Assessment of 

Outdoor Wood-fired Boilers

Prepared 

by 

NESCAUM 

(Northeast States for Coordinated Air Use Management) 

March 2006 

(revised June 2006)
Members of NESCAUM 

Arthur Marin, Executive Director 

NESCAUM 

Anne Gobin, Bureau Chief 

Connecticut Department of Environmental Protection, Bureau of Air Management 

James P. Brooks, Bureau Director 

Maine Department of Environmental Protection, Bureau of Air Quality 

Barbara Kwetz, Director 

Massachusetts Department of Environmental Protection, Bureau of Waste Prevention 

Robert Scott, Director 

New Hampshire Department of Environmental Services, Air Resources Division 

William O’Sullivan, Director 

New Jersey Department of Environmental Protection, Office of Air Quality Management 

David Shaw, Director 

New York Department of Environmental Conservation, Division of Air Resources

Stephen Majkut, Chief 

Rhode Island Department of Environmental Management, Office of Air Resources 

Richard A. Valentinetti, Director 

Vermont Department of Environmental Conservation, Air Pollution Control Division 
ii 

Assessment of 

Outdoor Wood-fired Boilers

Prepared 

by 

NESCAUM 

(Northeast States for Coordinated Air Use Management) 

March 2006

(revised June 2006)
iii 

Assessment of 

Outdoor Wood-fired Boilers

Project Manager

 Lisa Rector, NESCAUM 

Principal Contributors

 

George Allen, NESCAUM 

Philip Johnson, NESCAUM 

Lisa Rector, NESCAUM 

Editor

 

Paul Miller, NESCAUM 
iv 

Acknowledgments 

NESCAUM gratefully acknowledges the funding provided by the Massachusetts 

Executive Office of Environmental Affairs and NESCAUM member states’ dues to 

complete this report. This report was developed by staff at NESCAUM with assistance 

from Air Quality Technical Services, Essex, Vermont. 

NESCAUM also thanks the following individuals for contributing to this effort: 

Peter Babich, Connecticut Department of Environmental Protection 

Patrick Bowe, Connecticut Department of Environmental Protection 

David Cash, Massachusetts Executive Office of Environmental Affairs 

Marc Cohen, Massachusetts Department of Environmental Protection 

James Doucette, Massachusetts Department of Environmental Protection 

Phil Etter, Vermont Department of Environmental Conservation 

Jack Glenn, New Hampshire Department of Environmental Services 

Chris Jones, Vermont Department of Environmental Conservation 

Glenn Keith, Massachusetts Department of Environmental Protection 

Barbara Kwetz, Massachusetts Department of Environmental Protection 

Dave Manning, Vermont Department of Environmental Conservation 

Donald Squires, Massachusetts Department of Environmental Protection 

Printed: March 2006 

Revised: May 2006 

Revised: June 2006 
v 

TABLE OF CONTENTS 

Acknowledgments.............................................................................................................. iv 

Executive Summary.......................................................................................................... vii 

Executive Summary.......................................................................................................... vii 

1. Introduction................................................................................................................ 1-1 

1.1. OWB Description................................................................................................. 1-1 

1.2. Report Overview.................................................................................................. 1-2 

2. Issues Unique to OWBs............................................................................................. 2-1 

2.1. Unit Design .......................................................................................................... 2-1 

2.1.1. Combustion Cycle....................................................................................... 2-1 

2.1.2. Stack Height................................................................................................ 2-1 

2.1.3. Combustion Design..................................................................................... 2-1 

2.1.4. Efficiency.................................................................................................... 2-2 

2.2. Use Patterns ......................................................................................................... 2-2 

2.3. Fuel Quality ......................................................................................................... 2-2 

2.4. Application of Current Regulations to Address OWBs....................................... 2-3 

2.4.1. Federal Regulation...................................................................................... 2-3 

2.4.2. State Regulations ........................................................................................ 2-3 

3. Overview of OWB Industry....................................................................................... 3-1 

3.1. OWB Costs and Distribution ............................................................................... 3-1 

3.2. Sales Trend Analysis............................................................................................ 3-2 

4. Public Health Concerns.............................................................................................. 4-1 

