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Hearth & Home May 2017

Straight Talk

By James E. Houck

It’s time to face the truth about the EPA’s Standard of Performance for New Residential Wood Heaters.

It’s time to take the gloves off, stop being a gentleman and tell it like it is. The NSPS promulgated in 2015 is a bureaucratic nightmare conceived without good science. It has the potential to cripple the hearth industry and not do a very good job of improving air quality in the process.

As most in the hearth industry know, in order to be sold, wood heaters must be certified. The certification requirements are described in the regulation known as the New Source Performance Standard (NSPS) for wood heaters. The U.S. Environmental Protection Agency’s new NSPS for residential wood heaters was published in the Federal Register March 16, 2015 and became effective May 15, 2015.

It is only one of the NSPS’s that the EPA has published under the authority of Section 111 of the Clean Air Act. As of 2013 there were 90 NSPS’s for different categories and subcategories of air pollutant sources, mostly industrial. The original NSPS for wood heaters that was revised for the 2015 version was promulgated in 1988.

The 1988 NSPS was bad. The 2015 NSPS is bad. They are bad technically and they are bad for the hearth industry. Certainly they have and will provide some environmental and health benefits, but they are poorly written, they have loop holes, they have cost the hearth industry dearly, they have allowed gamesmanship, and an NSPS by its very nature is not a good regulatory vehicle to reduce air quality impacts for residential wood heaters. The blame cannot be put on regulators alone; those in the hearth industry also share some of it.

Garbage In, Garbage Out

Anyone who has had scientific training is familiar with the concept called propagation of errors. It refers to the fact that when a final number such as the certification number, i.e., the weighted particulate emission rate (g/hr), for a wood heater is generated by various measurements that are used as input into complex equations, the uncertainties in each measurement are propagated to the final number.

For calculating the certification number, many measurements are made such as flow rates, the concentrations of various gases, the mass of the filter with the particulate catch, the mass of the blank filter, the moisture content of the wood fuel, the mass of the wood fuel, the mass of ash remaining, several temperatures, etc. All need to be measured, all have uncertainties associated with them and all contribute to the uncertainty of the final calculated certification value.

Those of us that have conducted particulate emission testing on industrial smokestacks, such as a coal-fired power plant stack, that have nearly constant internal stack conditions know that this uncertainty is typically around ± 20% from the sampling procedure alone. For wood heaters the larger additional uncertainty is due to the difficulty in burning wood reproducibly in the heater. The large combined uncertainty in the measurement of particulate emissions, and the operation of the wood heater with the concomitant uncertainty in the certification number, have been clearly illustrated by EPA’s own proficiency program.

Specifically, as per §60.535(7) Standards of Performance for New Stationary Sources; New Residential Wood Heaters (Federal Register v. 53, n. 38, Feb. 26, 1988), accredited laboratories, “Agree to participate annually in a proficiency-testing program conducted by the Administrator.”

Between 1993 and 2000, a single stove model was used for this proficiency-testing program. The results were obtained from EPA for results submitted by eight laboratories during that time period. The variation in results for individual runs amazingly ranged from 0.82 g/hr to 54.14 g/hr for a non-catalytic stove model (same actual stove shipped among the laboratories) with a published certification number of 3.60 g/hr. (See the March 2011 issue of Hearth & Home for a more detailed discussion.)

Even more amazingly, the EPA did nothing to mitigate this seemingly very serious fundamental problem, and considerable effort was even required on the author’s part with a cooperative EPA employee after repeated requests to even find that data apparently buried in an unorganized fashion in various files.

Bottom line is that the uncertainties inherent in needed measurements combined with the variability in the operation of the wood heater do not allow for a very accurate determination of a certification value. For example, under the 1988 NSPS a stove model with a reported certification value of 3 g/hr could easily be, in reality, 2 g/hr or 4 g/hr or further afield. Sadly, many hearth industry manufacturers have bought off on the numbers at face value that were produced by laboratories (some of which strived to get a reputation of producing “low numbers” to win clients) and used them for marketing claims.

Even more absurd is that this kind of uncertainty was bad enough under the 1988 NSPS with a 7.5 g/hr standard, but with the new 2015 NSPS with an initial standard of 4.5 g/hr and eventually a standard of 2 g/hr it seems unlikely that even with improvements in wood stove operation protocols and test methods that the results will be compatible with needs of the new standards.

These concerns were provided to the regulatory community but were dismissed using “junk” statistics by some and ignored by others. It was clear that science took second seat to the pressure to make a more restrictive standard even though it was inherently inconsistent with the facts.

Testing a Jøtul F370 at Dirigo Laboratories, Clackamas, Oregon.

Flawed Reporting Convention

There are three common conventions for reporting particulate emissions. They are: (1) The mass of particles per time, (2) The mass of particles per mass of dry fuel burned, and (3) The mass of particles per unit of energy delivered. The convention mandated by the NSPS’s was the mass of particles per time, which is the worst of the three for assessing fundamental particulate emission levels characteristic of a heater.

