Rebuttal To:

 “Evaluation of Method 28 Wood Heater Burn Rates”

Prepared by James E. Houck, Jeremy Clark, and Thomas Christensen

OMNI Environmental Services, Inc.

Dated September 21, 2009

This rebuttal prepared for:

Gil Wood, Staff Lead for Residential Wood Heater NSPS

United States Environmental Protection Agency

Prepared by

Thomas P. Morrissey

Woodstock Soapstone Company, Inc., 66 Airpark Road, West Lebanon, NH  03768

‐and‐

Harold Garabedian

Energy & Environmental Analytics, 132 Northfield St., Montpelier, VT  05602

November 2, 2012
1

In September 2009, a report commissioned and paid for by the Hearth, Patio, & 

Barbecue Association (hereafter HPBA) and entitled “Evaluation of Method 28 Wood Heater 

Burn Rates” (hereafter the “Houck Paper”) was published by OMNI Environmental Services, Inc. 

and subsequently presented to EPA. The Houck Paper suggests a rationale for increasing the 

Low Burn Rate required by Method 28, or reducing the weighting applied to the Low Burn Rate, 

or both.  Raising the low burn rate requirement would be advantageous to stoves that do not 

burn cleanly at low outputs1.

The Houck Paper has been referenced repeatedly by some manufacturers, and by HPBA,

as evidence that burn rates used in Method 28 should be increased.  However, the conclusions 

of the Houck Paper are not supported by the data it cites.

The Houck Paper focused almost exclusively on (1) the “endpoint” determinations for 

certification tests conducted under Method 28 (hereafter “Method 28 Tests”), and (2) the 

“endpoint” for in‐situ tests using the automated woodstove emission sampler (AWES) 

conducted in Vermont, New York, and Oregon (hereafter “In‐Situ Tests”) which were used to 

determine burn rates and weighting in the original NSPS.  The Houck paper makes 

“hypothetical” calculations based on the assumption that “AWES and METHOD 28 conditions 

were directly comparable,” which they are not, as explained below.

The two respective “endpoint” conditions are as follows:

Method 28 Test Endpoint:

Method 28 Tests for EPA Certification are conducted in a laboratory setting with the 

appliance on a large scale.  Following the Method 28 Test protocol, a “pre‐burn” is conducted 

under prescribed conditions prior to each test, and a coal‐bed of a specified weight is required 

to be developed.  After the coal‐bed is developed, the scale is reset to zero (0.00 kgs) and the 

test fuel is loaded.  When the test fuel has been completely consumed and the scale again 

reaches 0.00 kg, the Method 28 Test endpoint has been reached.  Thus, the end of the test 

occurs when is all the fuel has been consumed, and the burn rate is defined as the weight of fuel

consumed divided by the time elapsed to consume that fuel. The weight of the coal‐bed is not 

                                                        1 A database prepared by HPBA in 2009 showed that of 110 “non‐catalytic” stoves still in production, the average 

emissions was 3.88 g/hr at low burn (Category 1), vs. 3.45 g/hr at Category 2; 2.99 at Category 3; and 4.43 at 

Category 4.  Any (a) increase in the low burn rate or (b) decrease in the weighting of low burns would boost the test 

results of these stoves.  The recently proposed ASTM Test Method would achieve both of these goals.
2

included in the calculation.  A majority of Method 28 Tests typically last between 2 and 8 hours, 

though some are shorter or longer.

In­Situ Test “Endpoint”:  

The In‐Situ Tests were 7‐day in‐home studies, conducted with an automated woodstove

emission sampler (AWES).  Since an In‐Situ Test stove could not be placed on a scale, the fuel 

was weighed separately before it was loaded.    The Houck Paper states that

“The endpoint of a “burn” was determined to be when the temperature dropped to 

below 100 degrees F in the chimney connector at the one‐foot monitoring point.” 2

However this “endpoint” definition was not actually used in the definition of the dataset.  

The 100oF marker was simply used to indicate that the stove was no longer in use, and to 

inform the AWES sampler to stop collecting emissions.  

