United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-98/017
February 1998
&EPA Project Summary
Emissions from Outdoor Wood-
Burning Residential Hot Water
Furnaces
Joseph C. Valenti and Russell K. Clayton
Modern outdoor residential wood-
burning hot water furnaces are free-
standing units situated outside the en-
velope of the structure to be heated.
They typically consist of a firebox and
water reservoir, assembled in a hori-
zontal configuration. Hot combustion
gases flow from the firebox at one end,
through channels or tubes in the water
reservoir, to the stack. The gases may
pass through the water reservoir once
to the stack at the end opposite the
firebox (one pass) or an additional set
of pipes may bring the gases back to
the stack located above but isolated
from the combustion chamber (double
pass). The heated water is pumped
through radiators in the dwelling or
through a heat exchanger in the heat-
ing, ventilation, and air-conditioning
(HVAC) duct in response to the home
thermostat. A separate pipe coil in the
water reservoir may be used to provide
domestic hot water, year-round if de-
sired. The furnace draft is controlled
by a thermostat monitoring the tem-
perature of the water in the reservoir.
Central heating furnaces of all types
are exempt from the EPA wood heater
(wood stove) standard. In this project,
emissions were measured from a
single-pass and a double-pass furnace
at average heat outputs of 15,000 and
30,000 Btu/hr (4.4 and 8.8 kW) while
burning typical oak cordwood fuel. One
furnace was also tested once at each
heat output while fitted with a proto-
type catalytic unit installed in the com-
bustion chamber. Emissions measured
included: EPA Method 5G participate,
semivolatile and condensible organics,
20 target polycyclic aromatic hydrocar-
bon (PAH) compounds, and carbon
monoxide (CO). Emission results are
presented in terms of rate per hour,
quantity per unit weight of wood
burned, and quantity per unit of heat
delivered. Delivered efficiencies are also
presented. Compared to a wide range
of residential heating options, these fur-
naces' emissions were of the same or-
der as other stick wood burning appli-
ances.
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
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
In the early 1980s, the State of Oregon
began developing methods for character-
izing source emissions from residential
wood combustion units. The developed
methods have since blossomed into test
methods used to audit and certify wood-
burning heaters. From these beginnings,
the U.S. Environmental Protection Agency
(EPA) has established emission perfor-
mance standards for residential wood heat-
ers.
The federal regulations established by
the EPA in 1988 limit emissions from resi-
dential wood heaters, such as wood
stoves, pellet stoves, and factory built fire-
place inserts. These regulations, however,
do not include all wood-fired heating ap-
pliances. For example, central heating fur-
nace/boilers are not covered under the
current regulations.
In general, emissions from the combus-
tion of wood in stoves and furnaces con-
tain significant levels of CO and fine par-
ticulate matter (PM) consisting, in part, of
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mutagenic PAHs. If atmospheric condi-
tions are conducive for accumulating
smog-like clouds of emissions, the wood
smoke could pose a health hazard. With
the potential for such a condition under
consideration, the EPA established maxi-
mum acceptable emissions levels for the
certification of most residential wood-fired
heaters.
Typically, the modern outdoor residen-
tial wood-burning hot water furnace is a
freestanding unit situated outside the en-
velope of the structure to be heated. The
unit consists of a closed combustion cham-
ber surrounded by a water tank and vented
through a stack. A wood burning fire is
contained and controlled in the combus-
tion chamber or firebox of the furnace.
During the combustion process, heat is
transferred through the walls of the cham-
ber into the water. The hot water from the
furnace tank can then be circulated through
radiators or air-handling heat exchangers
to transfer heat into the residence. Some
central heating furnaces are equipped with
additional plumbing to provide domestic
hot water.
Most commercial central heating fur-
naces are supplied with an 8- to 10-ft (2.4
to 3.0 m) tall stack. Typical indoor wood
burning stoves have chimneys which ex-
tend through the roof of a home to heights
of 20 to 30 ft (6.1 to 9.1 m). The relatively
low chimney height of the central heating
furnace/boiler, compared to the conven-
tional wood stove installations, creates a
greater potential for the localization of ob-
jectionable emissions in and around resi-
dences. Additionally, concerns have been
raised about the manner in which the com-
bustion process is controlled and how the
control affects the emissions.
The State of Wisconsin has asked the
Control Technology Center of EPA's Air
Pollution Prevention and Control Division
(APPCD) for assistance in determining
whether the need exists to regulate these
furnaces. Therefore, the EPA has under-
taken the task of evaluating the emissions
from the central heating furnaces and the
manner in which the combustion is con-
trolled. The objective is to develop baseline
emission factors for comparison with other
residential heating systems.
In the full report, Section 2 describes
the experimental approach and sampling
and analytical methods employed. Steps
to ensure project quality are described in
Section 3. Data, results, and discussion
are presented in Section 4. The appendi-
ces contain the detailed data.
Project Description
Two types of furnaces were selected as
representative of the industry. The type of
furnace is defined by the configuration of
the unit. The flue gases exit the combus-
tion chamber by way of a flue that passes
through the water tank. A single-pass fur-
nace allows the flue gases to pass once
through the flue in the water tank before
exiting through the chimney. As the hot
flue gases pass through the flue, heat is
transferred to the water in the tank. In a
double-pass furnace, flue gases pass
through the water tank twice before exit-
ing through the chimney. The second pass
of the stack provides more surface area
and more contact time between the hot
flue gases and the water in the tank. Rep-
resentative furnaces of both types were
provided to EPA/APPCD for testing.
