Opinions of top wood stove industry insiders revealed in 1998 interviews

The late Paul Tiegs, one of the
greatest authorities on wood
stoves, conducted the
interviews for the EPA. 

Long before the regulatory debate about wood stoves heated up in the 2010s, the EPA commissioned a series of fascinating interviews with the top wood stove experts in the country on a host of technical and policy issues.  These interviews give a glimpse of the opinions and philosophies of industry and academic leaders at a time when they apparently felt free to go on the record about what became controversial topics. 

The content of these interviews remains very relevant today for anyone interested in a behind-the-scenes look at many of the underlying issues in the 2015 EPA stove and boiler regulations.  The interviewers – Jim Houck and Paul Teigs (who are top experts themselves) – asked questions ranging from whether masonry, pellet, boiler and furnace appliances should be regulated, to the vulnerabilities of catalytic stoves, to how lab testing can better reflect real world use of stoves. 

These interviews remain a valuable resource because each of the nine experts was asked the exact same questions.  Thus, if you are interested in masonry heaters, or catalytic or pellet stoves, or how labs coax the best numbers from stoves, it is relatively easy to scroll down and see how each person answered the question.  Of the nine interviewees, four are from industry (John Crouch, Bob Ferguson, Dan Henry and Michael Van Buren), two from test labs (Rick Curkeet and Ben Myren), two from academia (Skip Hayden and Dennis Jaasma) and one from EPA (Robert C. McCrillis). Their full titles and affiliations are at the end of this blog along with the full list of questions asked.  The full set of questions and answers are in Appendix B on page 58 and can be downloaded here (pdf).

In general, Bob Ferguson and Dan Henry tended to oppose further regulation, and felt, for example, that pellet stoves and wood-fired central heating appliances did not need to be regulated.  Ben Myren tended to favor a blanket approach of closing loopholes and regulating all appliances.  This difference in views between two industry experts and one test lab expert can be viewed through their respective economic interests and how it would affect their livelihoods.  But these interviews also show deeper philosophical differences and illuminate the reasons for their positions, whether they concern the health impacts of wood smoke, consumer protection, profitability, practicality of test method changes, etc.

We have chosen to reproduce the answers to two questions and invite readers to refer to the full set of interviews to find issues that they may be more interested in, such as the impact of wood species on emissions, stress testing to see how durable stoves are, and options to promote or require education or maintenance of stoves by consumers. 

When the Alliance for Green Heat began ten years after these interviews in 2009, much of the content had already been seemingly lost or obscured.  Very few people, for example, knew of the origin or impact of the 35:1 air-to-fuel ratio loophole that allowed pellet stove manufacturers to make low efficiency stoves in order to avoid regulation.  Right up until 2014, state and federal government agencies, along with top industry outlets, continued to propagate myths about pellet stoves.  Even the EPA never advised consumers that uncertified pellet stoves were likely to have lower efficiencies due to the 35:1 loophole they created.  These interviews provide the best information anywhere on how this came to be and what impact it had on the pellet stove industry and consumers.

We chose the question about whether central heaters should be regulated because this turned into one of the biggest issues in the 2015 regulations.  Only one interviewee – John Crouch – saw a causal relationship between the rise of outdoor wood boilers and the 1988 emissions regulations. 

Question: The 35:1 air-to-fuel ratio cut-off for certification has produced two classes of pellet stoves — those that are certified and those that are not. The latter class may have models that are less efficient and have higher emissions than the former. Should the regulations be amended to close the loop-hole and discourage the practice of intentionally designing models with a higher air-to-fuel ratio to avoid certification?

John Crouch, HPBA's
foremost wood stove expert.

John Crouch, HPBA: I wouldn’t use the term “close the loop-hole”. I would say, “is the proper place to cut off the definition of a wood heater?” We all know the whole discussion during the Reg-Neg ignored this emerging category of pellet stoves. So this gets back into my other broader comment, which is, instead of going back in and changing the NSPS in a piecemeal fashion, there needs to be a true revision of the whole thing that deals with the category of pellets and masonry heaters and outdoor furnaces.

Rick Curkeet, Intertek: Yes. The way to amend the regulation is to simply remove the 35:1 air/fuel ratio exemption. This has never been required by fireplaces (they meet the 5 kg/hr minimum burn rate exemption criterion anyway). Pellet units are readily able to meet emissions requirements and the exemption only encourages making these units less efficient to avoid the regulation.