(1) The mass of particles per time – This is correctly referred to as an emission rate and is usually reported as g/hr. The NSPS certification testing uses emission rates. The problem with emission rates is that not only are the particulate emissions associated with combustion reflected in them, but they are also directly impacted by the burn rate. For example, a large stove that burns more wood and produces more heat is at a disadvantage as compared to a small stove, simply because less wood is burned in the small stove not because it is any “cleaner” per unit of fuel burned. An extreme example of the problem would be a home with two small, certified stoves producing in combination the same heat and more particles than a large uncertified stove that produces fewer particles.

Besides this issue, another problem with an emission rate reporting convention is that the efficiency of the wood heater is not reflected in it. Two stoves, one with a higher and one with a lower efficiency, could have the same reported certification value but the higher efficiency one would burn less wood and effectively produce fewer particulate emissions to satisfy the same heating demand.

(2). The mass of particles per mass of dry fuel burned – This is correctly referred to as an emission factor and is generally reported in units of g/kg, kg/Mg, or lb/ton. It reflects the amount of particles emitted per mass of fuel burned, which is more predictive of a wood heater’s “cleanliness,” but efficiency is still not reflected in it. It’s most useful for emission inventory purposes and is the convention used in EPA’s well-established emission inventory document AP-42.

Emission factors are used by state and local air quality regulators in the calculation of emission inventories and the relative significance of residential wood combustion to air quality. There are many technical reasons why it’s difficult to relate emission rates, as reported in the certification process, to emission factors. but a detailed discussion is outside the space limitations we have here.

(3) The mass of particles per unit of energy (heat) delivered – For assessing, ranking, and comparing the actual environmental impact, in this case the particulate emissions, for wood heaters, unquestionably the mass of particles per unit of heat delivered is the most appropriate. For the mass of particles per unit of heat delivered, the units of g/Mj are usually used. The mass of particles per unit of heat delivered also allows for the comparisons of heaters using very different fuels (e.g., natural gas versus cordwood).

Efficiencies need to be measured to report emissions in this convention. There are at least half a dozen standardized methods of measuring efficiencies; all seem to produce slightly different results and all have uncertainties associated with them. In the original 1988 NSPS there was a “place holder” for an efficiency method (Appendix J, 40 CFR §60.534 (d)) but one was never put in place.

Instead, almost arbitrary default efficiencies of 63% for non-catalytic cordwood heaters, 72% for catalytic wood heaters, and 78% for pellet stoves (40 CFR §60.536(i)(3)) were assigned. Interestingly, these efficiency numbers are inconsistent with the efficiencies listed in EPA’s often-quoted emission inventory guide (AP-42). Now, measured efficiencies rather than default efficiencies can be put on the list of certified stoves, but since they were developed under optimized laboratory tests they have little relation to efficiencies under actual in-home use and have even a higher relative uncertainty than particulate emission values.

The often-told story on how the gram per hour convention for evaluating certification for wood heaters was selected during the development of the 1988 NSPS is that a single very aggressive state regulator without data pushed for it and it was adopted. The author of this article was told by the EPA staff member responsible for development of the 2015 NSPS that, because the gram per hour convention was already in place, it would be too difficult to change it – so the process is still saddled with the poorest reporting method to evaluate particulate emissions.

The NSPS Is for New Heaters Only

The New Source Performance Standard, as the name implies, only has the authority to regulate new heaters. It does not require old, uncertified wood heaters to be replaced; it simply requires that only EPA-certified wood heaters can be sold and installed. The NSPS for residential wood heaters is a somewhat awkward adaption of a regulatory authority designed for new industrial facilities. Wood heaters are even referred to as the “affected facility.”

Independent survey data showed as of 2010 only 35.4% of the total number of freestanding cordwood stoves and pellet stoves were certified cordwood stoves or pellet stoves. The replacement rate of old, conventional, uncertified stoves has been less than 1.5% per year, suggesting that fewer than one-half of the wood heaters in homes are still uncertified cordwood or pellet stoves.

Further, stoves degrade with time due to gasket failures, metal warping, catalyst failure, etc., suggesting the earliest certified models are no longer producing emissions as low as when they were new. Bottom line is that the impact of the NSPS on particulate emissions levels from residential wood heaters will be slow. Local burn restrictions, home occupant educational programs, and financial incentives may be more effective alone or used in concert with the NSPS to affect a real change in particle emissions from residential wood combustion.

The in-home emissions of certified cordwood stoves do not correlate well with certification test values due to the wide range of conditions under in-home use. The black dots are the mean of runs for a specific certified stove model; the bars are the standard deviation around the means for multiple runs with each model, and the green squares are the reported certification value for each stove model. This graph first appeared in the March 2014 issue of Hearth & Home and is provided again here due to its importance

Real-World Particulate Reduction

Certification numbers for wood heaters are not predictive of their actual performance in homes or of the relative ranking of the performance of various wood heaters under real world conditions. The fundamental reason certification test procedures are not predictive of real-world emissions during in-home use is that there is a host of variables that make in-home usage quite different from the controlled test conditions in the laboratory.

