10/11/2012

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Proposed Revisions to the NSPS 

for Residential Wood Heaters –

Industry Perspective

Presented by the 

Hearth, Patio & Barbecue Association

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Common Goals

1. Improve Air Quality.

2. Replace uncertified stoves. 

3. Continue to work to improve real world 

performance of certified stoves.

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• Using data from annual nationwide consumer 

surveys, Dr. James Houck determined that in 

2010 there were more than 6 million 

uncertified freestanding wood stoves in 

homes nationwide. 

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Freestanding Stoves: Nationwide 

Inventory - 2010

Source: The Fraction of Freestanding Wood-Fueled Stoves in Current Use That are U.S. EPA 

Certified Cordwood Stoves and Wood Pellet Stoves, Dr. James Houck, July 2011

4A
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4B

4C
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• The six million conventional, 

uncontrolled wood stoves are the 

problem.

• The “common goal” solution is to work 

to accelerate their replacement!

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• How can we improve the real world 

performance of certified stoves?

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• First we have to understand the 

capabilities AND the limitations of the 

EPA test methods.

• Next we have to understand the 

relationship between EPA laboratory 

emission performance and in-home, realworld emission performance. 

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• What variability – precision- can we expect 

when testing the same wood stove:

– In the same test lab?

– In a different test lab?

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We start in the lab.
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• This is not a new issue in the wood stove 

regulation.

– The assumed “within-the-same-lab” precision 

when the current NSPS was promulgated was 

±1 g/hr.

– This determination was based on very limited 

precision data.

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This assumed precision is represented 

by a blunt arrow on current hang tags.

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What are the sources of variability in 

the EPA Wood Stove Emission Values?

1. EPA Test Methods, including:

– Particulate Measurement Methods 

• Dilution Tunnels Methods (5G-1, 5G-2, 5G-3)

• In-Stack Method (5H)

– EPA Fueling and Operating Test Method (M28)

2. The Inherent Variability in Burning Wood

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There is a robust database AND a standardized 

procedure for determining test method 

precision.

• EPA Proficiency Test Data

– To obtain and maintain their accreditation, EPA requires 

laboratories to conduct “proficiency tests” on EPAprovided wood stove samples.

– From 1987 to 2005, five different test stoves were shipped 

from lab to lab for proficiency testing. 

• 84 four-run data sets from 12 different laboratories were 

submitted to EPA. 

• ASTM E691-09 – “Standard Practice for Conducting an 

Inter-laboratory Study to Determine the Precision of a 

Test Method” was used to conduct the analyses.

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Conclusions from Precision Analyses of the 

EPA Laboratory Proficiency Data

• The “within the same lab” precision of the weighted

average emissions at the 95% confidence level is at

best ~±3 grams per hour.

• The “between labs” precision is poorer (at least ±4.5

grams per hour again at 95% confidence).

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Source: “EPA Wood Heater Test Method Variability Study - Analysis of Uncertainty, Repeatability and 

Reproducibility based on the EPA Accredited Laboratory Proficiency Test Database”, R. Curkeet and R. 

Ferguson. Submitted to EPA on October 6, 2010. The report is available upon request. The report was 

also summarized in Hearth and Home Magazine, March 2011 Issue.

• The dilution tunnel emission measurement methods 

have good precision and are not major contributors 

to the high overall variability. 

– M5H (in the stack) has shown much poorer precision but is 

still not a major contributor.

• Operational and fuel parameter variations (within 

the specified ranges in EPA M28) are also not major

contributors. 

– Tightening these parameters (moisture content range, 

for example) would increase costs, would not improve 

precision and would further increase the disconnect 

with real world operation.

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Conclusions from the Precision Analyses (cont.)
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• The high level of variability in wood heater emission 

testing results for any given appliance is most likely a 

function of the random nature of burning wood, no 

matter how tightly you try to control the process. 

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Conclusions from the Precision Analyses (cont.)

Source: “EPA Wood Heater Test Method Variability Study - Analysis of Uncertainty, Repeatability and 

Reproducibility based on the EPA Accredited Laboratory Proficiency Test Database”, R. Curkeet and R. 