Bob Ferguson, Consultant: The 35:1 cutoff was intended for fireplaces. However, pellet stoves are the only product that even take advantage of the air-fuel exemptions. Fireplaces generally use the burn rate exemption. Pellet stoves probably don’t need to be regulated at all. They are all quite clean burning. Let the marketplace decide if exempt stoves are acceptable. If pellet stove users demand products that use fewer pellets (more efficient), the manufacturers will respond. 

Skip Hayden, Researcher: Yes. In Canada, we recommend that people buy only EPA-approved pellet stoves. We have developed a high ash pellet stove that's operating around 85% and its emissions are about 0.3 g/hr or less. 

Dan Henry, a founder of Quadrafire
stoves is one of industry's most
articulate spokesmen.

Dan Henry, Aladdin: There is no data that indicates that even a poorly operating stove is a dirty burning appliance. They are inherently clean, becoming more and more reliable, and don’t fix them if they aren’t broken.

Dennis Jaasma, University of VA: Pellet stoves are inherently clean burning unless there is something very bad about their design. I am not concerned about regulating the currently uncertified units unless their field emissions are bad compared to certified stoves.

Robert C. McCrillis, EPA: Yes, all pellet stoves should be affected facilities and not subjected to that 35:1.

Ben Myren one of Amreica's most
thoughtful and experienced stove tester.

Ben Myren, Myren Labs:  I agree, no more loop-holes. The new technology stoves that are coming on the market are going to be totally new critters. I don’t think that turning down the air- to-fuel ratio, to make it whatever it is, should get you out of the loop. Some of those suckers have got to be just filthy. I mean you look at the flame. I’ve seen them burn at the trade show; you know, the glass is sooting up on the edges. You can just see it.

Michael van Buren was a technical
expert with The Hearth Products
Association, now HPBA

Michael Van Buren, HPBA: I don’t know what that loop-hole does, whether it really affects the operation of the stove and the efficiency of the stove.

Question: According to a Department of Energy survey out of the 20.4 million households that used a wood burning appliance in 1993, less than 0.3 million used a wood burning furnace as their primary source of heat. Are there enough wood-fired central heating furnaces in use to merit their closer evaluation? How many commercially available models are there? Are there emissions data for them? Should they be certified?

John Crouch: The [1988] EPA New Source Performance Standards killed the indoor furnace industry and created this little loop-hole which the outdoor furnace industry is beginning to exploit and kind of underscores the need for a more comprehensive wood burning regulation which sets out over a several year period to codify all forms of wood burning technology.

Rick Curkeet tested stoves for Intertek
labs and is one of industry's top experts.

Rick Curkeet: I don’t know how many new units are being produced but I’m sure it’s a very small number. Still, one really poor unit can be a significant problem if it’s in your neighborhood. There have never been any standards for testing this type of product for emissions and efficiency. However, we have adapted existing methods and can say that the performance range is very wide. Poor designs may be 30% or less efficient and produce nearly 100 grams/hr emissions rates. Good designs are able to approach certified wood stove performance levels.

Bob Ferguson: I don’t feel there are enough units being sold to merit any activity what-so-ever. There are only a handful of manufacturers. I don’t think there has been anything published--so if testing has been conducted, it is probably a good assumption that the numbers aren’t that good. They shouldn’t be certified, as you would have to develop test methods and standards. The country would be better off using the money to pay manufacturers to phase out of production, sort of like the agricultural method of paying farmers not to grow certain crops.

The late Skip Hayden also worked
at the US Federal Enercan Lab,
back when the federal government
focused more on wood heating.

Skip Hayden: The number of central wood furnaces in Canada, certainly in comparison to the United States, would be higher. In our Eastern provinces, it’s a relatively common add-on to existing oil furnaces. Generally, they are as dirty as can be.

Dan Henry: I think a lot of these are used in rural areas and considering the fuels that are out there, I don’t think they should be regulated. Maybe just a spot check of some sort. I think the only thing that would benefit would be the testing laboratories. If it emits particulate into an air shed where it can have an adverse effect on the industry (my ability to make a living), then yes.

Dennis Jaasma
also ran a combustion
 research lab at
Virginia Polytechnic
 Institute.