4.1. Wood Smoke Composition.................................................................................. 4-1 

4.2. Particulate Matter Health Effects and Populations at Risk.................................. 4-1 

4.3. Unique Factors Relating to OWB Emissions and Potential Public Health Threats4-4 

4.4. Cause for Concern................................................................................................ 4-5 

5. Emissions Information............................................................................................... 5-1 

5.1. Previous Test Data ............................................................................................... 5-1 

5.2. Near-Source Ambient Emission Field Monitoring.............................................. 5-4 

5.3. Results of NESCAUM OWB “In Use” Stack Test.............................................. 5-6 

5.3.1. Testing Methods.......................................................................................... 5-6 

5.3.2. Results and Discussion ............................................................................... 5-7 

5.4. ASTM Test Protocol.......................................................................................... 5-18 

5.5. Conclusions on Emissions Testing .................................................................... 5-18 

6. OWB Control Strategies ............................................................................................ 6-1 

6.1. Regulatory Action................................................................................................ 6-1 

6.1.1. Ban/Removal of OWBs .............................................................................. 6-1 

6.1.2. Emission Standards..................................................................................... 6-1 

6.1.3. Fuel Requirements ...................................................................................... 6-2 

6.1.4. Mandatory Removal.................................................................................... 6-3 

6.1.5. No Burn Days ............................................................................................. 6-3 

6.1.6. Nuisance Rules............................................................................................ 6-3 

6.1.7. Opacity Rules.............................................................................................. 6-3 

6.1.8. Removal Prior to Sale or Transfer of Property ........................................... 6-4 

6.1.9. Zoning Regulations..................................................................................... 6-5 
vi 

6.2. Voluntary Programs............................................................................................. 6-5 

6.2.1. Change-out Programs.................................................................................. 6-5 

6.2.2. Voluntary Industry Actions......................................................................... 6-5 

6.3. Court Actions....................................................................................................... 6-5 

6.4. Enforcement Actions ........................................................................................... 6-6 

6.4.1. Connecticut ................................................................................................. 6-6 

6.4.2. Massachusetts ............................................................................................. 6-6 

6.4.3. Vermont ...................................................................................................... 6-9 

7. Conclusions/Recommendations................................................................................. 7-1

Appendix A: Outdoor Wood-fired Boiler Correspondence 

Appendix B: Outdoor Wood-fired Boiler Manufacturers

Appendix C: Estimated National Sales of OWBs 

Appendix D: Michigan DEQ Modeling 

Appendix E: Field Report on Moisture Readings and Opacity 

Appendix F: Outdoor Wood-fired Boiler Regulations 

FIGURES 

Figure 1-1. Schematic of an OWB Installation............................................................... 1-2 

Figure 3-1. States representing 95% of the OWBs sold .................................................. 3-2 

Figure 5-1. PM Emission Comparison (g/hr) ................................................................. 5-3 

Figure 5-2. Field measurements of PM2.5 near outdoor wood boiler.............................. 5-5 

Figure 5-3. Field measurements of PM2.5 near outdoor wood boiler............................ 5-6 

Figure 5-4a-e. VT Outdoor Wood Furnace Stack Sampling ........................................ 5-13 

Figure 5-5. VT Outdoor Wood Furnace Stack Sampling Continuous PM.................... 5-19 

TABLES 

Table 3-1. Estimated Sales of OWBs since 1990 ........................................................... 3-3 

Table 5-1. Unevaluated OWB Emissions Data................................................................ 5-2 

Table 5-2. Filter Measurements Idle Burn Mode.......................................................... 5-10 

Table 5-3. Filter Measurement Full Burn Mode........................................................... 5-11 

Table 5-4. Comparison of Filter Data and DataRam .................................................... 5-12 
vii 

Executive Summary 

This report was undertaken by NESCAUM (Northeast States for Coordinated Air 

Use Management) to provide policymakers with an assessment of concerns relating to the 

growing use of outdoor wood-fired boilers (OWB), also known as outdoor wood-fired 

hydronic heaters or outdoor wood-fired furnaces. The increased use of OWBs in 

populated areas represents a potential public health problem in the Northeast because of 

the severity of health effects associated with residential wood smoke inhalation. 

This report 1) overviews unique features of OWB appliances, 2) estimates OWB 

appliance sales trends, 3) assesses emission considerations and potential health concerns 

of residential wood smoke, 4) presents results of OWB ambient monitoring and stack 

testing conducted by NESCAUM, and 5) provides recommendations for regulatory 

action. 