Ferguson. Submitted to EPA on October 6, 2010. The report is available upon request. The report was 

also summarized in Hearth and Home Magazine, March 2011 Issue.

• The EPA test methods are certainly capable of 

reliably distinguishing between good and bad 

performance, but they cannot reliably 

distinguish between “good, better and best” 

performance.

• The EPA test methods cannot reliably 

distinguish emissions performance differences

of less than 3 grams per hour. 

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What does this mean?
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Next we compare laboratory results 

with real world performance.

• EPA and State SIP Planners have long 

recognized that laboratory test values do not 

necessarily represent in-home performance.

– Some states have been calling for test method 

changes to address this issue. 

• EPA’s current AP-42 emission factors (g/kg) 

reflect ~50% emission reduction credit for 

certified versus conventional stoves.

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• Studies published over the past two decades 

that were used for determining AP-42 

emission factors also show an average 50% 

reduction for in-situ emission rate (g/hr) for 

certified models.

– These studies show that certified stoves are 

working.

• The average emission rate in these field 

studies is about 10 g/hr for certified stoves 

and the average lab value is below 4.5 g/hr.

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• The ability of wood stove NSPS certification 

values to predict in-home performance was 

further investigated in a recent paper by Dr. 

James Houck1.

• As part of the study, real-world emissions 

from certified wood stove models were 

compared to uncertified models.

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1A Comparison of Particulate Emission Rates from the In-Home Use of Certified Wood Stove Models with U.S. EPA 

Certification Emission Values and A Comparison between In-Home Uncertified and Certified Wood Stove Particulate 

Emissions, Dr. James Houck, February 2012

• A robust database comprised of 618 total 

measurements was used for this evaluation. 

– For the certified stoves: 

• 409 emissions tests 

• 85 wood stoves 

• 41 individual models 

– For the uncertified stoves:

• 209 emissions tests

• 62 wood stoves

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Major conclusions from this study

• There is no correlation between certification 

values and the real world emission rates.

• And further, rank orders change. 

– A stove with a higher certification value may 

perform better in the field than one with a lower 

certification value.

• Certified stoves perform significantly better 

than conventional stoves (50% PM reduction).

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22A
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22B

22C
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22D

22E
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• Lowering the NSPS passing grade will not 

improve the real-world performance of 

certified stoves. 

– We cannot reliably differentiate between models 

performing in the range of interest (i.e., below 4.5 

g/hr), because the precision of the lab test 

methods is no better than ±3 g/hr.

– Even if we could, emission rates from lab testing 

do not correlate well with field performance.

• The rank orders can change, with appliances that 

perform with higher lab emissions often performing 

better in the field.

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Conclusions

• Lower standards are also not cost 

effective.

– New Source Performance Standards (NSPS’s) are “technologybased” standards. Setting them requires EPA to balance two 

different kinds of inputs:

• technological feasibility

• cost effectiveness

– EPA has determined that standards below 4.5 g/hr are not cost 

effective.

– HPBA is conducting its own cost effectiveness analysis; our 

preliminary conclusions support EPA’s conclusions.

– These analyses assume that differences in EPA certification 

scores are “real”, 

• that we can meaningfully distinguish between appliances performing 

within the range of interest based on lab test data, 

• and that the lab test scores predict performance in the field.

– Neither of these assumptions is valid.

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Are lower standards an effective air 

quality management strategy?

• 25 years after the promulgation of the NSPS, there are still MORE 

THAN 6 million uncertified, high-emitting stoves in homes.

• These stoves have real-world emission rates at least 2 or 3 times 

higher than certified models. 

• The real air quality objective should be changing-out these old

stoves with certified models.

• Lowering the passing grade below 4.5 g/hr would slow change-outs 

by:

– increasing the cost of certified models

– reducing the numbers of models available to consumers

• Lowering the passing grade will produce only the appearance of 

progress, nothing more, and would not be cost effective even if the 

marginal gains involved were in fact real. We need to keep our eye 

on the ball here, and focus on what matters -- facilitating changeouts.

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How can we improve real-world, inhome performance?

• Would increasing the minimum required burn 

rate in EPA Method 28 have a beneficial 

impact?