Dennis Jaasma: Yes, central heaters merit further evaluation. I don't know how many models are available. I think EPA has done some work on them, but I do not know any results. Yes, they should be certified. They are in danger of becoming extinct if they don't wind up with a certification program.

Robert C. McCrillis: In some localities I think these furnaces are a problem; I don’t know how many are commercially available. I think I can name off six or eight companies and each one makes several models, but I don’t know what the total market is, maybe 10,000 - 15,000 a year. The little bit of testing that we did here, says that they are probably on a par with a conventional wood stove. The way those things work, they have a thermostatically operated draft and when the thermostat shuts off the draft closes, so you get this real smoldering burning situation. Secondary combustion technology probably wouldn’t work. Possibly a catalytic technology would, but I just don’t think it stays hot enough in there. I guess that really depends on the impact.

Ben Myren: I don’t think they should be exempt for any reason. As to the rest of it--are there emissions data for them? I suspect there are. Should they be certified? Yes they should be certified. Nobody should be exempt from the process.

Michael van Buren: I think there should be some type of testing on them.

List of Experts Interviewed

Mr. John Crouch, Director of Local Government Relations, Hearth Products Association (CA) [now HPBA]

Mr. Rick Curkeet, P.E., Manager, Intertek Testing Services (IL)

Mr. Bob Ferguson, President, Ferguson, Andors and Company (VT)

Dr. Skip Hayden, Director, Combustion and Carbonization Research Laboratory (Ontario, Canada)

Mr. Daniel Henry, Vice President, Aladdin Steel Products, Inc. (WA) [now Quadrafire]

Dr. Dennis Jaasma, Associate Professor, Department of Mechanical Engineering, Virginia Polytechnic Institute and State University (VA)

Mr. Robert C. McCrillis, P.E., Mechanical Engineer, Air Pollution Prevention and Control, Division, U.S. EPA (VA)

Mr. Ben Myren, President, Myren Consulting (WA)

Mr. Michael Van Buren, Technical Director, Hearth Products Association (VA) [now HPBA]

Interview Questions

RWC Technology Review

Environmental Protection Agency Order no. 7C-R285-NASX

prepared by

OMNI Environmental Services, Inc.

Beaverton Oregon 97005

1. State-of-the-art of wood stove combustion and emission control technologies.

1. 1.1  Are in-home emission reductions as compared to conventional stoves shown in Table 1 for catalytic and non-catalytic certified stoves reasonable?
2. 1.2  Are efficiencies shown in Table 2 for catalytic and non-catalytic certified stoves reasonable?
3. 1.3  Can catalytic technology for use in wood stoves be fundamentally improved?
4. 1.4  Is the use of manufactured fuel (densified and wax logs) a credible emission

reduction strategy? See Tables 1 & 2 .

5. 1.5  For non-catalytic stoves the heat retention adjustment with refractory material of various densities can reduce particulate emissions. How big an effect can this have?
6. 1.6  Approximately one half of the particulate emissions occur during the kindling phase for non-catalytic wood stoves and more than half for catalytic wood stoves. Are there improvements in technology that can mitigate this problem? Can specially designed high BTU wax logs be used to achieve a fast start and reduce kindling phase emissions?
7. 1.7  Should masonry heaters with tight fitting doors and draft control be classified as a wood stove and be subject to some type of certification even though most weigh more than 800 kg?
8. 1.8  Are the emissions and efficiencies for masonry heaters, based on in-home tests, shown in Tables 1 and 2 reasonable?

9. 1.9  The OMNI staff feels the emissions per unit of heat delivered (e.g., lb/MBTU or g/MJ) is a more appropriate way to rank the performance of wood burning appliances than emission factors (lb/ton or g/kg) or emission rates (g/hr). — Comments?
10. 1.10  Default efficiency values are used for wood stoves. This coupled with the fact that emission factors or rates (not g/MJ) are used to rank wood stoves does not provide an incentive for manufacturers to increase the efficiency of their stoves. — Comments? Should an efficiency test method as described (FR v. 55, n 161, p. 33925, Aug. 20,1990) be required to be used and the results listed?
11. 1.11  Have certified stove design and performance improved since the first certified stoves? If so, how?