Findings from this study show that: 

• OWBs, generally, do not use catalytic or non-catalytic emission control 

devices that other residential, wood-fired combustion devices, such as 

indoor wood stoves, commonly employ. 

• OWB use has become more prevalent, commonly replacing indoor wood 

stoves, and continued increases in sales are likely. 

• OWBs emit significantly more particulate matter than other residential 

wood burning devices and short term particulate matter spikes can be 

extremely high. 

• OWBs could contribute almost 900,000 tons of particulate matter by 

2010. 

• Local populations are likely subject to elevated ambient particulate matter 

levels from OWB smoke. 

• Current regulations do not provide surrounding areas with adequate 

protection from the use of OWBs in residential applications. 

• There is a lack of information relating to air toxic emissions, such as 

polycyclic aromatic hydrocarbons (PAHs), polycyclic organic matter 

(POM) and dioxin. 

OWBs present unique issues, unlike other residential wood burning devices, based on the 

following factors: 

• Year Round Operation – OWBs are designed to provide heat and hot 

water year round. Owners often use them in the warmer months not only 

for domestic hot water but also to heat their swimming pools and/or spas. 

• Cyclic Operation - The cyclic nature of OWB operations, unlike EPA 

certified wood stoves, does not allow for complete combustion and creates 

an environment conducive to increased toxic and particulate emissions. 
viii 

• Short Stack Heights – Stacks from OWBs, as per manufacturer’s 

installation instructions, are usually less than 12 feet from the ground, 

resulting in poor dispersion of smoke and are more likely to cause 

fumigation within surrounding areas. 

• Oversized Firebox – An OWB’s large firebox is built such that a user 

could burn a variety of inappropriate materials that could not be burned in 

wood stoves or fireplaces. Enforcement programs have discovered OWBs 

burning tires, large bags of refuse, and railroad ties. Even when used 

properly, overall OWBs emissions are greater than other residential wood 

burning devices. 

 With funding from the Massachusetts Executive Office of Environmental Affairs 

(EOEA), NESCAUM measured emissions from an OWB unit in the field under real 

world conditions. The test data indicate that the smallest OWB is likely to have an 

average in-use emission rate of approximately 161 grams of fine particulate matter per 

hour, which is twenty times higher than the average in-use emissions of an EPA certified 

wood stove. This report utilizes a gram per hour measurement in order to understand the 

potential ambient impacts and to make comparisons to other residential furnaces and 

heating units such as oil-fired furnaces, natural gas furnaces, and residential wood stoves. 

Furthermore, NESCAUM believes that given the health impacts associated with wood 

smoke and their use in residential locations (near at-risk populations such as children and 

the elderly), it is critical to assess particulate emissions from OWBs on a mass per unit 

time basis to fully understand potential health risks and appropriate protections. 

 Based on the test results in this report, the average fine particulate emissions from 

one OWB are equivalent to the emissions from 22 EPA certified wood stoves, 205 oil 

furnaces, or as many as 8,000 natural gas furnaces. To put these emissions into 

perspective, one OWB can emit as much fine particulate matter as four heavy duty diesel 

trucks on a grams per hour basis. Cumulatively, the smallest OWB has the potential to 

emit almost one and one-half tons of particulate matter every year. Based on sales 

estimates, OWBs could emit over 233,000 tons of fine particulate matter nationwide in 

2005. Considering sales trends, NESCAUM estimates that there could be 500,000 

OWBs in place nationwide by 2010. Based on that estimate, emissions from OWBs 

would reach 873,750 tons of fine particulate matter nationwide per year by 2010. 