In‐Situ Tests were conducted over a one‐week period, and ended at the end of seven 

days.  The AWES stopped collecting data when and if the stack temperature at 12 inches above 

the flue outlet dropped below 100 degrees, as noted above.  The vast majority of In‐Situ Tests 

ended with temperatures well above 100 degrees, since the end of the test was determined by 

time (7 days) and not flue temperature.  In some of the 7‐day In‐Situ tests, the woodstove being 

sampled never reached the 100‐degree flue temperature.  In other tests the woodstove reached 

the 100‐degree flue temperature in the early morning or late afternoon because the stove was 

not capable of sustaining a fire until the resident awoke in the morning and re‐loaded the stove, 

or returned from work to re‐load the stove before the fire went out.  In other cases, the 

woodstove reached the 100‐degree temperature and stayed at that temperature for 8‐10 hours 

(while the resident was away or otherwise chose not to operate the stove).  

The In‐Situ Tests were conducted over one week periods, with sampling interruptions 

only when the stack temperature at 12” declined below 100 degrees.  The In‐Situ Testing 

encompassed over 18,500 hours of data.  There were approximately 18,588 hours of woodstove 

operation in the In‐Situ Testing, and approximately 240 occasions when the stove being tested 

reached a 100 degree flue temperature.  In most cases where the stove reached 100 degrees, it 

was immediately refueled.  The average uninterrupted test period between these 100o events

was approximately 77.45 hours (18,588 total hours/240 one hundred degree occasions = 77.45 

                                                        2 Houck Paper, page 1
3

hours average).  The period of time consumed by the cooling of the stack to 100 degrees was a 

small percentage of the total time: no more than 5‐6%.

Test Duration:  In­Situ Tests vs. Method 28 Tests 

The average test duration of the In‐Situ Tests was approximately 77.45 hours. The 

Houck Paper compares these In‐Situ Tests to forty six Method 28 Tests that lasted an average of 

3.65 hours, or 4.7% of the length of an average In‐Situ Test.  This is a critical difference in the 

data sets and a difference not addressed in the Houck paper.

The Houck Paper then posits that a) since it “hypothetically” took the In‐Situ Test stoves 

about six hours to cool down to the 100 degree mark, then b) this six hour “cool down” time 

should be added to the Method 28 Tests (average length 3.65 hours) in order to c) get a real 

comparison of burn rates in the In‐Situ Tests and the Method 28 Tests.  This mathematical

contrivance (reconciling unrelated and dissimilar “endpoints” by mathematically adding long 

cool‐down times to Method 28 Tests) has the effect of 1) producing test times that are on 

average 3 times longer (the average test time in his forty six Certification Tests is 3.65 hours 3; 

the average test time in his “adjusted” tests is 9.5 hours4) and produces “Hypothetical 100 oF 

Burn Rates (Kg/h)” that are all approximately 0.5 kg/hr5 (as opposed to an average of  1.50 in 

the forty six Certification Tests).  In fact, the Houck Paper concludes; 

“The average hypothetical burn rate (had the Method 28 endpoint been a one‐foot 

stack temperature of less than 100oF) is more than 3 times lower than the burn rate 

calculated from the Method 28 endpoint.”6

This would be truly entertaining if it hadn’t been repeatedly advanced as justification for raising 

the burn rates in the Method 28 Tests.

The Houck Paper Argument:

Here is the argument put forth by the Houck Paper:

1. Since there is an apparent discrepancy between the Method 28 Test endpoint and the 

In‐Situ Test “endpoint”;

2. And since the (a) determination of Method 28 burn rates and (b) determination of 

weighting for these burn rates relied on the In‐Situ Testing;

                                                        3 This is the average of 46 Total Test Times in Table 1, pp 6‐7 of the Houck Paper 4 See Table 3, pg 9, Column entitled “Time to 100oF” in the Houck Paper) 5 See Houck Paper, Table 3, page 9, last column 6 Houck Paper, page 4
4

3. Then, if the two “endpoints” are directly comparable, the Method 28 Tests should 

“hypothetically” be extended until the stack temperature at 12 inches above the flue 

reached 100 degrees (when the coal‐bed would be largely consumed).   