2. State-of-the-art of fireplace emission control technology.
1. 2.1  Are the emission factors and efficiencies for the in-home use of fireplaces and inserts shown in Tables 3 and 4 reasonable?
2. 2.2  There appear to be only a few practical design or technology options for fireplaces that will potentially mitigate particulate emissions. — What designs and technologies are available? What retrofit options are there?
3. 2.3  The use of wax fire logs reduces emissions over the use of cordwood. Can the formulation of wax logs be changed to produce even less emissions?
4. 2.4  What are the distinctions between a masonry fireplace and a masonry heater?
5. 2.5  As with wood stoves, the OMNI staff believe that the mass of emissions per unit of heat delivered is a better way to rank the performance of fireplaces than emission factors or emission rates.
3. State-of-the-art of wood-fired central heating furnace emission control technology.

3.1 According to a Department of Energy survey out of the 20.4 million households that used a wood burning appliance in 1993, less than 0.3 million used a wood burning furnace as their primary source of heat. Are there enough wood-fired central heating furnaces in use to merit their closer evaluation? How many commercially available models are there? Are there emissions data for them? Should they be certified?

4. State-of-the-art of pellet-fired wood stove technology.

1. 4.1  Are the emissions and efficiencies for the in-home use of pellet stoves shown in Tables 1 and 2 reasonable?
2. 4.2  The 35:1 air-to-fuel ratio cut-off for certification has produced two classes of pellet stoves — those that are certified and those that are not. The latter class may have models that are less efficient and have higher emissions than the former. Should the regulations be amended to close the loop-hole and discourage the practice of intentionally designing models with a higher air-to-fuel ratio to avoid certification?
3. 4.3  Have pellet stove design and performance improved since the first models were introduced? If so, how?

1. Ramifications of ISO.

5.1 The International Organization for Standardization (ISO) has a technical committee for developing emissions, efficiency and safety test standards for wood-fired residential heaters and fireplaces. (See Table 5 for comparison of the draft ISO method 13336 with EPA methods 28, 5G and 5H.) Do you feel that the EPA methods should be replaced with or be made comparable to an international standard?

2. Correspondence between in-home and laboratory emission test results.
1. 6.1  How accurately do certification tests predict in-home performance?
2. 6.2  How would you design research testing in the laboratory to simulate in-home use?
3. EPA Method 28 strengths and weaknesses.
1. 7.1  Method 28 is in part an “art”. Fuel loading density, fuel moisture, fuel characteristics (old vs new growth, grain spacing, wood density) and coal bed conditioning can be adjusted within the specification range of the method to influence results. In your experience what things have the most effect on particulate emissions? How much influence can they have?
2. 7.2  Burn rate weighting is based on very limited data and the cities from where the data were obtained are not very representative of wood use nationwide (see Table 6). How can the weighting scheme be improved to be more representative of the nation as a whole?
3. 7.3  The equation for the calculation of the air-to-fuel ratio as in Method 28A is in error. The error produces a small but significant difference in the calculated air-to-fuel ratio. Should the method be corrected or should it be left as a “predictor” of the air-to-fuel ratio?

7.4 The assumed mole fraction of hydrocarbons (YHC) is defined as a constant in the air-to-fuel ratio calculations in Method 28A. The mole fraction of hydrocarbons in the vapor phase will vary significantly with fuel and combustion conditions. Should hydrocarbon vapors (more appropriately, organic compound vapors) be measured as part of the method?

4. EPA Methods 5G and 5H correlations.

8.1 The comparison data to demonstrate the correlation between 5G and 5H are limited. Should the correlation between the two methods be reevaluated?

5. Performance deterioration of EPA-certified wood stoves in the field.
1. 9.1  It is the opinion of many in the wood stove industry that catalysts last only five years and that a stove designed for a catalyst operated without a functioning catalyst can produce as much emissions as a conventional stove. — Comments?
2. 9.2  Field studies in Glens Falls, NY, Medford, OR, Klamath Falls, OR and Crested Butte, CO showed that emissions from some catalytic stoves became appreciably worse even after two to three years of use. Inspection of stoves in Glens Falls showed that catalyst deterioration and leaky bypass systems were responsible. Have improvements been made in the design of catalytic stoves to minimize these problems? Is it reasonable to require homeowner training on the proper use of catalytic stoves and/or to incorporate into their costs an inspection and catalyst replacement program?
6. Stress test pros and cons.
1. 10.1  A short-term laboratory woodstove durability testing protocol was developed to predict the long-term durability of stoves under conditions characteristic of in- home use (see EPA-600/R-94-193). It was concluded in that study that damage occurs during those occasional times when a woodstove is operated in the home at exceptionally high temperatures. The laboratory stress test was designed to operate a woodstove at very high temperatures over a one to two week period to predict long-term durability under in-home use. Is this a reasonable approach?
2. 10.2  Should a stress test be made part of the certification process?
7. Feasibility of developing separate emission factors for dry and wet wood and for

softwood and hardwood species classes.

