United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-00/100 December 2000
Long-Term of EPA-
2
and
Lawrence H. Fisher, James E. Houck, Paul E. Tiegs, and James McGaughey
Wood stoves have been identified as
a major source of particulate and poly-
cyclic organic matter (POM) emissions.
For this reason, new source perfor-
mance standards (NSPS) were promul-
gated for wood heaters. Wood heaters
sold after July 1, 1992, had to be certi-
fied for low emissions, meet the most
stringent requirements of NSPS, and
are referred to as phase 2 certified. Of
concern has been the fact that labora-
tory and field studies have shown that
certified wood heaters can physically
degrade with use and their air emis-
sions commensurately increase. The
objective of this study was to evaluate
the condition and air emissions from
old phase 2 certified wood heaters in-
stalled in homes and used regularly for
home heating since the 1992/1993 heat-
ing season or earlier. Sixteen stoves
were evaluated in the study, eight each
in Klamath Falls and Portland, Oregon.
An extensive data base from 43 week-
long test runs was developed. The par-
ticulate emission factors of the certified
phase 2 stoves evaluated in this study
appear to have increased with use but,
on average, after about 7 years still
have lower emissions than uncertified
conventional stoves. In addition, it was
clear from the results that emission
rates for phase 2 stove models reported
as part of the NSPS certification pro-
cess do not represent emission levels
of the same stove models in homes
after extended use.
This Project Summary was developed
by the National Risk Management Re-
search Laboratory's Air Pollution Pre-
vention and Control Division, Research
Triangle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Residential wood combustion (RWC)
has been identified as a major source of
particulate matter (PM) and POM air emis-
sions. During 1997, RWC contributed an
estimated 12% of the sum of the total PM
with aerodynamic diameters < 10 urn (PM10)
emissions attributed to all fuel combus-
tion, industrial processes, and transporta-
tion sources combined. RWC was also
identified as the largest single source of
POM during 1990. Approximately 72% of
the cordwood burned annually in the U.S.
in the category of residential wood com-
bustion was burned in wood stoves. (The
remaining 28% was burned mostly in fire-
places.) An estimated 9.3 million wood
stoves were used in homes during the
1997-1998 heating season.
Due to the level of emissions attributed
to wood stoves, standards of performance
were promulgated for new residential
wood heaters. All wood heaters sold after
July 1, 1992, have to meet the most strin-
gent phase 2 particulate emission limits
of the standards. These particulate emis-
sion limits are 4.1 g/hr for catalytic stoves
and 7.5 g/hr for noncatalytic stoves. The
limits for catalytic stoves are lower than
for noncatalytic stoves since the pre-
sumed deterioration of the catalyst over
time was estimated to result in emissions
from catalytic wood heaters over their use-
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ful lifetimes, approximately equal to those
for noncatalytic wood heaters.
Furthermore, there has been concern
about the overall physical deterioration of
wood stoves with use and the commen-
surate increase in air emissions. This con-
cern has been confirmed in both
laboratory and in-home studies: physical
degradation coupled with higher PM emis-
sions has been documented for some
stoves. Not only are accurate airshed in-
ventories of PM and POM fundamentally
important for health and environmental
assessments, state and local agencies in
areas of PM10 nonattainment have been
directed to consider performance degra-
dation in their State Implementation Plans
(SIPs) when calculating credits from re-
placing noncertified stoves with certified
stoves. The replacement of noncertified
stoves with phase 2 certified stoves re-
mains a viable option for reducing airshed
pollutant levels and obtaining PM10 SIP
credits because, as of 1997, more than
80% of the wood stoves in use were still
older non-certified units. In addition, be-
cause over 90% of the PM10 emissions
from residential wood combustion are also
PM with aerodynamic diameters < 2.5 jim
(PM25), emission credits may be very im-
portant for possible future PM25
nonattainment areas.
