By Jim Gillam, editor of SNEWS - The Chimney Sweep's Newsmagazine. This article originally appeared in the March 2001 issue of SNEWS, which is an independent trade journal for chimney professionals.
The U.S. Environmental Protection Agency says certified woodstoves degrade with use, but they still burn cleaner than uncertified stoves.
Jim Gillam says with conscientious woodburning and regular service by a professional chimney sweep, certified woodstoves in the real world nearly match their laboratory emission numbers.
Particulate emissions measured from woodstoves after extended use don't match the laboratory numbers reported for the same stove models when certified to the U.S. Environmental Protection Agency's (EPA) Phase II emissions standards, and in most cases are higher, according to an EPA-sponsored study released in December. "But, on average, after about 7 years they still have lower emissions than uncertified conventional stoves," the EPA report concluded.
Close analysis of the small sample in this study also suggests that conscientious woodburning and regular service of the stove and chimney system by a professional chimney sweep enables a woodstove in active use to burn so cleanly as to nearly achieve the laboratory emissions values.
Due to concerns about air pollution, a maximum level of particulate emissions is enforced for woodstoves. Before any stove model can be sold, manufacturers must submit a sample for testing under controlled conditions in an emissions laboratory.
Air quality regulators are also concerned about the levels of polycyclic organic matter (POM) produced by woodburning appliances.
Primarily due to the high operating temperatures characteristic of woodstoves, some components of woodstoves may become damaged with long-term use. These components include baffles, catalytic combustors, catalytic combustor bypasses, gaskets and seals.
This study attempted to test the suspicion among air quality regulators that air emissions from woodstoves with damaged or worn out parts would be higher than values accepted during the certification process. EPA hired OMNI Environmental Services, Inc. of Beaverton, Oregon, a leading emissions testing laboratory, to examine sixteen EPA Phase 2 certified woodstoves that had been installed in residences prior to autumn 1992. They were monitored "in order to assess the level of long-term degradation and potential increase in PM (particulate matter) and POM (polycyclic organic matter) air emissions of older Phase 2 certified stoves under actual in-home usage."
The testing took place in November and December of 1998.
The Testing Emissions in the Real World
A total of 43 test runs were performed during three one-week periods. The stoves were located in Klamath Falls and Portland, Oregon.
Though in the same state, the two locations have dissimilar climates. Klamath Falls, situated in an intermountain basin at 4100' elevation, is relatively arid and cold in the winter with an average heating degree-day (HDD) value of 6600. Portland has a mild maritime climate with rainy winters. Portland's HDD value is 4109. The intent was to produce results more widely applicable to the nation as a whole than if homes in a single city were selected.
Participants in the study burned locally available cordwood fuel and were reimbursed for its cost. Fuel moisture was measured with a Delmhorst moisture meter, or determined through drying/gravimetric analysis in the laboratory. Wood was div ided into pre-weighed bundles, and any leftover wood was also weighed.
A variety of species was burned. Softwoods and hardwoods including Douglas fir, maple, alder, oak, cherry, birch, and lodgepole pine were burned in the Portland woodstoves. In the Klamath Falls appliances, conifers including lodgepole pine, ponderosa pine, juniper and Douglas fir were consumed.
Wood moisture in this study ranged from 9% to nearly 53% on a wet basis. Wood moisture averaged considerably less in the Klamath Falls group.
A sampling device known as the AWES (Automated Woodstove Emissions Sampler) was attached to the stovepipe of each stove. The AWES was developed by OMNI Laboratories to measure emissions of residential woodburning appliances during normal in-home use. It has previously been used in studies quantifying emissions from woodstoves, masonry heaters, pellet stoves and fireplaces.
In appearance, the AWES was simply an aluminum box that sat near the stove. A stainless steel inlet probe was inserted into the stovepipe 30 centimeters above the flue collar of the stove.
During this study, the AWES was programmed to sample for two minutes every fifteen minutes whenever the temperature of the flue gases exceeded 100°F, indicating that the appliance was in use.
Samples of the flue gases flowed into the AWES through stainless steel tubing. Particulate samples were captured with a heated filter and the oxygen content of the flue gas was measured with an electrochemical cell. Flue gas temperature and room temperatu re were recorded and the flue gas was returned to the woodburning appliance chimney above the point where the sample was withdrawn.