4. Further, since this would generate a burn rate well below what was targeted by the In‐

Situ Tests and identified by the NSPS as “representative”, then the Method 28 Burn 

Rates should be increased.

5. This supports the HPBA position that Low Burn Rates and Medium Burn Rates for the 

Method 28 Test should be increased, and their weighting decreased.

Why the Houck Paper is Misleading:

The Houck Paper is incomplete, inaccurate, and misleading for the following reasons:

a. The Method 28 and In‐Situ Test “endpoints” are not comparable because the test 

duration of the in‐situ testing was one week (168 hours), and the average test 

time between 100 degree flue temperatures was approximately 77 hours whereas 

the average Method 28 tests presented in the Houck Paper was 3.65 hours;  

b. The Method 28 Test and In‐Situ Test “endpoints” are not comparable because the 

time spent cooling down to the 100 degree flue temperature in the In‐Situ Tests 

was a very small percentage of the total test time (5‐6%), whereas when this time 

is “hypothetically” attached to the end of a Method 28 Test it becomes a very large 

percentage of the test time (over 50%); 

c. The Houck Paper states that the “raw data” from the In‐Situ testing is unavailable, 

but graphs of each 7 day test showing  “Stove Temperature, Flue Oxygen, Fueling 

Practices and Heating System Use” are available on the EPA Web Site in a study 

entitled “Performance Monitoring of Advanced Technology Woodstoves:  Field 

Testing for Fuel Savings, Creosote Build‐up and Emissions”.7   The reason why the 

availability of data is important is that any person with a modicum of curiosity 

could go to the EPA website and confirm the data for himself/herself.

To discount the accuracy, usefulness and weighting of the Method 28 protocol based on 

the fact that AWES Sampling and Method 28 Test Sampling have different “endpoints” is error. 

The Houck Paper misrepresents the data for the benefit of its paying client, HPBA, which seeks 

                                                        7 For graphs of each 7day in‐situ study, see Appendix D, pp D1‐115 in Volume II of “Performance Monitoring of 

Advanced Technology Wood Stoves:  Field Testing for Fuel Savings, Creosote Build‐Up and Emissions”, prepared for 

New York State Energy Research and Development Authority by Omni Environmental Services, November 1987, and 

readily available as a pdf on the EPA web site.
5

an increase in Burn Rates defined by Method 28 Tests.  The data simply doesn’t support the 

“hypothetical calculations” proposed in the Houck Paper.   

FURTHER DISCUSSION

It is helpful to remember that the Houck Paper compares endpoints of forty six Method 

28 Tests where the Test Time ranged from 1.17 hours to 7.83 hours, and the average was 3.65 

hours, to the 113 In Situ Tests where the Test Time ranged from 84 hours to 168 hours; the 

average Test Time was 163.57 hours, and the average time between “endpoints” was 77.45 

hours, or an average of 21.2 times the length of the Method 28 Tests in question.  See Table 1.

Table 1

# of Shortest Longest Average Total

tests Test Time Test Time Test Time Test Hours

9/21/09 Houck Paper 46 1.17 hours 7.83 hours 3.65 hours 168.1

AWES In-Situ Tests 113 84 hours 168 hours 163.57hours** 18,588

**Overall average for In-Situ Tests was 163.57 hours. Average time between 100o 

"endpoints" for In-Situ Tests: 77.45 hours

Below is a sample of an In‐Situ Test with no “endpoints”:  the stove ran for seven days 

and never reached 100 oF.

Other In Situ Tests reached the 100o F point several times in a one‐week test period.  

