Project Summary Emissions From Outdoor Wood Burning Residential ...
United States
National Risk Management
Environmental Protection
Research Laboratory
Agency
Cincinnati, OH 45268
Research and Development
EPA/600/SR-98/017
February 1998
Project Summary
Emissions from Outdoor Wood-
Burning Residential Hot Water
Furnaces
Joseph C. Valenti and Russell K. Clayton
Modern outdoor residential wood-
presented in terms of rate per hour,
burning hot water furnaces are free-
quantity per unit weight of wood
standing units situated outside the en-
burned, and quantity per unit of heat
velope of the structure to be heated.
delivered. Delivered efficiencies are also
They typically consist of a firebox and
presented. Compared to a wide range
water reservoir, assembled in a hori-
of residential heating options, these fur-
zontal configuration. Hot combustion
naces’ emissions were of the same or-
gases flow from the firebox at one end,
der as other stick wood burning appli-
through channels or tubes in the water
ances.
reservoir, to the stack. The gases may
This Project Summary was developed
pass through the water reservoir once
by the National Risk Management Re-
to the stack at the end opposite the
search Laboratory’s Air Pollution Pre-
firebox (one pass) or an additional set
vention and Control Division, Research
of pipes may bring the gases back to
Triangle Park, NC, to announce key
the stack located above but isolated
findings of the research project that is
from the combustion chamber (double
fully documented in a separate report
pass). The heated water is pumped
of the same title (see Project Report
through radiators in the dwelling or
ordering information at back).
through a heat exchanger in the heat-
ing, ventilation, and air-conditioning
Introduction
(HVAC) duct in response to the home
In the early 1980s, the State of Oregon
thermostat. A separate pipe coil in the
began developing methods for character-
water reservoir may be used to provide
izing source emissions from residential
domestic hot water, year-round if de-
wood combustion units. The developed
sired. The furnace draft is controlled
methods have since blossomed into test
by a thermostat monitoring the tem-
methods used to audit and certify wood-
perature of the water in the reservoir.
burning heaters. From these beginnings,
Central heating furnaces of all types
the U.S. Environmental Protection Agency
are exempt from the EPA wood heater
(EPA) has established emission perfor-
(wood stove) standard. In this project,
mance standards for residential wood heat-
emissions were measured from a
ers.
single-pass and a double-pass furnace
The federal regulations established by
at average heat outputs of 15,000 and
the EPA in 1988 limit emissions from resi-
30,000 Btu/hr (4.4 and 8.8 kW) while
dential wood heaters, such as wood
burning typical oak cordwood fuel. One
stoves, pellet stoves, and factory built fire-
furnace was also tested once at each
place inserts. These regulations, however,
heat output while fitted with a proto-
do not include all wood-fired heating ap-
type catalytic unit installed in the com-
pliances. For example, central heating fur-
bustion chamber. Emissions measured
nace/boilers are not covered under the
included: EPA Method 5G particulate,
current regulations.
semivolatile and condensible organics,
In general, emissions from the combus-
20 target polycyclic aromatic hydrocar-
tion of wood in stoves and furnaces con-
bon (PAH) compounds, and carbon
tain significant levels of CO and fine par-
monoxide (CO). Emission results are
ticulate matter (PM) consisting, in part, of
mutagenic PAHs. If atmospheric condi-
furnace is defined by the configuration of
Results
tions are conducive for accumulating
the unit. The flue gases exit the combus-
Two basic furnace designs (single- and
smog-like clouds of emissions, the wood
tion chamber by way of a flue that passes
double-pass boiler heat exchangers) were
smoke could pose a health hazard. With
through the water tank. A single-pass fur-
chosen for these tests to see if the design
the potential for such a condition under
nace allows the flue gases to pass once
impacted emissions. Table 1 presents the
consideration, the EPA established maxi-
through the flue in the water tank before
particulate and PAH emission factor data
mum acceptable emissions levels for the
exiting through the chimney. As the hot
and efficiency aggregated by furnace and
certification of most residential wood-fired
flue gases pass through the flue, heat is
operating mode. Furnace B showed much
heaters.
transferred to the water in the tank. In a
less variability in operation and emissions
Typically, the modern outdoor residen-
double-pass furnace, flue gases pass
data compared to Furnace A. Whether
tial wood-burning hot water furnace is a
through the water tank twice before exit-
this is due to (1) furnace design, (2) the
freestanding unit situated outside the en-
ing through the chimney. The second pass
way the fuel was loaded, and/or (3) the
velope of the structure to be heated. The
of the stack provides more surface area
differences in the draft on/off cycles can-
unit consists of a closed combustion cham-
and more contact time between the hot
not be determined without further tests;
ber surrounded by a water tank and vented
flue gases and the water in the tank. Rep-
more than likely, all three variables ex-
through a stack. A wood burning fire is
resentative furnaces of both types were
erted significant influence.
contained and controlled in the combus-
provided to EPA/APPCD for testing.