1. 11.1  Optimum wood moisture for low particulate emissions seems to be in the 18% to 20% range. Are you aware of any data that will allow the impact of wood moisture to be isolated from other variables? Could it be different for wood from different tree species?
2. 11.2  Wood from different tree species clearly burns differently. The chemical make-up and density of wood from different tree species is different. For example wood from coniferous trees has more resin than wood from deciduous trees. It is believed that particulate emission factors will be different for wood from different tree species. If this is true different parts of the country may have different emissions factors for residential wood combustion. Are you aware of any data that document different emission factors for wood from different tree species?

8. Routine maintenance.

12.1 Would routine maintenance of stoves once they were in a home reduce particulate emissions? Would this be more relevant for catalytic stoves than non-catalytic stoves? Would this be relevant for pellet stoves with electronic and moving parts?

2. 12.2  Should the home owner be provided with a maintenance manual or a training course at the time of purchase? Should a maintenance program be part of the purchase price particularly for catalytic stoves?
3. 12.3  What would the key elements of routine maintenance be?

- end -

An Analysis of Stove Emissions and the Proposed NSPS

Alliance for Green Heat

April 1, 2014

Conventional wisdom says that cat stoves burn cleanest on a low air setting and non-cats burn cleanest on a high air setting.  Conventional wisdom also says that pellet stoves are cleaner than wood stoves.  However, the database used by industry and the EPA to analyze stoves shows that the reality is much more complicated than conventional wisdom suggests.  And, the implications for designing and testing cleaner stoves that hit a 1.3 standard, or whatever number the EPA arrives at, may be significant. 

This analysis shows that stoves in all categories-cat, non-cat and pellet – usually burn dirtiest on their high air setting, which under a weighted average in the existing NSPS, is largely discounted.  Under the proposed NSPS that high burn rate becomes all-important, and will be the focus of most cat, non-cat and pellet stove testing.  This may lead to manufacturers trying to reduce the high air settings to get their stoves to pass.  This in turn could increase start-up emissions and make it harder to get stoves up to temperature quickly.

This also raises the question about whether it makes sense to test stoves at their highest air setting, when homeowners usually use them at the lowest air setting.  With 40% of non-cat stoves emitting the highest emissions in Category 4, should the new NSPS really test them based on Category 4 emissions and completely ignore Category 1 emissions?  We think that weighting of emissions can still make sense under the new NSPS, although the weighting of wood stoves may be different from the weighting of pellet stove emissions, based on data of where homeowners typically use their respective type of stoves. Weighting should at least be considered to include all the test burns required by the NSPS. If four test burns are required, one high and one low and two more at the dirtiest burn rate, they could all be weighted equally. 

This analysis is based on a database of 147 stoves, compiled by the Hearth Patio and Barbecue Association (HPBA) in 2010.  It became public in January of this year when the EPA released it among the scores of documents they used to determine what emission levels to set for wood and pellet stoves in the new NSPS. 

Summary notes:

·      * When the 5G correction factor is removed, as it will be under the new NSPS, more stoves will likely be able to pass stricter limits than what is often being reported.  Many who cite figures about how many stoves can pass the new NSPS limits appear to be unaware of this.

·     *  Five stoves – 2 catalytic, 2 non-catalytic and 1 pellet–appear to be able to pass a 1.3 gram per hour (g/hr) standard with the 5G correction using crib wood. More will likely pass without it. 

·      * If the EPA settled on a 2 g/hr standard, 16 stoves would appear to pass: 8 pellet, 4 catalytic and 4 non-catalytic.

·     *  On average, non-cat stoves tend to burn cleanest on Category 3, the medium high burn rate and dirtiest on Category 4, the high burn rate.

·     *  Cat stoves burn consistently cleanest on Category 1, the low burn rate and dirtiest on Category 4.

