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, 20091 Introduction

OMNI Environmental Services, Inc. (OMNI) wa s contracted by the H earth, Patio & Barbecue Association (HPBA) to evaluate U.S. EPA Met hod 28 wood heater burn rates. Specifically, the veracity of the procedure to weigh burn rates an d associated particulate emission rates used for
the calculation of the overall cer tification emission rates was assessed.

Burn rates are simply calculat ed by dividing the dry mass of w ood 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 probab ilities 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 as sociated emission rates as measured following the Method 28 procedures.

The burn rate weighted probabilities for cal culating 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 w oodstove emission sampler (AWES) 1-3 (see Figure 1). The AES/AWES was developed and deployed by OM NI. 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-f oot monitoring point.

This approach for weighting burn rates was explic itly stated in the Proposed Rule and Notice of Public Hearing of the Federal Re gister “Standards of Performan ce for New Stationary Sources; Standards of Performance for New Sources; Residential Wood Heaters,” which states:

“Burn rate data collected during in-home studies in th e states of Vermont, New York, and Oregon were compiled to devel op a frequency distribution of burn rate patterns and were used as the basis for de veloping a weighting scheme for use in this NSPS. The burn rate frequency dist ribution demonstrated that approximately 50 percent of the measured burn rates were less than 1.2 kg/hr.

“The committee agreed that the freque ncy distribution developed from the inhome studies reasonably repr esented the full range of act ual 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 highe r burn rates that would be found in colder climates and on the coldest days. This distributi on was felt to be more representative overall than other averaging schemes considered. For these reasons, the committee selected this as the appropriate weighting scheme.” 22 In contrast to the “one-foot, 100 °F” endpoint us ed to calculate burn rates from the AES/AWES data, the NSPS Method 28 test procedures use an e ndpoint 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 abov e 100 °F when the remaining weight of the test fuel charge is 0.00 Kg. This dichotomy will cau se a burn rate measured and calculated by the Method 28 test procedure to be in actuality mu ch smaller when defined by the “one-foot, 100 °F” endpoint used to develop the wei ghted 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 da ta from 43 homes in New York, Vermont, and Oregon to the Nation as a whole, and the effect s on burn rates of: (1) in-home use of multiple tree species for fuel, including hardwoods, in contra st to Douglas fir, (2) the use of cordwood in
contrast to dimensional lumber made into structur ed cribs, (3) the various moisture contents of in-home fuel, (4) the inclusion of both hot a nd cold starts, and (5) various in-home draft conditions influenced by differing chimney hei ghts, 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 laborator y 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 (e lev.: 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 certificat ion. Test homes were selected that met local building and safety codes, coul d accommodate and prope rly operate the stoves, and could allow access to the AWES sampling equipment. Home owners 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 coal bed conditions prior to loading the stove. AWES sampling was conducted only during the st ove operation time, which was defined as stack temperature greater than 100 °F at one foot above the flue co llar. 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 record s, endpoint temperatur es at eight feet (a bove 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 cons tructed. 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 co rrelation between the one-foot and eight-foot temperatures. In addition to thes e four tests, one run for each wood heater was previously conducted by OMNI and th ese data are also reviewed here . In total there are data for Method 28 categories 2, 3, and 4 burn rates for e ach wood heater. The category 2 burn rates provided here were the lowest bur n 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 a nd eight-foot temperatures at the en dpoint 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 al ong 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 cate gory, are summarized in Table 2. A plot of end-of-test eight-foot temper atures versus burn rates are shown as Figure 1.

The average and instantaneous end-of-test one-f oot and eight-foot temp eratures for the two
certified wood heaters that underwent the six test s at category 2, 3 and 4 burn rates are provided in Table 3. Plots of one-foot, ei ght-foot, and room temperatures ve rsus time for all six tests are provided as Figures 2-7.4 Summary

The mean and median end-of-test eight-foot stack temperatures fo r 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 temperat ures 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 co rresponded to higher burn rates. A hypothetical calculation was run as though the Method 28 burn rate s were to be taken at face value and the
AES/AWES and Method 28 conditi ons 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 Networ k, Emission Measurement Center, CFR Promulgated Test Methods, Method 28, Cer tification and Auditing – Wood Heaters, page 1426, www.epa.gov/ttn/emc/promgate.html , February 2000.