Table 2 lists the emission results for
tion chamber or firebox of the furnace.
The outdoor residential wood-burning
various residential combustion devices.
During the combustion process, heat is
hot water furnaces were tested following
The results from this investigation (see
transferred through the walls of the cham-
EPA Reference Method 28 (M28–40 CFR
bottom row in Table 2) were included as
ber into the water. The hot water from the
Part 60, Appendix A), the test method
an average from all the tests. Based on
furnace tank can then be circulated through
used to certify and audit wood-fired heat-
this very limited test, it appears that the
ers (stick and pellet burning woodstoves).
radiators or air-handling heat exchangers
total particulate emission factor is compa-
The method specifies fuel preparation, fur-
to transfer heat into the residence. Some
rable to that for conventional wood stoves.
nace operation, and the reporting of the
central heating furnaces are equipped with
Note that all particulate values have been
results. Method 28 requires Method 5G or
additional plumbing to provide domestic
converted to the EPA Method 5H equiva-
5H (CFR Part 60, Appendix A) to deter-
hot water.
lent. The PAH emission factor appears to
mine the concentrations of oxygen (O ),
Most commercial central heating fur-
2
be generally the same as that for EPA
carbon dioxide (CO ), CO, and PM in the
naces are supplied with an 8- to 10-ft (2.4
2
certified wood stoves. The data presented
emissions.
to 3.0 m) tall stack. Typical indoor wood
in Table 2 were originally generated by
For these tests, some of the fuel prepa-
burning stoves have chimneys which ex-
different researchers using a variety of
ration procedures under Method 28 were
tend through the roof of a home to heights
sampling and analytical methodologies. A
modified in favor of preparing the fuel and
of 20 to 30 ft (6.1 to 9.1 m). The relatively
number of assumptions had to be made
operating the furnace as recommended
low chimney height of the central heating
to “normalize” the data for comparison.
by the manufacturer. Cordwood was used
furnace/boiler, compared to the conven-
instead of the dimensioned lumber speci-
Consequently, only order of magnitude dif-
tional wood stove installations, creates a
ferences should be considered significant.
fied for wood heater certification. Method
greater potential for the localization of ob-
28A was used to calculate the stack gas
Readers are encouraged to review the
jectionable emissions in and around resi-
dry molecular weight, as required for flow
reference cited in the footnote for a more
dences. Additionally, concerns have been
measurements. Method 5G was the pri-
thorough understanding of the data.
raised about the manner in which the com-
mary sampling method used for the test.
bustion process is controlled and how the
Conclusions
The sampling method, Method 5G, was
control affects the emissions.
modified by adding an XAD-2 absorbent
There were several data quality prob-
The State of Wisconsin has asked the
trap to collect organics; this modified sam-
lems with tests of Furnace A, all of which,
Control Technology Center of EPA’s Air
pling method will hereafter be referred to
though significant, are thought to be small
Pollution Prevention and Control Division
as Modified Method 5G (MM5G). The col-
enough to not bias the results for Furnace
(APPCD) for assistance in determining
lected MM5G samples were analyzed for
A sufficiently to cause an order of magni-
whether the need exists to regulate these
total PM, total semivolatile organics [some-
tude error. Tests of Furnace B had no
furnaces. Therefore, the EPA has under-
times referred to as total chromatographable
reported data quality problems. All tests
taken the task of evaluating the emissions
organics (TCOs)], condensible organics as
of Furnace B particulate matter emissions
from the central heating furnaces and the
measured by gravimetric analysis (GRAV),
were in the range of 36.5 to 37.6 g/hr
manner in which the combustion is con-
and PAHs. The efficiencies of the units
(high heat removal rate - tests B-1 and B-
trolled. The objective is to develop baseline
were measured as a secondary objective
2) and 14.3 to 15.5 g/hr (low heat removal
emission factors for comparison with other
for reporting emissions relative to the input
rate - tests B-3 and B-4). Particulate mat-
residential heating systems.