·     *  Pellet stoves burn almost equally cleanly on Categories 1, 2 and 3, but are significantly dirtier on Category 4.

·      * To pass future standards, many manufacturers may look to reducing the air in Category 4, which could have an impact on efficiency and may have effect of reducing maximum BTU output.

·     *  By testing at its dirtiest burn rate, many wood stove would be tested at burn rates which the consumer does not often use.  This may make testing less characteristic of real world use, rather than more.

·      * There is a very slight negative correlation between firebox size and emissions with non-cat stoves showing slightly lower average emissions from larger fireboxes.  Larger non-cat stoves are known to be harder to tune and get to pass.

·      * There is a very slight positive correlation between firebox size and emissions with catalytic stoves showing slightly higher average emissions from larger fireboxes.

·      * On the EPA list of certified stoves, there is a high concentration of stoves that tested just less than 4.5 g/hr, and relatively few that tested above 4.5.  This may indicate that stove companies are able to fine-tune their stoves to hit stricter emission targets.

The Database

The main database used by industry and the EPA has extensive details about 147 stoves, showing emission rates at the 4 burn levels.  It was compiled by Bob Ferguson, a consultant for HPBA who collected data from manufacturers who agreed to share it. The EPA independently also has this data and more.  Legally, emissions data is not protected by the confidential business information (CBI) label, but all manufacturers submit it as CBI, which then requires the EPA to undergo a lengthy process to remove the label. The data was compiled in 2010, so it’s a bit out of date and not an exhaustive list, but it still provides useful and important data to understand how a 1.3 g/hr standard may impact the stove industry in 5 or 8 years, when and if that standard becomes law.  There are up to 100 stoves not on this list, including many that became certified after 2010, which are being analyzed by the EPA. 

5G, 5H and the Correction Factor

One of the biggest hidden features of the NSPS is that stoves at 3.7 or 4.4 g/hr under the existing NSPS could come out a gram or even 2 grams less under the new NSPS.  This is because the EPA is getting rid of an adjustment or correction factor that has been used for stoves tested under certain test methods.  Most of the industry does not realize this and it makes the numbers proposed by the EPA appear stricter than they actually are.  In this analysis here, we are just using the numbers in the existing NSPS and we have not re-adjusted them. If we had, it would make many of them show much lower emission numbers. (For more info on this, see the emission testing method discussion here.) Seventy-two of the 147 stoves on this list used some version of 5G (5G1, 5G2 or 5G3). Twenty-nine did not specify whether they used 5G or 5H.

Stoves that can meet 1.3 grams per hour

This analysis is solely about meeting 1.3, or whatever standard the EPA arrives at, using crib wood testing.  Crib wood testing will continue to be used for five years before transitioning to cord wood.  Once manufacturers start designing for cordwood testing, stoves may operate in people’s homes more like they were tested in the lab, and become cleaner.

For the next five years, the EPA will maintain a 4.5 standard that average stoves can meet.  The proposed 1.3 standard to take effect in five years purposefully seeks to achieve the best demonstrated technology and get manufacturers to change the design of their stoves.

Of the 147 stoves on the list, 5 of them tested under 1.3 g/hr on low and high rates and are likely to pass the proposed new EPA standards using crib wood – but not necessarily cordwood.  The Alliance commented on how the EPA can set a standard for cordwood.  Two of them are cat stoves, 2 are non-cat and one is a pellet stove.

Overall Average Emission Rankings

The overall average weighted emission rate of all the stoves on the list is 2.01 for pellet stoves, 2.05 for cat stoves and 3.51 for non-cat stoves.  On average, all three categories of stoves were dirtiest on Category 4, which is the highest air setting and the most BTU output.  Cat stoves were the only type that was uniformly cleaner at Category 1 and uniformly dirtier at Category 4.  For non-cats, the cleanest average burn was Category 3, but there was only about a one g/hr difference between all 4-burn rates.  Conversely, cat stoves were consistently and significantly cleaner at one burn rate compared to others, with more than a 2.5 g/hr range.  Pellet stoves were slightly cleaner on Category 2, but Categories 1, 2 and 3 were very similar.  There was about a 1.5-g/hr range between cleanest and dirtiest burns.