The cumulative impact of OWBs to ambient air quality is only one part of the 

problem. Because OWBs are used primarily in residential applications, they have the 

potential to emit fine particulate matter and air toxics at levels that could create elevated 

risk to nearby populations. A second pilot study conducted by NESCAUM measured 

ambient PM2.5 within 150 feet of an OWB device. Relative to background levels, the 

study documented high 15-second average values (>1,000 micrograms per cubic meter 

(μg/m3

)) with spikes greater than 8,000 μg/m3 throughout the course of normal daily 

OWB operating modes. These data suggest that buildings located near OWBs can 

experience high PM2.5 levels during typical boiler operating conditions. Health studies 

have found associations of acute and chronic exposure to wood smoke with adverse 

health outcomes such as increases in respiratory symptoms, decreases in lung function, 

visits to emergency departments, and hospitalization. 
ix 

States have requested several times that the U.S. Environmental Agency (EPA) 

develop federal regulations for OWBs but EPA has yet to act. Critical to the 

development of a federal emissions standard is the need to develop a technology forcing 

standard that would require manufacturers to employ wood combustion controls 

commonly employed by other residential and industrial wood-fired devices. In addition, 

the increasing use of OWBs necessitates that action be taken as quickly as possible. 

Federal action would likely take five to seven years to put protective measures into place. 

In the absence of federal action, states will need to act on this emerging issue in a 

timely manner. Given the significant health effects OWB emissions may pose and the 

lack of action on the federal level, NESCAUM believes that states should take action 

immediately to control OWB emissions by establishing technology-forcing standards that 

will lead manufacturers to develop cleaner burning OWBs. 

 
Assessment of Outdoor Wood-fired Boilers Page 1-1

1. INTRODUCTION 

As the price of home heating oil, natural gas and liquefied petroleum gas (LPG) 

increases, many households are looking for heating alternatives. Increasingly, people are 

turning to wood-burning devices to replace oil and gas heating.1 Wood-fired devices 

include masonry heaters, indoor wood stoves, indoor wood furnaces, pellet stoves and 

outdoor wood-fired boilers. The recent increase in the use of outdoor wood-fired boilers 

(OWBs) are of particular concern to state environmental agencies because the cumulative 

stack emissions from these appliances are higher than other wood burning appliances 

and, unlike other wood burning devices, currently are unregulated. In the early 1990’s, 

states became aware of OWBs, although their use at that time was primarily limited to 

rural settings. However, states have noted an increasing trend of OWB installation in 

suburban and urban neighborhoods to provide space heating, year-round heating of hot 

water, and heating of swimming pools, Jacuzzis, and hot tubs. Use of OWBs in 

commercial applications is also increasing beyond traditional use in agricultural 

operations. 

1.1. OWB Description 

An OWB is a wood-fired furnace that is usually housed within a small insulated 

shed located some distance from a house. Manufacturers design OWBs to burn large 

amounts of wood over long periods of time.2

 OWBs vary in size ranging from 115,000 

Btu/hr up to 3.2 million Btu/hr, although residential OWBs tend to be less than 1 million 

Btu/hr. According to sales data, the size of the most commonly sold unit is 500,000 

Btu/hr. OWBs heat buildings ranging in size from 1,800 square feet to 20,000 square 

feet. Typically, the dimensions of an OWB are three to five feet wide, six to nine feet 

deep, and six to ten feet tall, including the height of the chimney. Inside the OWB is an 

oversized firebox that can accommodate extremely large loads. Firebox sizes will vary 

with each unit but tend to range in size from 20 cubic feet up to 150 cubic feet. Industry 

literature indicates that a commonly sized residential unit can easily accommodate wood 

pieces that are 30 inches in diameter and 72 inches long.3 Surrounding the firebox is a 

water jacket that can be heated to temperatures up to 190°F. The OWB cycles water 

through the jacket to deliver hot water to the building. Water pipes run underground to 

deliver hot water for both space heating and domestic use. 

Figure 1-1 provides a schematic of an OWB installation. OWBs have a cyclical 

operating pattern; when the water temperature in the water jacket reaches a given 

temperature, an air damper closes off air to the unit until the temperature drops and the 

air damper opens, creating an on/off cycle. 

 

1

http://www.usatoday.com/news/nation/2005-11-13-woodburning-pollution_x.htm 

2

 Schreiber, Judith et al. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State. Office of the 

Attorney General; Albany, New York, 2005. Available at 

http://www.oag.state.ny.us/press/2005/aug/August 2005.pdf 

3

http://www.centralboiler.com/eadvantage.php 
Assessment of Outdoor Wood-fired Boilers Page 1-2

Figure 1-1. Schematic of an OWB Installation 

Manufacturers advertise OWBs for operation with wood fuel, however, many 

states have found that users sometimes fuel OWBs with yard waste, packing materials, 

construction debris, and domestic wastes.4

 While most manufacturers of OWBs instruct 

owners of OWBs to burn wood, anecdotal data suggest that distributors instruct 

purchasers of OWBs they can burn “whatever they want” -- even household garbage and 

tires.5 

1.2. Report Overview 

This report provides data on issues unique to OWBs, an analysis of sales and 

distribution of OWBs, an explanation of wood smoke pollutants and their associated 

public health risks, analysis of OWB emissions, an overview of regulatory options, and 

conclusions. 