heating value of the wood and to their heat
ter emissions from Furnace A appear con-
In the full report, Section 2 describes
output from the furnace.
sistently higher; but, within the limits of
the experimental approach and sampling
Each furnace was tested at two heat
these tests, experimental error, and con-
and analytical methods employed. Steps
output levels, 15,000 and 30,000 Btu/hr
sidering the testing problems previously
to ensure project quality are described in
(4.4 and 8.8 kW). Each test was run in
discussed that may have compromised
Section 3. Data, results, and discussion
duplicate for a total of four runs per fur-
the data quality for Furnace A, a direct
are presented in Section 4. The appendi-
nace. In addition, two high heat output
comparison of Furnace A and Furnace B
ces contain the detailed data.
scoping runs were performed on Furnace
emissions is without adequate foundation
A. Furnace A was also tested once at
and, therefore, is not meaningful. How-
Project Description
each heat output while fitted with a proto-
ever, from Table 2, it is evident that all
Two types of furnaces were selected as
type catalytic device in the combustion
wood-burning home heating combustion
representative of the industry. The type of
chamber, giving a total of 12 runs.
equipment, including wood stoves, boil-
2
ers, or fireplaces, has much higher par-
ticulate matter emissions than gas- or oil-
fired home heating furnaces.
Table 1. Comparison Data Aggregated by Operating Mode and Furnace [Range in ()]
Furnace
Operating
Parameter
Mode
A
B
High Heat
19.6 (14.8-24.5)
12.0 (10.8-13.3)
M5G Particulates, g/kg
0.347 (0.216-0.478)
0.319 (0.315-0.324)
PAH, g/kg
45.6 (38.8-53.4)
53.8 (50.5-57.1)
Delivered Efficiency, %
Low Heat
16.6 (15.9-17.3)
9.35 (9.2-9.5)
M5G Particulates, g/kg
0.236 (0.228-0.245)
0.283 (0.235-0.332)
PAH, g/kg
44.4 (42.4-46.4)
55.2 (55.1-55.4)
Delivered Efficiency, %
Table 2. Overall Comparison of Residential Wood, Oil, and Gas Combustion Emissionsa
M5H Particulate
PAHs
Mutagenicityb
Combustion Device
mg/MJ input
mg/MJ input
krev/MJ input
Natural gas furnace
Conventional
0.44
0.000124
0.007c
High Efficiency
0.43
0.000028
NDc,d
Oil furnace
Retention head
3.2
—e
6
Conventional
15.1
—
20
Conventional wood stove
786
40
600
Certified wood stove
Non-catalytic
383
28
100
Catalytic
425
24
—
Pellet (certified)
110
0.082
—
Pellet (exempt)
176
0.014
—
Fireplace 907 41
—
Wood furnace
Cordwood - Swedish lab tests
Intermittent firing
1862
—
—
Continuous firing
182
15.3
148f
Chips (dry)
45.3
<0.02
0.48f
US EPA lab tests
Furnace Ag
1048
15.6
—
Furnace B
681
16.1
—
a All data except that in italics taken from: McCrillis, R.C., “Review and Analysis of Emissions Data for
Residential Wood-Fired Central Furnaces,” In Proceedings of the 88th Annual Meeting of the AWMA,
Air & Waste Management Association, San Antonio, TX, June 1995, Paper No. 95-RP137.04.
b Microsuspension assay, TA98+S9 unless otherwise noted.
c Ames plate incorporation assay, TA98+S9.
d ND means not detected.
e No data available for this parameter.
f
Ames plate incorporation assay, TA100+S9.
g Only includes comparison data.
3
J. Valenti and R. Clayton are with Acurex Environmental Corporation, Research
Triangle Park, NC 27709.
Robert C. McCrillis is the EPA Project Officer (see below).
The complete report, entitled “Emissions from Outdoor Wood-Burning Residential
Hot Water Furnaces,” (Order No. PB98-127087; Cost: $41.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
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
United States
BULK RATE
Environmental Protection Agency
POSTAGE & FEES PAID
National Risk Management Research (G-72)
EPA
Cincinnati, OH 45268
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/SR-98/017
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