It is important to know that EPA emissions numbers do not equally average the 4 burn rates to come up with a final number.  They are a “weighted average,” so that the high burn rate counts for very little and the low burn rate counts a lot.   Since the Category 4 high burn doesn’t affect the weighted average much, most manufacturers don’t pay that much attention to it.   This is one reason why the average Category 4 burns were the dirtiest for all stove types. On the other hand, the low burns are heavily weighted, so stoves are designed to perform well at those levels.  High burns that previously counted for 1-10% of a weighted average will now count heavily.  The good news for stove manufacturers is that high burn emissions are often easier and less costly to reduce than low burn emissions.  But the impact in the field is likely to be increased emissions during start-up.

Its also relevant that the median numbers for each burn rate for each stove type are lower than the average, showing that there are more stoves on the cleaner end of the spectrum. 

Cat Stove Emission Characteristics

Cat stoves were the only stove type to have an average under 1.3 on any burn category.  Cat stoves had an average of 1.1 g/hr on Category 1, the low burn.  But under the new EPA proposal, stoves have to meet 1.3 on their dirtiest setting, which will be Category 4 for cat stoves.  In this stove sample, the average on Category 4 was 3.6.

Under the proposed new method of testing at the dirtiest burn level, stoves that consistently have the greatest range between Categories 1 and 4 may be penalized, and stoves that have the least range may benefit.  While cat stoves have the cleanest burn at their “sweet spot” which helped them pass with flying colors over the past 2 decades, they will lose some of that advantage under the new rule which does not average in the clean burns, much less give them greater weight.

Of the 15 cat stoves:

·            *  13 burned cleanest on Category 1, the low burn rate

·            * 12 burned dirtiest on Category 4, the high burn rate

·            * The highest emitter was 9.7 g/hr on Category 4

Non-cat Emission Characteristics

Non-cats had the dirtiest average weighted burn rate of 3.5 and had the dirtiest average on any single burn category.  They had an average of 4.04 on high burn, and the cleanest average was 2.9 on medium high.  This contradicts the much-repeated conventional wisdom that non-cats are cleanest on their highest air setting and dirtiest on the lowest air setting.  Under the new NSPS, these stoves will usually have to be tested at the highest air setting, which is their dirtiest and have the furthest to come down towards 1.3. This may result in non-cats (and cats) being tested at burn rates that homeowners don’t often use.  Unlike cat stoves, that are consistently cleanest on Category 1 and consistently dirtiest on Category 4, the following table shows that cat stoves do not show any similar consistency:

We question whether this data supports the EPA’s proposal to test on a stove’s dirtiest burn rate.  Another option would be to maintain the weighted average for wood stoves (not pellet stoves) but put a cap on emissions on any test run as the EPA proposes to do with outdoor boilers.  Thus, even if the EPA finalized on 1.3 or 2.0 g/hr, the stove could not emit more than 3 or 4 g/hr on any burn rate.

Out of the 110 non-cats on the list:

·      8 were cleanest on Category 1

·      28 were cleanest on Category 4

·      54 were dirtiest on Category 4, the highest burn rate, and

·      33 were dirtiest on Category 1, the low burn rate

·      The highest emitter was 17.4 g/hr on Category 4

Pellet stove Emission Characteristics

As noted above, the cleanest weighted average included one pellet stoves that emitted under 1.3 g/hr on high and low burn rates. Even though nearly a third of pellet stoves had a weighted average under 1.3, with the weighting removed and testing focused on the dirtiest emission rate, many pellet stoves would have to redesign to get their high burn rate emissions down. This is why many manufacturers are currently getting the pellet stove certified so they have a 5-year certificate and won’t have to retest under the new testing protocol for 5 more years. 

Emissions were virtually flat on Categories 1, 2 and 3 (between 1.65 and 1.8) and about the same number of stoves had their cleanest run on Category 1, 2 and 3.  Pellet stoves were consistently dirtiest in Category 4, where emissions jumped to an average of 2.9. One pellet stove put out 11.9 g/hr on Category 4.  However, many experts believe that pellet stoves have a lot of room for improvement and have been intentionally “de-tuned” to meet the 35 to 1 air to fuel ratio. 

Of the 22 pellet stoves:

·      10 burned cleanest on Category 1, the low burn rate

·      12 burned dirtiest on Category 4, the high burn rate

·      5 burned dirtiest on Category 1

·      The dirtiest emitter was 11.9 on Category 4

There is greater certainty around pellet stoves, as they are not undergoing such a drastic switch in testing methods from crib wood to cordwood.  Their test fuel is not remaining the same however, as the EPA will start to require the use of PFI certified pellets, which may perform slightly different than the pellets used before.