 

4

 Conversations with representatives from enforcement staff at the New York Attorney General’s Office, 

Connecticut Department of Environmental Conservation, Maine Department of Environmental Protection, 

and Massachusetts Department of Environmental Protection. 

5

 Staff from a variety of environmental agencies have spoken with distributors at fairs and other distribution 

venues to gather information on the units. 
Assessment of Outdoor Wood-fired Boilers Page 2-1

2. ISSUES UNIQUE TO OWBS 

While there are a variety of concerns surrounding wood burning devices, several 

issues make OWBs unique. The largest number of complaints voiced to Northeast state 

air quality enforcement programs relate to excessive smoke and nuisance conditions. 

These conditions arise due to the unique design and operating features of OWBs. 

2.1. Unit Design 

The smoke from OWBs appears to be greater than other wood burning appliances 

for several reasons. Primarily, it is because of the basic design of OWBs. Even when 

operated properly with seasoned wood, OWBs typically generate significant amounts of 

smoke. When an operator dampers down the unit, the lack of oxygen to support 

combustion creates a build up of materials such as creosote. When opening the damper, 

these materials burn and release immediately to the air. Excessive loading and/or low 

demand for heat further aggravates this problem. In addition, OWBs emit more 

pronounced smoke than wood stoves due to the short stack height, which does not 

disperse smoke above living spaces in neighboring homes. The addition of sophisticated 

combustion controls could address many of the smoke issues, however, the vase majority 

of units sold do not employ any catalytic or non-catalytic emission controls. 

2.1.1. Combustion Cycle 

The primary reason for the large amounts of smoke is the cyclic nature of the 

device. When an OWB is in the “off” cycle and does not need to generate heat, the air 

damper closes to cut off the air supply. This creates an oxygen-starved environment in 

which the fire smolders, creating smoke and creosote that condenses on the internal steel 

surfaces. When heat needs to be produced, the air damper opens and natural draft forces 

air into the firebox, pushing the smoke and air pollutants out the stack. Measured 

emissions peak when the unit has received a fresh load of fuel and the wood has not yet 

reached a charcoal stage. In the field test conducted by NESCAUM, the unit’s internal 

stack temperature never reached levels that would have resulted in complete combustion 

(see Section 5.3). 

2.1.2. Stack Height 

Another contributing factor to OWB smoke is the short stack height of OWBs. 

Manufacturer installation literature specifies stack heights that are generally eight to 

twelve feet from the ground. Stack heights this short typically fail to disperse smoke 

adequately, resulting in excessive ground level smoke. Because OWBs rarely provide 

stovepipe fans to increase the upward velocity of the smoke, there is only limited vertical 

dispersion of OWB emissions. Certain weather conditions aggravate this situation, such 

as cold weather inversions when the smoke does not rise but stays close to the ground. 

2.1.3. Combustion Design 

Most OWBs do not have any combustion controls, such as catalytic devices and 

secondary combustion. Incorporating these features could significantly reduce particulate 

emissions. Almost all indoor wood stoves sold today use catalytic or secondary 
Assessment of Outdoor Wood-fired Boilers Page 2-2

combustion to reduce emissions. NESCAUM’s review of OWB manufacturers’ data 

finds that only a limited number of manufacturers utilize common wood burning 

combustion controls. Discussions with wood combustion experts indicate that these units 

could incorporate combustion controls into their design in a relatively short period of 

time, which would result in reduced emissions, increased efficiency and an overall 

improvement in OWB performance. 

