United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S 7-87/026 Feb. 1988
Project Summary
The Northeast Cooperative
Woodstove Study
Paul Burnet
This report gives results of a 2-year
study in Vermont and New York,
monitoring woodstove performance.
The objective of the study was to
determine the effectiveness of catalytic
and non-catalytic low-emissions wood-
stove technology in reducing wood use,
creosote accumulation, and paniculate
emissions. Wood use and creosote
accumulation in chimney systems were
measured in 68 houses over two
heating seasons. Of these houses, 42
were instrumented to measure partic-
ulate emissions and directly measured
wood use. Catalytic woodstoves, cata-
lytic add-on/retrofit devices, and non-
catalytic low-emission stoves were
provided by various woodstove manu-
facturers for use by volunteer
homeowners during the study period.
Conventional technology stoves were
also included to provide baseline data.
Averaged results indicate that the
low-emission non-catalytic stoves and
catalytic stoves had lower creosote
accumulation, wood use, and particu-
late emissions than conventional tech-
nology stoves, but the range of values
was quite large. Particulate emissions
reductions by the catalytic and low-
emission stoves were not as great as
could be expected based on laboratory
tests. The many variables affecting
stove performance in real world con-
ditions make it difficult to identify
causative factors. Additional analysis
of data and further tests are currently
planned.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory. Research
Triangle Park. NC. to announce key
findings of the research project that is
fully documented in two separate
volumes of the same title (see Project
Report ordering information at back).
Introduction
Woodstove performance was studied
during the 1985-86 and 1986-87 heating
seasons in the Northeast. In the Water-
bury, VT, and Glens Falls, NY, areas, 68
homeowners were provided with
selected "advanced technology" stoves
or asked to use their existing (conven-
tional) stoves for the study. The stoves
were monitored for wood use, creosote
accumulation in the chimney system,
and paniculate emissions. Three
advanced technology stove categories
(catalytic stoves, add-on/retrofit devices,
and low-emission, non-catalytic stoves)
were compared with conventional tech-
nology stoves. Objectives of the study
were to evaluate the performance of the
advanced technology stoves for safety
factors (creosote), efficiency (wood use),
and environmental impacts (particulate
emissions). The effectiveness of catalytic
combustors was emphasized.
Creosote and woodpile volumes were
measured on all 68 houses. Creosote
accumulation was measured by period-
ically sweeping the chimney system and
weighing the collected material. Wood
use was monitored by measuring wood
piles during the heating season and
normalizing for moisture content and fuel
species.
Additionally, 34 houses were routinely
sampled for particulate emissions over
1 -week periods. These houses had data
logging systems to record stove temper-
atures, flue gas oxygen concentrations,
and wood weights. Particulate samples
consisted of integrated samples collected
every half hour during each week-long
sampling period. Fl ue gas flow rates were
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\
calculated based on combustion stoichi-
ometry: burn rates, fuel species, flue gas
oxygen measurements, and estimated
CO/C02 levels.
Note that many variables were found
in field stove installations: chimney
systems, fuel characteristics, user prac-
tices, stove maintenance, etc. The range
of values recorded in all categories was
quite large. Reported data, while repres-
enting the values recorded during this
study, may not be representative of other
climates, fuel woods, stove or catalytic
combustor models, chimney systems, or
stove use patterns. Great care should be
used in extrapolating these findings to
other circumstances.
Due to the high variability and large
range of data, averages from advanced
technology stove groups, in most cases,
were not statistically different from the
conventional stove group. "Student's t"
tests showed that only the low-emission
non-catalytic stove group had a mean
particulate emission rate with a greater
than 90% probability of being different
and hence lower than those from the
conventional stove group. Emissions
from individual stove models, however,
often were statistically different from the
mean of the conventional stoves. All
advanced technology devices (catalytic,
add-on/retrofit, and low-emission non-
catalytic) showed lower average partic-
ulate emission rates, wood use, and
creosote accumulation than the conven-
tional technology. Figure 1 summarizes
averaged results from the stove technol-
ogy groups.
The stove technology group data
represent averages, and reflect a wide
range of values. In general, all stove
categories, including conventional
stoves, had models and specific instal-
lations with low (and high) particulate
emissions. It is therefore most approp-
riate to evaluate stove performance on
a model-by-model basis, recognizing that
(due to the relatively few installations
and stove models) values may not be
representative of "typical" stove
performance.
Although the number of samples is
high, the wide range of values and the
many variables make it difficult to identify
causative factors. Results given in this
report are from a number of stove types
and models in different installations, in
which homeowners used different fuels
and operating procedures. A thorough
review of stove burn rates, fuel loading
practices, catalyst operation times, and
frequencies of alternate heating systems
did not identify a single factor responsible
for emission patterns. This indicates that,
while many factors can affect particulate
emission rates, no single factor appears
to be dominant in all stove types or
models. In general, however, it appears
that stoves with smaller fireboxes,
regardless of technology type, tend to
have lower emission rates.