Experimental Procedures
The basis of the experimental study
was the use of the Automated Woodstove
Emission Sampler (AWES), which was
developed to quantify emissions of par-
ticles for residential wood-burning appli-
ances while they are in normal in-home
use. The AWES is small and operates
unattended in home settings. Due to the
temporal variability in emissions from
wood-burning appliances, the AWES is
also designed to collect long-term inte-
grated samples necessary to provide
mean values. Studies conducted with the
AWES have provided the majority of the
data base used for particulate emission
factor development by EPA for residential
wood combustion. The AWES has been
used to quantify emissions from wood
stoves, masonry heaters, pellet stoves,
and fireplaces. Due to its extensive use,
the AWES has undergone EPA-supported
quality assurance evaluation.
For sampling purposes, the AWES is
placed adjacent to the wood-burning ap-
pliance in study homes. For wood-stove
applications, a stainless steel inlet probe
is typically attached to the chimney (stove
pipe) 30 cm above the flue collar of the
stove. The sample is withdrawn at a rate
of approximately 1 L/min. The flow rate is
maintained by a calibrated orifice. Par-
ticulate samples, including condensible
particles, are captured with a heated filter
followed by an XAD-2® resin cartridge. All
interconnecting tubing, holders, and hard-
ware exposed to the sample are made of
either stainless steel or Teflon® to main-
tain sample integrity. After sample collec-
tion, the chimney gas is passed through
silica gel to protect downstream compo-
nents from condensate. The oxygen con-
tent of the chimney gas is measured with
an electrochemical cell. The sample flow
is then returned to the wood-burning ap-
pliance chimney above the point where
the sample was withdrawn. Room tem-
perature and chimney gas temperature
are measured with Type K thermocouples.
The chimney gas temperature is mea-
sured within the chimney at the same
location as the sample is withdrawn.
The data logging system is a key com-
ponent of the AWES. The data logging
system records date, time, oxygen con-
tent, room temperature, and chimney gas
temperature at regular intervals. The oxy-
gen content of the chimney gas, along
with the mass of wood burned, allows for
the calculation of total chimney gas flow
during sampling which is needed for the
subsequent calculation of emission rates
and emission factors. The record of chim-
ney gas temperatures allows for the total
time of appliance operation over the
course of the sampling duration to be
determined. In addition to data recording,
the system is programmed to control the
sampling frequency, sampling duration,
and sampling period. For this study, the
AWES was programmed to sample for 2
minutes every 15 minutes for 1 week.
The system is further programmed to turn
on the sampling pump during the pro-
grammed 2-minute sampling time only if
the wood stove is in operation (as deter-
mined by the chimney temperature), in
order to avoid collection of sample mate-
rial when the appliance is not in opera-
tion. A threshold chimney temperature of
100° F (38° C) was used as an indicator
of wood stove operation.
Results and Discussion
The primary objective of the study was to
select phase 2 stoves that were installed
in homes prior to the fall of 1992 in order
to assess the level of long-term degrada-
tion and potential increase in PM and
POM air emissions of older phase 2 certi-
fied stoves under actual in-home usage.
Wood stoves in homes in both Portland
and Klamath Falls, Oregon, were selected
for this study. The average heating de-
gree day (HDD) value for Portland is 4109
and the average HDD for Klamath Falls
is 6600. Stoves in the two climatologi-
cally dissimilar cities were selected to pro-
duce results more widely applicable to
wood-stove usage in the nation as a
whole than if homes in a single city were
selected. In addition, nine phase 2 stoves
installed in homes in Klamath Falls were
previously studied during the 1989-1990
and 1991-1992 heating seasons. There-
fore, a secondary objective of the study
was to utilize as many of these homes as
possible in the current study to help docu-
ment phase 2 stove degradation and com-
mensurate emission increase.
Sixteen homes were targeted for study
during the 1998-1999 heating season.