If the EPA were ultimately to set a 2 g/hr standard after 5 years, this database shows that pellet stoves would be the category that shines between 1.3 and 2 g/hr, with about a third of them emitting less than 2 g/hr on all four burn rates.  If the EPA were to set a 2.0 limit, there are hardly any more cat or non-cat stoves that would be less than 2 g/hr four burn runs on this list (more would likely pass after taking away the 5H correction).

Implications for Efficiency

Under the proposed NSPS, there will be no minimum efficiency standard, but manufacturers will have to test and report efficiencies using the CSA 415.1 stack loss efficiency test.  This is a method that measures how hot the flue gasses are coming out of the stack, compared to the heat that was transferred from the stove to the room.  All other things being equal, the hotter the gas coming out the chimney, the less efficient the stove.

Stoves tend to burn more efficiently at lower burn rates and less efficiently at higher burn rates.  To design a stove that meets 1.3, or whatever number the EPA arrives at, on high burn, many manufacturers may focus R&D on lowering their highest burn rate.  Under this scenario, a stove’s efficiency may rise.

However, if efficiency calculations under B415.1 are done using a weighted average of all four burn rates, that data will not even be available and efficiency will only be calculated using high and low burn rate data.

One negative implication of reducing air in Category 4 is that stove start up may be more difficult and could result in greater emissions as Category 4 is most consistently used during the start up period. This problem could be reduced if the operator leaves the door slightly ajar, which is already a very common practice and recommended as an option in some owner’s manuals.  However, this cannot be done during certification testing.

Pellet stoves are likely to see the most rapid rise in efficiencies because many of them are currently exempt because of the 35 to 1 air to fuel ratio exemption.  By using this exemption from certification, manufacturers have penalized the efficiency of many pellet stoves.  By removing that exemption to certification in the proposed NSPS, those stoves will have to reduce airflow through the combustion chamber, which can significantly raise efficiency.  A 5 – 20% rise in efficiency, or more, is possible for many exempt stoves.  This will result in significant fuel savings for thousands of consumers.

Tuning a pellet stove for maximum efficiency could cause problems in stoves that are vented through the wall instead of through the ceiling.  Pellet stoves are tested with a vertical stack set-up and a side vented unit will not have the benefit of that increased draft.  The lower efficiency and higher airflow of some existing pellet stoves can help them in the field when they are side vented.

Cat Stove Emission Correlation

We found that there is a small positive correlation between the usable firebox volume and EPA weighted emissions. This means that as the volume of the firebox increases, so does the emission of the stove in g/hr. The value of R (correlation) is 0.3356. Although technically a positive correlation, the relationship between the two variables is weak (the nearer the value is to zero, the weaker the relationship) using an alpha of 0.05, the correlation is not deemed to be statistically significant. The scatter plot below depicts the slight positive slope.

Non-cat Stove Emission Correlation

 The same correlation between firebox volume and EPA weighted emissions evaluated for non-cat stoves shows a small negative correlation. The sample size is much larger at 110 stoves, and the correlation between volume and emissions has an R-value of -0.2566. This negative correlation means that as the firebox size goes up, the weighted emissions of the stove go down, for a cleaner burn. Although technically a negative correlation, the relationship between these two variables is also statistically weak with an alpha level of 0.05. The graph above shows the downward sloping relationship. Firebox sizes are not as relevant with pellet stoves and very few of 22 pellet stoves on the list even included their firebox size, so we did not perform a correlation analysis for them.

Distribution of Tested Emissions in Non-Cat Stoves

 Using the list of EPA certified stoves that was updated in December 2013, we see a very high concentration of stoves that were tested right under 4.5 g/hr.  Conversely, there are very few stoves tested right above 4.5 g/hr.  This may indicate an ability of stove manufacturers to fine tune their stoves to just barely meet stricter EPA standards.  The EPA stove list does not include the lab where they were tested, so it is not known at which EPA approved labs the testing was done.  Similar emission charts for catalytic stoves and pellet stoves did not show any similar concentration around 2.5 or 4.5 g/rh.  Cat stoves are held to a 2.5 g/hr standard in Washington and Oregon and in some change-out programs.