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Background:

• EPA Method 28 requires a certification test run at a low burn rate of 

< 1.0 dry kg/hr.

• This requirement was based on an analysis of in-situ test data during 

the development of the current NSPS.

• Dr. Houck has demonstrated conclusively that this analysis was 

severely flawed.1

– Test run duration (end of test) was defined very differently for the field 

testing (100°F Flue Gas) vs. Method 28 lab testing (100% Test Fuel Burned).

– The field tests had LONGER durations than Method 28 testing for the same 

amount of wood burned.

– This duration determination difference skewed the in-situ burn rate down 

when compared to M28 lab testing. This skew was not accounted for when 

the M28 minimum burn rate was determined. 

– Dr. Houck estimates that the impact of this error was significant and would 

certainly have resulted in equivalent M28 minimum burn rates that are 

appreciably greater than 1.0 kg/hr.

27A

1

Evaluation of Method 28 Wood Heater Burn Rates, James E. Houck, Jeremy Clark, and Thomas Christensen,

OMNI Environmental Services, Inc., September 2009

Background:

• EPA Method 28 requires a certification test run at a low burn rate of 

< 1.0 dry kg/hr.

• This requirement was based on an analysis of in-situ test data during 

the development of the current NSPS.

• Dr. Houck has demonstrated conclusively that this analysis was 

severely flawed.1

– Test run duration (end of test) was defined very differently for the field 

testing (100°F Flue Gas) vs. Method 28 lab testing (100% Test Fuel Burned).

– The field tests had LONGER durations than Method 28 testing for the same 

amount of wood burned.

– This duration determination difference skewed the in-situ burn rate down 

when compared to M28 lab testing. This skew was not accounted for when 

the M28 minimum burn rate was determined. 

– Dr. Houck estimates that the impact of this error was significant and would 

certainly have resulted in equivalent M28 minimum burn rates that are 

appreciably greater than 1.0 kg/hr.

27B

1

Evaluation of Method 28 Wood Heater Burn Rates, James E. Houck, Jeremy Clark, and Thomas Christensen,

OMNI Environmental Services, Inc., September 2009

Hypothetical Example

M28 Test Complete (100% Test Fuel Weight Burned), Flue Gas Temp = 300° F

10 kg fuel/8 hours = 1.25 kg/hr burn rate

M28 Test Extended to Reach 100°F Flue Gas Temp

10 kg fuel/10 hours = 1.0 kg/hr burn rate

(10 hours = time to burn 100% of Test Fuel AND THEN reach 100°F Flue Temp) 

An M28 test at 1.25 kg/hr is equivalent to a field test at 1.0 kg/hr
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• The ASTM committee that developed the replacement for 

EPA Method 28 tried to solve this problem. The “fix” was 

to modestly increase the low burn rate requirement by 

15%, from <1.0 kg/hr to <1.15 kg/hr. 

• This is not a matter of trying to game the system. 

– Every certified stove model has satisfied the <1.0 kg/hr burn 

rate requirement.

– The minimum burn rate tested is the lowest burn rate 

achievable by the stove design.

• Stove designs with burn rate capabilities lower than 1.15 kg/hr will be 

tested at their lowest burn rate.

– Increasing the lowest air setting would help homeowners to 

more consistently achieve expected levels of emissions 

performance with real world installations and with real world 

fuel.

– We believe that the current M28 low burn rate requirement is 

frustrating the implementation of BDT, not reflecting it.

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Colville WA Wood Stove Low Burn Rate Project

• To show the impact of the EPA M28 minimum 

burn rate requirement, two identical non-cat 

stoves were run in several series of comparison 

tests.

• In one series, EPA M28 test conditions were run 

with the exception that one stove had EPA fuel 

cribs and the other was fired with D. fir 

cordwood. Emissions were measured using a 

dilution tunnel.

• The comparison of the crib fuel to cordwood fuel 

is shown on the following slide. 

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Test Fuels

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EPA M28 Test Fuel Crib – 7 lb/ft3 D. fir Cordwood Test Fuel – 7 lb/ft3

Note: The steel bars represent the interior side walls of the stove.

