 

16-17, 1986. 
Figure 1. AES/AWES frequency distribution of burn rates: VT, NY, & OR (43 homes). Figure redrafted from reference 5
Table 1 

End-of-test stack temperature at eight feet, normalized end-of-test stack temperature at eight feet, burn rates, a

EPA certified wood heaters tested by OMNI

End of Burn Temperatures (°F) Stove Code Run 

Stack @ 8 ft Stack @ 8 ft Corrected* Room Burn Rate (Kg/h, dry) Total Test T

1 143 137 74 0.91 3.17 2 274 263 79 2.17 1.33 3 159 147 80 1.16 2.50 

320S931 

4 182 175 75 1.56 2.00 

1 152 154 66 0.98 5.00 2 203 204 67 1.23 3.83 3 479 480 67 3.90 1.33 

310S013 

4 287 289 66 1.84 2.50 

1 122 119 71 0.92 7.83 2 141 140 69 1.08 6.67 3 358 358 68 3.34 2.08 4 227 225 70 1.67 4.33 

337S160 

5 184 187 65 1.06 6.67 

1 124 109 83 0.89 6.83 2 369 372 65 2.81 2.17 3 262 258 72 1.89 3.33 4 194 185 77 1.23 4.83 

347S820 

5 147 144 71 0.98 6.00 

1 149 140 77 0.84 3.33 2 291 277 82 1.82 1.50 3 141 138 71 0.88 3.17 

311S512 

4 306 299 75 2.07 1.33 

 *Corrected stack temperatures are normalized to 68 °F room temperature.
Table 1 (cont.) 

End of Burn Temperatures (°F) Stove Code Run 

Stack @ 8 ft Stack @ 8 ft Corrected* Room Burn Rate (Kg/h, dry) Total Test T

1 156 153 71 1.00 4.67 2 162 164 66 1.24 4.17 3 390 389 69 3.06 1.67 4 191 191 68 1.50 3.50 

320S514 

5 179 177 70 1.15 4.50 

1 165 166 67 0.95 4.67 2 210 212 66 1.27 3.50 3 199 202 65 1.17 3.83 

331S831 

4 365 364 69 1.79 2.50 

1 199 196 71 0.93 2.17 2 383 377 74 1.76 1.17 3 245 248 65 1.23 1.83 4 246 246 68 1.22 1.83 5 284 284 68 1.32 1.67 

381S291 

6 189 192 65 0.90 2.67 

1 192 181 79 0.99 7.83 2 181 169 80 1.01 7.67 3 362 343 87 2.96 2.67 4 237 227 78 1.50 5.17 

322S722 

5 216 211 73 1.10 7.17 

1 154 143 79 1.04 3.67 2 148 134 82 0.94 4.00 3 319 313 74 2.15 1.67 

360S172 

4 259 248 79 1.78 2.17 

 *Corrected stack temperatures are normalized to 68 °F room temperature.
8

Table 2 

Means, standard deviations, medians, and n values of normalized end-of-test stack temperatures at 

eight feet and associated burn rates – EPA certified heaters tested by OMNI*

Overall Category 2 Category 3 Category 4 

Mean 225 °F 

(1.50 Kg/h) 

169 °F 

(1.04 Kg/h) 

260 °F 

(1.64 Kg/h) 

352 °F 

(2.81 Kg/h) 

St. 

Dev. 

86 °F 

(0.72 Kg/h) 

35 °F 

(0.13 Kg/h) 

62 °F 

(0.21 Kg/h) 

66 °F 

(0.65 Kg/h) 

Median 199 °F 

(1.23 Kg/h) 

165 °F 

(1.01 Kg/h) 

253 °F 

(1.72 Kg/h) 

350 °F 

(2.89 Kg/h) 

n = 46 26 12 8 

*Burn rate statistics are in parentheses (in Kg/h, dry basis), stack temperatures normalized to 68 °F room 

temperature

Figure 2. End-of-test stack temperature at eight feet plotted against burn rate for each run on ten 

EPA certified stoves tested by OMNI; stack temperatures normalized to 68 °F room temperature.
Table 3 

Summary of average eight-foot and one-foot flue temperatures to the end of Method 28 test, instantaneous tempera

Method 28 test, and time until 100 °F temperature was reached at one foot

Average Temperature to 

Method 28 Endpoint (°F) Temperature at Endpoint (°F) Stove 

Model 

Burn 

Rate 

(Kg/h) 

Category 

Room 8 ft* 1 ft* Difference Room 8 ft* 1 ft* Difference

Time to 

Method 28 

Endpoint 

1.07 2 89 248 404 156 88 169 263 94 5 h, 20 min 

A 1.49 3 74 327 527 200 73 230 373 143 3 h, 40 min 

2.85 4 86 529 738 209 84 387 521 134 1 h, 50 min 

1.26 2 82 305 450 145 83 234 348 114 4 h, 10 min 

B 1.59 3 79 350 494 144 78 293 414 121 3 h, 5 min 

2.85 4 87 572 758 186 86 411 535 124 1 h, 45 min 

Average 1.85 - 83 389 562 173 82 287 409 122 3 h, 18 min 

*Normalized to 68 °F. **It is clear the hypothetical burn rate does not reflect the burn rate calculated by the AES/AWES method becauconsiderations, including: percent moisture variation, hot vs. cold starts, cordwood vs. dimensional lumber and hsoftwood fuel, multiple cooling “tails” in the AES/AWES data due to multiple fuel loads, draft variation due to cconfiguration, indoor/outdoor temperature differentials, etc. Note: The “1 ft” measurement is defined as one foot above the top of the heater. 

The “8 ft” measurement is defined as eight feet above the floor/scale. The difference between the two measurements for heater model A is 4 feet, 7 3/8 inches. 

The difference between the two measurements for heater model B is 4 feet, 3 inches.
Figure 3. Stack temperatures, heater model A, category 2 (1.07 Kg/h); represents the lowest burn rate achievable 
Figure 4. Stack temperatures, heater model A, category 3 (1.49 Kg/h). Note: Test was not run to 100 °F at one foot.
Figure 5. Stack temperatures, heater model A, category 4 (2.85 Kg/h).
Figure 6. Stack temperatures, heater model B, category 3 (1.26 Kg/h); represents the lowest burn rate achievable 
Figure 7. Stack temperatures, heater model B, category 3 (1.59 Kg/h). Note: Test was not run to 100 °F at one foot.
Figure 8. Stack temperatures, heater model B, category 4 (2.85 Kg/h).