Two of the study group were homes in
Klamath Falls that had phase 2 wood
stoves that were part of the earlier stud-
ies. Emission samples were collected for
three 1-week periods from wood stoves
in each home using the AWES. The AWES
was developed specifically for the in-
home collection of air emission samples
from residential wood burning appliances,
and data developed from its use have
previously been used to calculate par-
ticulate emission factors published in AP-
42. Samples collected with the AWES
were analyzed for particulate matter and
organic compounds. The specific organic
compounds analyzed included the seven
and sixteen POM compounds needed to
calculate the 7-Polycyclic Aromatic Hy-
drocarbon (PAH) and 16-PAH values, re-
spectively, which are used as surrogate
indicators for POM. The PM and POM
surrogate emission factors (mass of pol-
lutant emissions per unit mass of fuel)
were compared against the emission fac-
tors tabulated in AP-42 for wood stoves.
The PM emission rates (mass of pollutant
emissions per time of stove operation)
measured under actual in-home use for
each wood-stove model were compared
against their certified emission values
listed by EPA.
The PM emissions from this study and
from the previous studies were compared
for the stoves in the Klamath Falls homes
that were part of earlier studies.
Cordwood tree species, cordwood mois-
ture, the amount of cordwood burned,
burn rates, ambient temperature during
testing, a description of wood stove use
in each home, chimney characteristics,
and the condition of the stoves were also
documented as part of the study.
Conclusions
Of the 16 stoves inspected, all showed
the effects of use. However, only six stoves
were degraded to the point that it was
speculated that their condition would
significantly affect air emissions. Rou-
tine maintenance or minor repairs could
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have kept all units in good operating con-
dition if the maintenance or repairs had
been done.
An extensive data base was developed
from the 43 week-long test runs on 16
homes in Klamath Falls and Portland. No
direct statistical correlation between emis-
sions 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 wood stoves.
The particulate emissions for stoves in
Portland homes were, on average, higher
than for stoves in Klamath Falls homes.
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 particulate emission factors for the
certified phase 2 stoves evaluated in this
study appear to have become higher with
use, but after about 7 years, on average,
the certified phase 2 stoves still have
lower emissions than uncertified conven-
tional stoves (Table 1).
The emission rates for phase 2 stove
models reported as part of the NSPS cer-
tification process do not represent emis-
sion levels of the same stove models in
homes after extended use.
Particulate emissions cannot be used
as a surrogate measure of POM emis-
sions for wood stoves. POM emission fac-
tors, as based on the 7-PAH and 16-PAH
surrogates, determined from the in-home
use of wood stoves in this study, were
lower than the POM emission factors tabu-
lated in AP-42. This observation is signifi-
cant because the AP-42 emission factors
are the basis for the national emission
inventory of POM for which residential
wood combustion was identified as the
single largest source.
Table 1. Comparison of average
particulate emission factors (5H
adjusted) to AP-42 values.
Stove Group
Catalytic Stoves
(5 Stoves, 1 3
Runs)
Noncatalytic
Stoves (1 1
Stoves, 30 Runs)
Catalytic
Noncatalytic
Conventional
Method 5H
Equivalent
Emission Factor
g/kg (Dry)
10.8
9.23
8.1
7.3
15.3
L. Fisher, J. Houck, and P. Tiegs, are with OMNI Environmental Services, Inc,
Beaverton, OR 97005, and J. McGaughey is with Eastern Research Group,
Morrisville, NC 27560.
Robert C. McCrillis, the EPA Project Officer, is no longer with the Agency, but for details
contact John Kinsey (see below).
The complete report, entitled "Long-Term Performance of EPA-Certified Phase 2
Woodstoves, Klamath Falls and Portland, Oregon: 1998/1999," (Order No. PB2001-
101921; Cost: $54.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: (703) 605-6000
(800) 553-6847 (U.S. only)
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection Agency
CenterforEnvironmental Research Information
Cincinnati, OH 45268
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