5. Radian Corporation, OMNI Environmental Se rvices, 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, 1986.5Figure 1. AES/AWES frequency distribution of burn rates: VT,NY, & OR (43 homes). Figureredrafted from reference5.6Table 1End-of-test stack temperature at eight feet, normalized end-of-test stack temperature at eight feet, burn rates, and total testtime –EPA certified wood heaters tested by OMNIEnd of Burn Temperatures (°F)Stove CodeRunStack @ 8 ftStack @ 8 ft Corrected*RoomBurn Rate (Kg/h, dry)Total Test Time (h)1 143 137 74 0.91 3.172 274 263 79 2.17 1.333 159 147 80 1.16 2.50320S9314 182 175 75 1.56 2.001 152 154 66 0.98 5.002 203 204 67 1.23 3.833 479 480 67 3.90 1.33310S0134 287 289 66 1.84 2.501 122 119 71 0.92 7.832 141 140 69 1.08 6.673 358 358 68 3.34 2.084 227 225 70 1.67 4.33337S1605 184 187 65 1.06 6.671 124 109 83 0.89 6.832 369 372 65 2.81 2.173 262 258 72 1.89 3.334 194 185 77 1.23 4.83347S8205 147 144 71 0.98 6.001 149 140 77 0.84 3.332 291 277 82 1.82 1.503 141 138 71 0.88 3.17311S5124 306 299 75 2.07 1.33*Corrected stacktemperatures are normalized to 68 °F room temperature.7Table 1 (cont.)End of Burn Temperatures (°F)Stove CodeRunStack @ 8 ftStack @ 8 ft Corrected*RoomBurn Rate (Kg/h, dry)Total Test Time (h)1 156 153 71 1.00 4.672 162 164 66 1.24 4.173 390 389 69 3.06 1.674 191 191 68 1.50 3.50320S5145 179 177 70 1.15 4.501 165 166 67 0.95 4.672 210 212 66 1.27 3.503 199 202 65 1.17 3.83331S8314 365 364 69 1.79 2.501 199 196 71 0.93 2.172 383 377 74 1.76 1.173 245 248 65 1.23 1.834 246 246 68 1.22 1.835 284 284 68 1.32 1.67381S2916 189 192 65 0.90 2.671 192 181 79 0.99 7.832 181 169 80 1.01 7.673 362 343 87 2.96 2.674 237 227 78 1.50 5.17322S7225 216 211 73 1.10 7.171 154 143 79 1.04 3.672 148 134 82 0.94 4.003 319 313 74 2.15 1.67360S1724 259 248 79 1.78 2.17*Corrected stacktemperatures 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 temper ature at eight feet plotted agai nst burn rate for each run on ten EPA certified stoves tested by OMNI; stack temperat ures normalized to 68 °F room temperature.9Table 3Summary of average eight-foot and one-foot flue temperatures tothe end of Method 28 test, instantaneous temperatures at the end of theMethod 28 test, and time until 100 °F temperature was reached at one footAverage Temperature toMethod 28 Endpoint (°F)Temperature at Endpoint (°F)StoveModelBurnRate(Kg/h)CategoryRoom8 ft*1 ft*DifferenceRoom8 ft*1 ft*DifferenceTime toMethod 28EndpointTime to100 °F@ 1 ftHypothetical100 °FBurn Rate**(Kg/h)1.07 2 89 248 404 156 88 169 263 94 5 h, 20 min11 h,39 min0.4901.49 3 74 327 527 200 73 230 373 143 3 h, 40 min nd ndA2.85 4 86 529 738 209 84 387 521 134 1 h, 50 min8 h,0 min0.6521.26 2 82 305 450 145 83 234 348 114 4 h, 10 min9 h,50 min0.5331.59 3 79 350 494 144 78 293 414 121 3 h, 5 min nd ndB2.85 4 87 572 758 186 86 411 535 124 1 h, 45 min8 h,23 min0.596Average 1.85 - 83 389 562 173 82 287 409 122 3 h, 18 min9h,28 min0.568*Normalized to 68 °F.**It is clear the hypothetical burn rate does not reflect the burn rate calculated bythe AES/AWES method because of multipleconsiderations, including: percent moisture variation, hot vs. cold starts, cordwood vs. dimensional lumber and hardwood vs.softwood fuel, multiple cooling “tails” inthe AES/AWES data due to multiple fuelloads, draft variation due to chimneyconfiguration, indoor/outdoor temperature differentials, etc.Note: The “1 ft” measurement is definedas one foot above the top of the heater.The “8 ft” measurement is defined aseight feet abovethe floor/scale.The difference between the two measurements for heater model A is 4 feet, 73/8inches.The difference between the two measurementsfor heater model B is 4 feet, 3 inches.10Figure 3. Stack temperatures, heater modelA, category 2 (1.07 Kg/h); represents thelowest burn rate achievable with the heater.11Figure 4. Stack temperatures, heatermodel A, category 3 (1.49 Kg/h).Note: Test was not runto 100 °F at one foot.12Figure 5. Stack temperatures, heater model A, category 4 (2.85 Kg/h).13Figure 6. Stack temperatures, heater modelB, category 3 (1.26 Kg/h); represents thelowest burn rate achievable with the heater.14Figure 7. Stack temperatures, heatermodel B, category 3 (1.59 Kg/h).Note: Test was not runto 100 °F at one foot.15Figure 8. Stack temperatures, heatermodel B, category 4 (2.85 Kg/h).