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Particulate Emissions, g/hr
Wood Use, kg/HDD
Creosote Accumulation, kg/1000/HDD
(HDD = Heating
Degree Days)
Figure 1. Performance comparison by stove technology.
2
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Conclusions
General conclusions are listed below
by category: Advanced Technology Per-
formance, Catalyst Performance, Oper-
ator Practices, Technology Factors, and
Other Findings.
Advanced Technology
Performance
1. Most stoves in the advanced tech-
nology categories (catalytic, add-
on/retrofit, low-emission non-
catalytic) episodically demon-
strated lower emissions than the
baseline conventional stoves under
field use conditions. Good perfor-
mance in at least one installation
for most of the stove models indi-
cates that factors, such as stove
maintenance andfueling practices,
may be as important as stove
technology features in achieving
low emission rates. Stove firebox
size, regardless of technology
group, was a prime factor in deter-
mining emission rates; smaller
stoves had lower emissions.
2. In general, performance of the
stove technology groups appeared
to be consistently ranked in terms
of particulate emission rates, wood
use, and creosote accumulation;
low-emission non-catalytic stoves
had the lowest particulate emis-
sion rate, wood use, and creosote
accumulation, while conventional
stoves had the highest. Note that
only low-emission non-catalytic
stoves showed a mean emission
rate which was statistically differ-
ent from that of conventional
stoves. Also note that creosote
accumulation is strongly influ-
enced by flue system type, and
wood use appears to be influenced
by burning patterns and firebox
size.
3. All advanced technology stove
groups averaged lower wood use
and creosote accumulation rates
when households switched from
conventional stoves between
heating seasons. Average reduc-
tions by stove group ranged from
about 10 to 35% for creosote, and
from about 15 to 30% for wood use.
4. The low-emission stoves, as a
group, had the lowest average
emissions. Each model had differ-
ent burning characteristics; most
showed relatively good perfor-
mance. Average results from this
technology group are strongly
influenced by the good perfor-
mance of two stoves (M and N)
which may be EPA 1990-
certifiable. Furthermore, excluding
one high-emission house (V18,
using non-EPA-certifiable Stove K)
would reduce average emissions in
this category from 13.4 to 10.0 g/
hr, and reduce the standard devi-
ation (a) from 10.2 to 5.7.
5. User satisfaction was generally
high with the advanced technology
stoves provided to study houses. In
particular, homeowners with cata-
lytic and low-emission stove mod-
els were frequently pleased with
the units. (In some cases, user
satisfaction remained high even
though the catalytic combustor had
deteriorated.) Some add-on devices
also received positive comments.
The add-on with the lowest aver-
age particulate emission rate also
received homeowner complaints
about smoke spillage.
Catalyst Performance
1. Catalytic stoves showed variable
performance. Most individual mod-
els performed well in some houses.
Other installations had relatively
high emissions. Overall, perfor-
mance of these stoves did not
match the expectations created
under ideal laboratory conditions,
although only one of the catalytic
models may be EPA 1990-
certifiable. The mean emission
rates of existing catalytic stoves
and new catalytic stoves were
virtually identical User education
and further technology refine-
ments remain possible factors
which could help improve the
performance of catalytic stoves.
2. Add-on/retrofit devices did not
perform well overall, but two
devices reduced emissions consid-
erably. The stoves on which these
devices were mounted are a major
factor in measured emission rates.
Retrofit F, which consistently had
high emissions, is no longer being
produced.
3. Catalyst durability was quite var-
iable. Rapid deterioration was
noted in some combustors, all of
which were cordierite-based, with
corresponding increases in emis-
sions. In one stove model (which
apparently accelerated combustor
deterioration), replacement with
second generation, non-cordiente
combustors appeared to virtually
eliminate the deterioration trend.
Emissions from this stove model
were reduced by about 30% by
using second generation combus-
tors during the second year,
although it is not clear whether this
was from improved catalytic per-
formance or reduced degradation.
4. Overall, there did not appear to be
a consistent increase in particulate
emissions from catalytic devices
over the 2-year testing period. No
clear trend of long-term loss of
effectiveness was noted. However,
a number of combustors
(cordierite-based) were discovered
to be deteriorating. These combus-
tors were replaced; emission
values reported in this study reflect
relatively frequent catalyst inspec-
tions and replacement when
necessary. Note, however, that not
all cordierite-based combustors m
the study indicated signs of dete-
rioration of the substrate. A
cordierite-based combustor from
an existing stove with an estimated
6000 hours of use showed rela-
tively low emissions m laboratory
retestmg. All combustors retested
m the laboratory had reduced
performance relative to new
combustors.
5. Condensation of moisture and
organic material in flue systems
and subsequent drainage or leach-
ing of condensate was a problem
in some houses during very cold
« 20°C) weather Only catalytic
stoves experienced this problem.