For example, the stove graphed below dropped to the 100 degree “endpoint” three times in the 

first half of the one week test, each time at approximately 6:00 AM when the stove ran out of 

fuel and needed to be reloaded.  The last 3.5 days have no such occurrences.
6

The Houck Paper would have a credulous reader believe that every Method 28 Test 

should have a long “cool down endpoint” attached to it to be consistent with the original In‐Situ 

Testing.  To get an idea of what this would mean, imagine adding the tail on the graph below (in 

blue) to the Method 28 Test (a sample Method 28 Test is superimposed below, in red).  Note the 

obvious difference in scale (5.5 hours for the Method 28 Test (in red) vs. 168 hours for the In‐

Situ Test).  
7

If the Houck Paper were correct, and the “endpoints” really were comparable, and the 

“endpoints” really did occur at similar frequencies, then each In­Situ Test would be an endless 

series of endpoints, which of course they are not. Therefore the 100O F criterion is not a 

meaningful definition of an endpoint: as a practical matter it was infrequently attained, and the 

endpoint of these tests was defined as occurring at the end of a one week interval.

ADDITIONAL IN­SITU STUDIES

A decade after the in‐situ testing in Vermont, New York, and Oregon, additional in‐situ 

testing was performed in Klamath Falls and Portland, OR, using the same AWES monitoring 

system, with test periods running for 7 days (for a summary, see APPENDIX A, pp 9‐10).

The Klamath Falls study included 2,881 hours of data collection with an average burn 

rate of 0.98 kg/hr.  The stoves installed in Portland were run for 2,509 hours at an average 

burn rate of 1.19 kg/hr.  Summaries of the In‐Situ Tests results from Klamath Falls and 

Portland are printed on pages 9‐10. If anything, these results merely confirm the accuracy of the 

original study of stoves in Vermont, New York, and Oregon, which was used to determine 

weighted burn rates for the original NSPS.

Burn rates in the 1987‐88 in‐situ studies in Vermont, New York, and Oregon  showed 

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

8  The committee 

negotiating the original NSPS agreed that the frequency distribution developed from the in‐

home studies reasonably represented the full range of actual consumer wood use.  No 

subsequent study has invalidated this original finding;  to the contrary, later tests confirmed it.

The original committee negotiating NSPS also noted that some heaters “could easily be 

modified by the consumer either by removing damper stops or through the use of a stack 

damper to achieve longer burn times, and thereby create high emissions.”9

Consumer advertising is additional evidence that burn rates around 1.00 kg/hr are 

intended throughout the industry.   A review of stove advertising shows that most stoves with 

fireboxes between 1.8 and 3.0 cubic feet advertise burn times between 10 and 22 hours.  These 

burn times, advertised by manufacturers and expected by consumers, cannot be reasonably 

attained without operating the stoves at low burn rates, exactly as the In‐Situ Tests confirm.

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

9 Federal Register, Vol. 52, No. 32, Page 5002, February 18, 1987
8

SUMMARY

Method 28 defines burn rate in a very straightforward way – you build a bed of coals, 

you add fuel, you measure the time it takes to consume that fuel and divide the weight of fuel 

charged by the time it takes to consume that fuel. The Houck paper considers an alternative 

endpoint, contrived from In‐Situ Testing.  To compensate for the difference between the 

Method 28 Test endpoint and the contrived In‐Situ “endpoint” , the Houck Paper suggests 

adding a hypothetical long “cool‐down tail” to comparatively short Method 28 tests, and then 

recalibrating burn rates and weighting.  This is error, because it ignores (1) the duration of the 

In‐Situ Tests (168 hours); (2) the infrequency of “100 degree endpoints” in the In‐Situ Testing 

(on average once every 77.45 hours); and (3) the small percent of the overall In‐Situ Test time 

occupied by these so‐called “endpoints (approximately 6%).   In summary, there is nothing in 

the Houck Paper to suggest that burn rates determined for the original NSPS were too low, or 

weighted too heavily.  Subsequent In‐Situ Tests and industry advertising also confirm that the 

original burn rates and weighting were correctly determined.
9

APPENDIX A

Summary of 1998‐99 Klamath Falls, OR stoves using AWES Sampling Equipment

2,881 total hours; average burn rate 0.98 Kg/hr; Average Stack Temp 443o.
10

Summary of 1998‐99 Portland, OR stoves using AWES Sampling Equipment

2,509 total hours; average burn rate 1.19 Kg/hr; Average Stack Temp 371o.