Those variables include: (1) wood moisture, (2) burn rate, (3) the species of tree used for fuel, (4) wood fuel piece size and shape, (5) kindling practices and wood addition patterns, (6) chimney draft – chimney height, chimney condition, geometry of chimney, chimney connectors and chimney cap, home exhaust fans (e.g., kitchen, bathroom, clothes drier), air infiltration/tightness of home, and creosote accumulation, (7) the stove’s condition (new versus various levels of wear), (8) wind gust creating pressure pulses, (9) elevation as it affects available oxygen for combustion, (10) barometric pressure as it affects draft (home elevation, and meteorological conditions), and (11) hot versus cold starts.

Two of these variables are notable. One is the fact that surveys of home wood piles found that a large fraction of wood burned in residential wood heaters is not seasoned and very wet, making for poor combustion and high particulate emissions. Similarly, it has been well documented that the startup of a fire produces a large fraction of the overall particulate emissions.

Consumer surveys sponsored by the Hearth, Patio & Barbecue Association (HPBA) showed that a national average length of wood stove use per occasion was between 4.8 hours and 5.8 hours. From that average short duration it’s clear that a cold start is part of most wood heater use events.

When analyzed without agenda, two very large databases have shown that, on average, one can expect close to a 50% reduction in particulate emissions with in-home usage of currently-certified wood stoves as compared to uncertified ones. The results from the two independent databases analyzed agreed well. One showed a 46% reduction and one showed a 48% reduction. The two databases are (1) from work conducted in Libby, Montana, where there was a complete replacement of uncertified stoves with certified ones, and (2) from numerous in-home field studies combined with in-laboratory studies simulating in-home use conditions.

These two large databases consist of literally thousands of data points, involving dozens upon dozens of researchers and technicians, and sponsored by multiple agencies/organizations. They do not support the statement in the proposed new NSPS document that “…a wood heater complying with the NSPS would emit 75 to 86 percent less than conventional wood heaters.” Nor do they support the 90% reduction one often hears being bandied around the hearth industry. As the old saying goes, “It is what it is.”

Bottom line is that the differential in emissions between a certified and uncertified wood heater is lessened when the certified wood heater is used under less than ideal, optimized laboratory conditions.

Significantly, the second database was provided to the regulatory community and again was dismissed using “junk statistics” or ignored all together. The junk statistics were conducted by a junior staff member of a local air quality jurisdiction that ostensibly wanted to use the larger reduction credit to support future air pollution reductions as described in their state implementation plan.

It needs to be emphasized that the second database was funded largely by various governmental agencies; it was reviewed without agenda, the results were clear, and it was a robust database comprised of 618 total measurements that included a total of 409 emissions tests on 85 certified wood stoves comprised of 41 models, and of 209 emissions tests on 62 uncertified stoves. There is no better or more compelling information available.

Not only did this database reveal a smaller reduction in particulate emissions offered by the replacement of older, uncertified stoves with certified ones, it also showed that certification values were not a good predictor of the in-home emissions or the relative ranking of their emissions in the home.

Bureaucracy Time

Anyone who has ever dealt with government agencies knows that they are slow, deadlines are seldom met, and that if private companies operated in the same fashion they would be out of business. The EPA is certainly no exception. Case in point, the Clean Air Act requires that the NSPS’s be reviewed every eight years. The 1988 NSPS rule for wood heaters was certainly well more than eight years prior to the 2015 NSPS – actually, it was 27 years from the 1988 rule.

Similarly, a letter jointly written by the air quality organizations of WESTAR and NESCAUM dated April 29, 2008 is generally recognized as the spark initiating the new NSPS process by EPA. It took EPA more than seven years to promulgate the new NSPS from that date, with many assurances along the way from EPA personnel that it would be done much sooner. Moreover, when litigation becomes part of the story, as it appears to be, all bets are off, leaving future benchmarks even further out in the future. Why does this matter now that the new NSPS has been promulgated?

The answer could be quite damaging to the hearth industry. There are many future deadlines embedded in the rule that require future interactions with the EPA. History would suggest that it’s unlikely that EPA will do its part in a timely fashion, particularly with the current projected budgetary cuts in the agency. Delays and uncertainties certainly leave many business decisions in limbo. It has been estimated that the 1988 NSPS caused several hundred (albeit smaller) wood manufacturers to go out of business.

To date the hearth industry’s response to the new NSPS has been targeted. It has not challenged the overall systemic efficacy of the rule. Given the current regulatory climate, perhaps such a proposal would not fall on deaf ears.

About the Author

Dr. James E. Houck has been involved in environmental research and the hearth industry for over 30 years. He currently is an independent consultant and can be reached at:

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