This appears to be related to
inappropriate installation and is not
necessarily a technology limitation
6. Catalyst AT (temperature change
across the combustor) and percent
operation time are not good indi-
cators of stove particulate emis-
sions Factors such as fueling
cyclesflong burn-down "tails") and
measurement difficulties may pre-
clude the use of these parameters
for predicting emission rates.
Operator Practices
1. Operator practices, in combination
with other parameters, appear to
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be a significant factor in stove
performance. Specific practices
which may result in lower emis-
sions from all stoves have not been
identified from available data.
However, routine maintenance
inspections of the combustor,
gasketing, and overall stove system
can help identify deteriorated com-
ponents in need of repair or
replacement.
2. Burn rates did not demonstrate a
strong correlation with emission
rates for any of the stove technol-
ogy groups, although general
trends were observed. Often, as
with conventional stoves, the trend
was opposite that which was
expected; emissions increased
with burn rate. This may be related
to field conditions, in which lower
burn rates may include longer
"charcoal phase" burning periods.
3. Mean fuel loading frequencies
were identical for the low-emission
and conventional stove groups,
although the average low-emission
stove fuel load was 56% that of the
average conventional stove fuel
load. This indicates that smaller
firebox capacity (typically asso-
ciated with low-emission stoves)
does not necessarily require more
frequent fueling of the stove. User
satisfaction was generally high
with the low-emission stoves.
4. Average emission factors (g/kg) for
all the stove categories were quite
similar. Differences in average
emission rates (g/hr) were there-
fore driven by burn rates. The low
average burn rate of the low-
emission stoves, and resulting low
average emission rate, may be due
to more frequent "charcoal phase"
burning periods.
5. Fuel loading frequencies did not
correlate well with particulate
emissions. However, loading fre-
quencies did increase with smaller
fuel loads for all technology groups,
as was expected.
6. Fuel loading frequencies were
significantly different between
houses, even those using the same
stove model.
7. The lack of strong correlations
between particulate emissions and
other variables indicated that many
parameters have significant, if
unquantified, effects on stove
performance. Fueling and burning
cycles are thought to be areas for
further investigation.
Technology Factors
1. Firebox size is a major factor in
determining particulate emissions
from woodstoves; emission rates
increased with firebox volume,
regardless of stove technology.
2. Preliminary results from stove
inspections conducted after the
second heating season (September
1987) indicate that significant
leakage of smoke around combus-
tors may be a cause of high emis-
sions in some stoves. (A separate
report on this work will be issued.)
Stove inspections showed that
gasketing, especially around the
bypass damper and combustor,
was the most frequent component
in need of maintenance and the
apparent cause of leakage. Leak-
age rates and particulate emissions
do not appear to correlate well
overall, but show some correlation
for individual stove models.
3. Using a qualitative measurement
methodology, insulated metal
chimney systems accumulated the
least amount of creosote. Masonry
chimneys located on outside walls
accumulated the most.
Other Findings
1. This study did not show that one
stove model is necessarily better
than another. As stated previously,
a wide range of results were
recorded. For a given stove model,
the most emission samples was 19;
the fewest was 1. The most instal-
lations for a given stove model was
four; the fewest was one. The high
degree of variability in performance
and the relatively small sample
populations make comparisons
inappropriate.
2. Conventional stoves in this study
may be cleaner-burning heaters
than are "typical." Four of the six
conventional stoves had relatively
small fireboxes ( < 2.4 ft3), and two
of these had small effective fire-
boxes ( < 1.5 ft3). Emissions from
these stoves therefore may not be
typical of existing stove technology.
Additionally, the cold Northeast
climate and commensurately
higher burn rates preclude direct
comparison to stove performance
in milder climates.
3. Alternate heating system use did
not correlate well with particulate
emission rates or burn rates,
although heating system use was
monitored only in the room with the
stove. In general, most houses in
the study used their alternative
heating system less than 3.5% of
the time (while the stove was
operating). This amounts to less
than 1 hour per day. Many of the
houses used no backup heat at all.
4. Polycyclic organic material (POM]
emissions were variable and non-
conclusive. Testing method and
analytical method limitations, and
a very limited database, preclude
any ranking of POM emissions by
stove type.
4
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—_ \OH\O,
Paul Burnet is with OMNI Environmental Services, Inc., Beaverton, OR 97005.
Robert C. McCrillis is the EPA Project Officer (see below).
The complete report consists of two volumes, entitled "The Northeast
Cooperative Woodstove Study:"
"Volume I," (Order No. PB 88-140 769/AS; Cost: $32.95)
"Vo/umell. TechnicalAppendix,"(OrderNo. PB88-140 777'/AS; Cost: $19.95)
The above reports will be available only from: (costs subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use S300
EPA/600/S7-87/026
ps
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