In the September 19, 2009 paper entitled “Evaluation of Method 28 Wood 

Heater Burn Rates”, three authors from OMNI Environmental Services assess 

the “veracity of the procedure to weigh burn rates and associated particulate 

emission rates used for the calculation of the overall certification emission rates”. 

The authors conclude that “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.”

I respectfully disagree with both the analysis and the conclusion of this 

paper for the reasons outlined below.

In coming to this conclusion, the authors correctly describe the “different 

techniques” used for ending a test run during (a) AWES sampling in 43 homes, 

and (b) Method 28 test protocol:

(a) In AWES sampling the test ended when the temperature one foot 

above the flue outlet reached 100 degrees F1;

(b) In the Method 28 protocol, the test ends when the “remaining weight of 

the test fuel charge is 0.00 Kg (0.0lb).” The authors note that in the 

Method 28 test protocol, the temperature at the endpoint of the test is 

substantially above 100 degrees F. 

In fact, the temperature at the end of every single Method 28 test ever 

conducted, regardless of burn category, is several hundred degrees more than 

100F. The authors attempt to remedy this apparent inconsistency in the end 

point determination by grafting the amount of time that it would take for the stack 

temperature to decline to 100F onto a series of sample Method 28 runs. This 

effectively doubles the duration of the Method 28 tests, and cuts the already low 

burn rate by half. (See “Evaluation of Method 28 Wood Heater Burn Rates”, pp. 

 

1 In the AWES test, a chimney temperature of over 100 degrees was simply used as a threshold 

to indicate that the stove was being used (since the sampling equipment was “automated, and no 

technician was present). In a study in Klamath Falls/Portland Oregon in 1998-99, a similar 

methodology was used and some stoves never reached 100 degrees, because they were 

operated continuously and the test period (168 hours, or one full week) ended without stack 

temperatures ever dropping to 100 degrees. Other stoves dropped to 100 degrees and were 

restarted, with the total test time below 200 degrees still a small percentage of total test time. 

The OMNI authors admit, "one test cycle may consist of multiple fuel loads."
9-15) The authors then posit that since the burn rates with the artificial grafts are 

well below “real-world” averages, the weighting must be incorrect (i.e. 

“overweighting of lower burn rates”).

What the authors fail to mention is that the AWES sampling was 

approximately one week long (168 hours) and the cool down period in the 

AWES sampling represented a small fraction (5%, more or less) of the total test 

time, if any time at all (see footnote 1) whereas in the Method 28 Testing (5-6 

hours) the same cool down period, when grafted onto the test results, represents 

over 50% of the test time. 

A more reasonable approach to reconciling this apparent discrepancy 

would be to artificially graft on an additional 5% duration to the Method 28 Tests, 

rather then the 50%+ that the authors erroneously add. Alternatively, maybe the 

Method 28 burn rates need to be adjusted upward by 5%, more or less, to 

account for distinction between the 100F and zero mass endpoints.

To understand why the conclusion of this paper is in error, one can look at 

the Klamath Falls/Portland Oregon Study conducted by EPA in 1998-1999.2 In 

this study, 16 stoves were operated over 5,488 hours and 5,802 kgs (12,764 lbs) 

of wood were burned. The average burn rate was 1.1 kg/hr. The average stack 

temperature for the 5,488 hours of testing was over 400 degrees F, which is 

consistent with the idea that the period of time when the stoves were idling down 

to a 100-degree stack temperature was small. 

Current Category 1 and Category 2 burn rates in Method 28 are 

reasonable slices of typical low burns. The zero-mass endpoint rule is not a bad 

way to approximate the burn rates seen in the long multiple-load AES test with its 

100F endpoint rule; the addition of a cool-down tail to a two to four -hour test 

(see “Evaluation of Method 28 Wood Heater Burn Rates”, page 9) is a terrible 

(and misleading) way to adjust for this difference. To discount the accuracy, 

usefulness and weighting based on the fact that a 168-hour field test (AWES 

Sampling) and a 6-8 hour lab test (Method 28) have different end points is an 

error.

On a related note: The Sept 9 ASTM draft proposes to do away with the Category 1 burn 

(see 9.5.1) and increase (but not decrease) the amount of start-up air (see 9.5.5.4), both 

of which would push the test protocol away from actual “in-home” burn rates.

 

2

 “Evaluation of Method 28 Wood Heater Burn Rates” indicates that raw data from original studies 

in VT, NY and OR are no longer available. . However, Paul Tiegs at OMNI indicated that the 

original tests were “weeks long” and used the same equipment as the Klamath study. In this 

sense the Klamath study may be representative.
ADDENDUM 1 

The current (May 22, 2009) EPA list of certified woodstoves consists of 

approximately 657 certified designs (after deleting duplicate listings for the 

same stove under multiple company names). The 657 total consists of 432 

non- catalytic certifications, and 225 catalytic certifications. 

Of the non-catalytic certifications 190 (44%) were certified with a low 

output of less than 0.8 kg/hr (the current standard for a low burn). 95% of noncatalytic stoves (412 of 432) had a low burn of less than 0.9 kg/hr. 

Of the catalytic certifications 183 (81%) were certified with a low output of 

less than 0.8 kg/hr (the current standard for a low burn). 92% of non-catalytic 

stoves (412 of 432) had a low burn of less than 0.9 kg/hr. 

Greater than 98% of all stoves had a burn rate of less than 1.00 kg/hr. 

Only two stoves (both catalytic) had a low burn rate of greater than 1.06 kg/hr. 

Low Burn: Non-Catalytic Certifications

Burn Rate (Kg/hr) Adjusted 

Particulate 

Emissions 0-.49

.50-

.59 .60-.69 .70-.79 .80-.89

.90-

.99 1.00+ Total

0.00-3.00 2 3 5 27 37 3 0 77

3.01-4.0 0 2 13 33 60 0 2 110

4.01-5.0 0 0 14 21 59 1 0 95

5.01+ 5 8 24 33 66 10 4 150

TOTAL 7 13 56 114 222 14 6 432

1.6% 3.0% 13.0% 26.4% 51.4% 3.2% 1.4% 100.0%

Data from May 22, 2009 EPA list

Low Burn: Catalytic Certifications

Burn Rate (Kg/hr) Adjusted 

Particulate 

Emissions 0-.49 .50-.59 .60-.69 .70-.79 .80-.89

.90-

.99 1.00+ Total

0.00-3.00 10 18 24 45 12 2 2 113

3.01-4.0 4 9 25 21 6 9 3 77

4.01-5.0 2 3 5 9 3 0 1 23

5.01+ 1 1 3 3 2 2 12

TOTAL 17 31 57 78 23 13 6 225

7.6% 13.8% 25.3% 34.7% 10.2% 5.8% 2.7% 100.0%

Data from May 22, 2009 EPA list