The outdoor residential wood-burning
hot water furnaces were tested following
EPA Reference Method 28 (M28-40 CFR
Part 60, Appendix A), the test method
used to certify and audit wood-fired heat-
ers (stick and pellet burning woodstoves).
The method specifies fuel preparation, fur-
nace operation, and the reporting of the
results. Method 28 requires Method 5G or
5H (CFR Part 60, Appendix A) to deter-
mine the concentrations of oxygen (O2),
carbon dioxide (CO2), CO, and PM in the
emissions.
For these tests, some of the fuel prepa-
ration procedures under Method 28 were
modified in favor of preparing the fuel and
operating the furnace as recommended
by the manufacturer. Cordwood was used
instead of the dimensioned lumber speci-
fied for wood heater certification. Method
28A was used to calculate the stack gas
dry molecular weight, as required for flow
measurements. Method 5G was the pri-
mary sampling method used for the test.
The sampling method, Method 5G, was
modified by adding an XAD-2 absorbent
trap to collect organics; this modified sam-
pling method will hereafter be referred to
as Modified Method 5G (MM5G). The col-
lected MM5G samples were analyzed for
total PM, total semivolatile organics [some-
times referred to as total chromatographable
organics (TCOs)], condensible organics as
measured by gravimetric analysis (GRAV),
and PAHs. The efficiencies of the units
were measured as a secondary objective
for reporting emissions relative to the input
heating value of the wood and to their heat
output from the furnace.
Each furnace was tested at two heat
output levels, 15,000 and 30,000 Btu/hr
(4.4 and 8.8 kW). Each test was run in
duplicate for a total of four runs per fur-
nace. In addition, two high heat output
scoping runs were performed on Furnace
A. Furnace A was also tested once at
each heat output while fitted with a proto-
type catalytic device in the combustion
chamber, giving a total of 12 runs.
Results
Two basic furnace designs (single- and
double-pass boiler heat exchangers) were
chosen for these tests to see if the design
impacted emissions. Table 1 presents the
particulate and PAH emission factor data
and efficiency aggregated by furnace and
operating mode. Furnace B showed much
less variability in operation and emissions
data compared to Furnace A. Whether
this is due to (1) furnace design, (2) the
way the fuel was loaded, and/or (3) the
differences in the draft on/off cycles can-
not be determined without further tests;
more than likely, all three variables ex-
erted significant influence.
Table 2 lists the emission results for
various residential combustion devices.
The results from this investigation (see
bottom row in Table 2) were included as
an average from all the tests. Based on
this very limited test, it appears that the
total particulate emission factor is compa-
rable to that for conventional wood stoves.
Note that all particulate values have been
converted to the EPA Method 5H equiva-
lent. The PAH emission factor appears to
be generally the same as that for EPA
certified wood stoves. The data presented
in Table 2 were originally generated by
different researchers using a variety of
sampling and analytical methodologies. A
number of assumptions had to be made
to "normalize" the data for comparison.
Consequently, only order of magnitude dif-
ferences should be considered significant.
Readers are encouraged to review the
reference cited in the footnote for a more
thorough understanding of the data.
Conclusions
There were several data quality prob-
lems with tests of Furnace A, all of which,
though significant, are thought to be small
enough to not bias the results for Furnace
A sufficiently to cause an order of magni-
tude error. Tests of Furnace B had no
reported data quality problems. All tests
of Furnace B particulate matter emissions
were in the range of 36.5 to 37.6 g/hr
(high heat removal rate - tests B-1 and B-
2) and 14.3 to 15.5 g/hr (low heat removal
rate - tests B-3 and B-4). Particulate mat-
ter emissions from Furnace A appear con-
sistently higher; but, within the limits of
these tests, experimental error, and con-
sidering the testing problems previously
discussed that may have compromised
the data quality for Furnace A, a direct
comparison of Furnace A and Furnace B
emissions is without adequate foundation
and, therefore, is not meaningful. How-
ever, from Table 2, it is evident that all
wood-burning home heating combustion
equipment, including wood stoves, boil-
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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
Mode
Parameter
B
High Heat
Low Heat
19.6(14.8-24.5)
0.347(0.216-0.478)
45.6 (38.8-53.4)
16.6(15.9-17.3)
0.236 (0.228-0.245)
44.4 (42.4-46.4)
12.0(10.8-13.3)
0.319(0.315-0.324)
53.8(50.5-57.1)
9.35 (9.2-9.5)
0.283 (0.235-0.332)
55.2(55.1-55.4)
M5G Particulates, g/kg
PAH, g/kg
Delivered Efficiency, %
M5G Particulates, g/kg
PAH, g/kg
Delivered Efficiency, %
Table 2. Overall Comparison of Residential Wood, Oil, and Gas Combustion Emissions3
Combustion Device
M5H Particulate
mg/MJ input
PAHs
mg/MJ input
Mutagenicityb
krev/MJ input
Natural gas furnace
Conventional 0.44
High Efficiency 0.43
Oil furnace
Retention head 3.2
Conventional 15.1
Conventional wood stove 786
Certified wood stove
Non-catalytic 383
Catalytic 425
Pellet (certified) 110
Pellet (exempt) 176
Fireplace 907 41 —
Wood furnace
Cordwood - Swedish lab tests
Intermittent firing 1862
Continuous firing 182
Chips (dry) 45.3
US EPA lab tests
Furnace AS 1048
Furnace B 681
0.000124
0.000028
40
28
24
0.082
0.014
0.007C
NDc>d
6
20
600
100
15.3
<0.02
75.6
16.1
148f
0.48f
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.
9 Only includes comparison data.
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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
Environmental Protection Agency
National Risk Management Research (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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EPA
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EPA/600/SR-98/017
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