2.1.4. Efficiency 

Review of available data indicates that the operating efficiencies of OWBs are 

extremely low, often half the efficiency of other residential wood burning devices such as 

wood stoves. Several manufacturers have made claims that their OWBs are up to 95 

percent efficient. Review of the data, however, suggests that heating efficiencies range 

from as low as 28 percent to not higher than 55 percent.6

 Test results obtained by 

Freedom of Information Act (FOIA) requests to EPA indicate that, in general, most units 

will have operating efficiencies in the range of 30 to 40 percent. A comparison with 

wood stove efficiencies finds that their efficiencies range from 60 to 80 percent efficient.7

 

Low efficiencies translate into increased amounts of wood burned to generate heat, which 

in turn increases emissions. 

2.2. Use Patterns 

Unlike indoor wood stoves and fireplaces, manufacturers design OWBs for use 

year round. OWBs not only heat space but provide domestic hot water and heat 

swimming pools and spas. In the summer months, the smoke may seem more apparent 

because the smoke may be less likely to disperse as the lower flue velocities from the 

stack may keep the smoke closer to the ground. NESCAUM observed this condition 

during its stack test conducted in June 2005. 

Another critical difference in OWB operations from other wood burning devices 

relates to long burn times. Manufacturers instruct owners that they can completely fill 

the OWB firebox and leave it burning for as long as 96 hours. However, completely 

filling the combustion chamber creates a smoldering fire that burns inefficiently, 

especially when warm weather or a depressed thermostat during mid-day or nighttime 

hours reduces demand for heat. The lack of tending for long periods of time compounds 

the problem, leading to inefficient burning conditions over extended periods. In contrast, 

owners of conventional indoor wood stoves tend them more often to optimize 

combustion. 

2.3. Fuel Quality 

While most manufacturers provide OWB owners with best burn practices and 

instruct users that they should only burn dry seasoned wood, the design of the OWBs 

encourages burning of inappropriate materials. State agencies have documented the 

burning of wet, large, unsplit wood, wood waste, yard waste, refuse, tires and railroad 

 

6

 Schreiber, Judith et al. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State. Office of the 

Attorney General; Albany, New York, 2005. Available at 

http://www.oag.state.ny.us/press/2005/aug/August%202005.pdf. 

7

http://www.epa.gov/woodstoves/efficiently.html. 
Assessment of Outdoor Wood-fired Boilers Page 2-3

ties. In fact some manufacturer’s websites state the ability to burn green or scrap wood 

as one of the benefits of OWBs. For example, the Pacific Western website states that 

their unit, “easily burns junk wood including pallets.”8 

2.4. Application of Current Regulations to Address OWBs 

Enforcement programs have attempted to use existing regulations to address 

OWB complaints. A detailed description of various regulatory options can be found in 

Section 6.1 of this report. This section provides a more general overview of the current 

efforts used by states to address OWBs. 

2.4.1. Federal Regulation 

While EPA requires indoor wood stoves, pellet stoves, and small wood-fired 

industrial boilers to adhere to federal air pollution standards of 7.5 g/hr for non-catalytic 

appliances and 4.1 g/hr for catalytic appliances, there are no federal standards for OWBs. 

To date EPA has received several requests from state agencies to develop federal 

standards. EPA has yet to act on these requests and a June 2005 letter sent by EPA to 

OWB manufacturers, and an EPA statement provided to the Clean Air Act Advisory 

Committee (CAAAC) on August 11, 2005, indicates that any action in the near-term is 

unlikely (see Appendix A). The August 11, 2005 statement to the CAAAC advised that 

EPA was reviewing the state petitions and did not expect a decision on action until spring 

2006. In this same statement, EPA also indicated that it would take a minimum of five to 

seven years to develop and implement a federal standard. 

2.4.2. State Regulations 

In the absence of federal standards, the states have attempted to use several 

avenues to address OWB problems. Two states and several municipalities have 

attempted to address OWBs by adopting regulations limiting emissions or banning the 

sale of OWBs. Regulations in the state of Washington limit emissions from all solid fuel 

burning devices rated less than one million Btu/hr. Sales data obtained from 

manufacturers show that only a small number of OWBs have been sold in Washington. It 

is unlikely that the OWBs sold in Washington could meet the standard. Correspondence 

between Central Boiler and the Washington Department of Ecology indicates that the 

manufacturers dispute the application of Washington’s regulation on OWBs (see 

Appendix A). In 1997, Vermont adopted regulations that require setback and stack height 

standards for OWBs. This regulation has not eliminated the OWB problem; and in fact 

the VT DEC continues to receive complaints. 