• The stoves and their chimney exits were 

video-taped to create both a data and visual

record.

• A visual evaluation of smoke opacity was also 

made by experienced technicians at regular 

intervals over the course of the test runs.

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1 8

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399.6°F 407.4°F

What are we looking at?

Run #’s View of Fire

Chimney Exits

Run Time from 

Start of Test

Ave. Stove Temp. at Start

Opacity Chart

Video Cue

Ave. Stove Temp. at Start

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• Additional comparative test runs were 

conducted under conditions better 

representing homeowner use. 

– ~21 feet of chimney height, vented though the 

roof, to create more realistic draft conditions. 

– D. fir cordwood was used.

– Start-up times were increased from the five 

minutes allowed in EPA Method 28.

– Fuel load density was increased from 7 to 10 

lb/ft3. 

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• One test series included 15 minute start-up times 

(to allow the fuel load to become more fully 

ignited) before the Primary Air Control (PAC) was 

set to the run position.

• At the end of the start-up, one stove’s PAC was 

set at its minimum setting while the second had a 

partially open PAC.

• A video synopsis of the first two hours of that 

series is shown on the next slide along with a plot 

of the opacity readings.

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• The results of this test series were as 

anticipated. 

– The stove with the PAC at its minimum setting 

smoked for most of the test.

– The stove with the partially open PAC ran with 

little or no smoke for most of the test run.

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• In another test series under the same 

conditions, a different result was achieved.

– The emission performance of the stove with its 

PAC at the minimum setting was exceptionally 

good – better even than the stove with the 

partially open PAC.

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• A plot of the opacity readings for the first 

two hours for these runs are shown below.

• This result shows that under favorable inhome conditions, the stoves can perform 

well.
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• Additional test runs were conducted comparing wet 

and dry cordwood and shorter start-up times. 

• This example shows the opacity readings for a wet 

cordwood test run.

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Other Test Runs

• A wet crib run was also conducted. 

• A summary of all the test runs plus video and 

test run data can be made available upon 

request. 

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What did the Colville project show?

• To achieve a cleaner burn more consistently, there are 

significant differences in the minimum combustion air settings 

required for EPA crib fuel compared to cordwood.

• The current minimum burn rate requirement in EPA M28, is 

negatively impacting real-world low burn rate performance.

• Modest increases in the minimum combustion air setting will 

allow homeowners to achieve better, more consistent 

performance. 

• Wet wood performance was worse than drier wood even 

when given a longer start time. No surprise!

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Industry’s Concerns about EPA’s 

Proposed Efficiency Standards

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Is Overall Efficiency a Surrogate for Carbon 

Monoxide?

• The short answer is no! CO like PM correlates with

combustion efficiency, not overall efficiency. 

– Overall efficiency is the combination of combustion 

efficiency plus heat transfer efficiency. These can work in 

opposite directions.

• EPA found 25 years ago that separate standards for 

CO were not needed. 

– PM controls that require improvements in combustion 

efficiency would also improve CO.

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Are the proposed efficiency standards 

supportable?

• Adding “Overall Efficiency” standards to the 

NSPS occurred late in the process. This 

presents several issues:

– EPA has done NO technological feasibility or 

incremental cost effectiveness analysis that 

include BOTH PM and Overall Efficiency standards. 

– Without that required analysis, EPA attempted a 

short cut, by selecting efficiency standards it 

believed could be met by the vast majority of 

appliance models that met the PM standards. 

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• The Hearth, Patio & Barbecue Association 

(HPBA) undertook an efficiency data gathering 

effort for wood heaters in response to EPA’s 

initiative. The study was completed in 2011.

• CSA B415.1-2010 overall efficiency data, along 

with corresponding EPA emissions certification 

values for 68 currently produced non-cat* 

models that meet Washington State emission 

limits were obtained. 

*Too little data were obtained to allow any meaningful analysis for 

catalytic wood heaters. 

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Figure 1 is a bar chart showing the distribution of overall weighted average efficiencies 

by efficiency range for the 68 non-catalytic heater models. 

