United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

Project Summary

Research and Development EPA/600/SR-98/017 February 1998

Modern outdoor residential woodburning hot water furnaces are freestanding units situated outside the envelope of the structure to be heated.

They typically consist of a firebox and

water reservoir, assembled in a horizontal 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 heating, 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 desired. The furnace draft is controlled

by a thermostat monitoring the temperature 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 prototype catalytic unit installed in the combustion chamber. Emissions measured

included: EPA Method 5G particulate,

semivolatile and condensible organics,

20 target polycyclic aromatic hydrocarbon (PAH) compounds, and carbon

monoxide (CO). Emission results are

Emissions from Outdoor WoodBurning Residential Hot Water

Furnaces

Joseph C. Valenti and Russell K. Clayton

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 furnaces’ emissions were of the same order as other stick wood burning appliances.

This Project Summary was developed

by the National Risk Management Research Laboratory’s Air Pollution Prevention 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 characterizing source emissions from residential

wood combustion units. The developed

methods have since blossomed into test

methods used to audit and certify woodburning heaters. From these beginnings,

the U.S. Environmental Protection Agency

(EPA) has established emission performance standards for residential wood heaters.

The federal regulations established by

the EPA in 1988 limit emissions from residential wood heaters, such as wood

stoves, pellet stoves, and factory built fireplace inserts. These regulations, however,

do not include all wood-fired heating appliances. For example, central heating furnace/boilers are not covered under the

current regulations.

In general, emissions from the combustion of wood in stoves and furnaces contain significant levels of CO and fine particulate matter (PM) consisting, in part, of
2

mutagenic PAHs. If atmospheric conditions 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 maximum acceptable emissions levels for the

certification of most residential wood-fired

heaters.

Typically, the modern outdoor residential wood-burning hot water furnace is a

freestanding unit situated outside the envelope of the structure to be heated. The

unit consists of a closed combustion chamber surrounded by a water tank and vented

through a stack. A wood burning fire is

contained and controlled in the combustion chamber or firebox of the furnace.

During the combustion process, heat is

transferred through the walls of the chamber 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 furnaces are supplied with an 8- to 10-ft (2.4

to 3.0 m) tall stack. Typical indoor wood

burning stoves have chimneys which extend 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 conventional wood stove installations, creates a

greater potential for the localization of objectionable emissions in and around residences. Additionally, concerns have been

raised about the manner in which the combustion 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 undertaken the task of evaluating the emissions

from the central heating furnaces and the

manner in which the combustion is controlled. 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 appendices 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 combustion chamber by way of a flue that passes

through the water tank. A single-pass furnace 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 exiting 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. Representative 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 heaters (stick and pellet burning woodstoves).

The method specifies fuel preparation, furnace operation, and the reporting of the

results. Method 28 requires Method 5G or

5H (CFR Part 60, Appendix A) to determine the concentrations of oxygen (O2),

carbon dioxide (CO2

), CO, and PM in the

emissions.

For these tests, some of the fuel preparation 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 specified 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 primary 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 sampling method will hereafter be referred to

as Modified Method 5G (MM5G). The collected MM5G samples were analyzed for

total PM, total semivolatile organics [sometimes 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 furnace. 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 prototype 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 cannot be determined without further tests;

more than likely, all three variables exerted 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 comparable to that for conventional wood stoves.

Note that all particulate values have been

converted to the EPA Method 5H equivalent. 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 differences 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 problems 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 magnitude 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 B2) and 14.3 to 15.5 g/hr (low heat removal

rate - tests B-3 and B-4). Particulate matter emissions from Furnace A appear consistently higher; but, within the limits of

these tests, experimental error, and considering 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. However, from Table 2, it is evident that all

wood-burning home heating combustion

equipment, including wood stoves, boil-
3

ers, or fireplaces, has much higher particulate matter emissions than gas- or oilfired 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.
4

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

EPA/600/SR-98/017

Official Business

Penalty for Private Use $300

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35