The Results
The sixteen used certified woodstoves in this study emitted an average of 11 grams per hour (g/hr) of particu late matter. This is more than double the certification value for those stove models, but substantially lower than the emissions measured from uncertified woodstoves in similar tests.
"Out of the 16 stoves inspected, all showed the effects of use," the report concluded. "Routine maintenance or minor repairs could have kept all units in good operating condition if they had been done," the authors (Lawrence H. Fisher, James E. Houck and Paul E. Tiegs of OMNI Environmental Services, Inc., Beaverton, OR and James McGaughey of the Eastern Research Group, Inc., Morrisville, NC) added.
Although this was not specifically tracked in the study, I have personal knowledge of five of these woodstove systems, having serviced them annually as Ash Bros. Chimney Sweep for a number of years.
The maintenance history of the remaining three Klamath Falls stoves, and of the eight Portland stoves, is unknown.
The average emissions rate for the five chimney sweep maintained woodstove systems was 4.8 grams per hour (g/hr). This is very close to the average certification value, 4.2 g/hr, derived in laboratory testing for those stove models. All of these stoves were non-catalytic models. The certification threshold for EPA Phase 2 certified non-catalytic stoves is 7.5 g/hr.
The average emissions rate for the 11 stoves with unknown maintenance histories was 13.8 g/hr, while the average certification value for those stoves was 3.9 g/hr. These stoves included five catalytic models. The certification threshold for Phase 2 catalytic stoves is 4.1 g/hr.
Conscientious Wood Burning
One objective of this study was to document changes in emissions performance of woodstoves over time. An attempt was made to return to woodstoves studied in real-world emissions monitoring conducted in 1989/1990 [see SNEWS July 1990, p. 20] and 1991/1992. This proved to be possible in only two cases. In only one case were the same homeowners (stove operators) involved.
In the only case where emissions from the same stove with the same operators were compared, emissions actually were lower in 1998 testing than they were when the stove was new. In this instance, the same woodstove with the same operators was tracked through three studies over a nine-year period.
The authors of the EPA report said, "The higher emission factor [for this stove] in the 1989/1990 study cannot be readily explained. However, it is probably simply a reflection of the variability often seen in woodstove emissions when different fuels are burned and different burning patterns are used."
"We've learned about the stove," said Karine Neubert, the owner of the Haughs 171E non-catalytic woodstove that emitted fewer particulates than did any other in this study.
"It burns really well," Mrs. Neubert continued, "but we were concerned about how it would measure up in this study, because there is a crack in the baffle."
Fuel Moisture
Mrs. Neubert gave some of the credit for their stove's low emissions in the tests to their firewood. "We always burn good wood," she said. "We keep it dry and covered."
The report documents that the Neuberts burned mostly lodgepole pine during the 1998 study, lodgepole and Douglas fir in the 1991/1992 tests, and juniper during emissions testing in 1989/1990. In each of the three tests, the Neuberts' wood moisture measured between 13-17%.
The EPA report noted that particulate emissions for stoves in Portland were higher on the average than the stoves in Klamath Falls. "This result is consistent with the average higher fuel moisture content and burn rate characteristics of the Portland portion of the study as compared with the Klamath Falls portion of the study," the authors said.
Drawing Conclusions
"The emission rates for phase 2 stove models reported as part of the NSPS (new source performance standards) certification process do not represent emission levels of the same stove models in homes after extended use," the authors concluded. "On the average, they still have lower emissions than uncertified conventional stoves."
The authors were careful to note that "no direct statistical correlation between emissions and wood moisture, burn rate or stove condition could be made due to the number of variables associated with real-world in-home use of woodstoves."
With this in mind, their conclusion that "the particulate emissions factors of the certified phase 2 stoves evaluated in this study appear to have become higher with use" would appear to be questionable. The disappointingly small subset of two stoves that participated in previous emissions tests and thus are available for comparison doesn't lend support to this view. One of the two stoves actually had lower emissions numbers in the later tests.
However, observation and photographic evidence definitely confirm that the stoves 'showed the effects of use."
The Bottom Line
With the authors' hint that "routine maintenance or minor repairs could have kept all units in good operating condition" we are led to the most important conclusion available from this small study, although they did not explicitly state it, or perhaps even realize it:
With proper installation and conscientious operation, including use of covered, seasoned wood and regular service by a competent professional chimney sweep, certified woodstoves after years of use burn nearly as cleanly in the real world as they did under laboratory conditions for certification.