Figure 1

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• Of the 68 non-catalytic wood heater models in 

this database, 21 models have weighted 

average overall efficiencies that are below the 

proposed level of 70%. This represents 31% of 

the heater models in the database. 

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• The relationship between weighted average overall 

efficiency and weighted average particulate emissions

for the 68 non-cat models was evaluated using 

standard statistical practices1. 

• The poor R2 value (0.05) for the linear regression line 

and the corner score analysis show there is no 

correlation or association between these parameters.

1Mark’s Standard Handbook for Mechanical Engineers, Eighth Edition, 17-22

A linear regression line was fit to the data and a corner score analysis conducted.

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• These analyses clearly show that EPA’s proposed 

overall efficiency standard doesn’t meet their 

“low bar” design criteria and is also inappropriate 

for regulating CO.

– About 1/3 of this significant sub-set of current non-cat 

models meeting a 4.5 g/hr PM standard won’t meet 

EPA’s proposed 70% weighted average overall 

efficiency standard. 

• The actual percentage, if all heaters meeting 4.5 g/hr are 

considered, could be even greater. 

– CO and PM correlate with combustion efficiency, not 

overall/delivered efficiency.

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• There is, however, a sound alternative to what 

EPA is proposing: 

– require efficiency testing using CSA B415.1-10, but 

don’t set enforceable efficiency standards.

• The efficiency values would be disclosed to the 

buying public to aid their purchasing decision.

– on hang tags 

– in advertising materials. 

• Marketplace pressures will ultimately drive 

manufacturers to improve efficiency levels for 

their lower performing models. 

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Restating the Common Goals

1. Improve Air Quality

2. Replace uncertified stoves. 

3. Continue to work to improve real world 

performance of certified stoves.

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Final Conclusions

• Certified stoves work in the field!

• There are over six million uncertified wood 

stoves still in American homes. These 

uncontrolled stoves have emission rates at 

least two or three times higher than EPA 

certified stoves.

• 86% of Wood Stove PM emissions (~135M 

Tons) are from uncertified stoves.

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• Lowering the NSPS passing grade will not 

improve the real-world performance of 

certified stoves. 

– The precision study shows that that we cannot 

reliably differentiate between models performing 

in the range of interest (i.e., below 4.5 g/hr), 

based on lab testing.

– Even if we could, Dr. Houck’s study demonstrates 

that the emission rates from lab testing do not 

correlate well with field performance.

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Final Conclusions (cont.)

• Even if we could differentiate between stoves with lab 

emissions in the range of interest and if the lab testing 

correlated with field performance, EPA has determined 

that standards below 4.5 g/hr are not cost effective.

– HPBA’s independent analysis is coming to the same conclusion.

• Lowering the passing grade will produce only the 

appearance of progress in improving the in-home 

emission performance of new stoves – nothing more. 

• It will slow the rate of change-outs, by increasing the cost of 

new stoves and reducing the number of models available to 

consumers.

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• Increasing the minimum tested burn rate will 

have a beneficial impact real world 

performance. 

– The Colville Project demonstrates that increasing 

the minimum burn rate in EPA M28 will result in 

new stove models with more robust in-home 

performance over a wider range of real-world 

conditions.

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Final Conclusions (cont.)

• EPA’s proposed efficiency standards are not a 

sound way to lower carbon monoxide 

emissions.

– CO and PM correlate with combustion efficiency, 

NOT overall efficiency.

– Overall Efficiency is therefore a very poor choice 

as a regulatory surrogate for CO. PM is the better

choice.

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Final Conclusions (cont.)
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• The proposed efficiency standards are not 

supportable.

– EPA’s effort to shortcut NSPS decision criteria for

efficiency standards doesn’t work.

• There is a better option: Don’t regulate 

efficiency. Require testing and reporting of 

efficiency for point of sale use. Let the 

marketplace provide the pressure to improve 

efficiency ratings.

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Final Conclusions (cont.)

What is our most important common 

goal?

• Changing out the more than six million 

uncertified stoves still in American homes 

because this will improve air quality.

– Lowering the passing grade below 4.5 g/hr will 

slow change-outs and produce only the 

appearance of progress – nothing more.

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