Many states have opacity regulations that could apply to OWBs (a detailed 

explanation of opacity regulations can be found in Section 6.1.7.). Based on the 

experience of state enforcement staff, it is unlikely that an OWB could meet most states’ 

opacity requirements even under ideal operating conditions. In fact, several states have 

attempted to work with homeowners to find methods to operate OWBs within state 

opacity limits but none to date have been able to comply. Given this scenario, the only 

solution is removal, which comes at great cost to the homeowner because it requires 

 

8

http://www.outdoorfurnaces.com/article/pacific-western-details-6.asp. 
Assessment of Outdoor Wood-fired Boilers Page 2-4

removal of the OWB and installation of a new heating system. Opacity regulations place 

a significant burden on the homeowner who, often unwittingly, purchases a unit that 

cannot meet the state regulatory opacity requirements. Further compounding the problem 

is the limited ability of many state enforcement agencies to enforce against residential 

OWBs. Most state enforcement methods aim towards mitigating conditions at industrial 

facilities, leaving the states reticent and ill-equipped to enforce opacity requirements at 

residential OWBs. 
Assessment of Outdoor Wood-fired Boilers Page 3-1

3. OVERVIEW OF OWB INDUSTRY 

NESCAUM has identified twenty-seven manufacturers of OWBs (see Appendix 

B). Manufacturers are located throughout the United States with eight in Minnesota, 

three in Pennsylvania, two in Wisconsin, two in North Carolina, and one each in 

Missouri, Nebraska, New Hampshire, New York, Tennessee, and Washington. In 

addition, NESCAUM identified six manufacturers based in Canada. These companies 

range in size from one to over 140 employees. Some of these manufacturers participate 

in the Hearth, Patio and Barbecue Association (HPBA), which is an industry trade 

association that represents wood stove, pellet stove and fireplace manufacturers as well. 

 The Northeast states have focused attention on OWBs since the late 1990’s. 

Several states have attempted to work with manufacturers to address complaints and 

nuisance issues with little success. The New York State Department of Environmental 

Conservation held a meeting with manufacturers in 2003 to gain a better understanding of 

the industry and to determine the direction of manufacturers. At these meetings, 

manufacturers stated that cleaner OWBs would be on the market in the near future. 

Almost three years later, there has been little change in OWB design. In fact, recent 

statements from the largest producer of OWBs indicate that it does not believe that 

OWBs emit significant amounts of pollution. Testimony prepared by Central Boiler for 

the Vermont draft OWB regulations9

 and statements by its vice-president Rodney 

Tolufsen declared that its OWBs pollute no more than EPA-certified wood stoves.10

3.1. OWB Costs and Distribution 

The total cost to purchase and install the smallest OWB can range from $8,000-

$10,000, with costs increasing with the size of the unit. The OWB itself costs 

approximately $5,000, excluding installation. Installation usually includes laying a 

concrete foundation, putting in a power source, installing underground piping from the 

unit to the house, and other additional piping. 

Commonly, manufacturers sell OWBs directly or through a national distribution 

network. Distribution venues commonly include hardware stores, fairs, and direct sales. 

Typically, stores that sell indoor wood stoves or fireplaces do not sell OWBs. Marketing 

literature from manufacturers typically claims that OWBs have many advantages over 

gas, oil and other wood burning devices, including:

• Eliminating heating bills 

• Improving indoor air quality 

• Reducing the incidence of asthma or allergies 

• Benefiting the environment by reducing the greenhouse emissions 

• Increasing safety of heating by removing the heating unit from the building 

• Requiring less time to operate than other wood burning devices 

• Reducing dependence of fossil fuels11

 

9

 Transcripts from public hearings on Vermont’s proposed regulations. 

10 http://groups.yahoo.com/group/woodheat/message/6422. 

11 http://www.shol.com/mahoning/reason.HTM and http://www.centralboiler.com/eadvantage.php. 
Assessment of Outdoor Wood-fired Boilers Page 3-2

In its review of emissions and efficiency data, NESCAUM found limited or no 

documentation to support manufacturers’ claims. Manufacturers often claim that their 

OWBs have relatively high efficiency ratings. Some manufacturers have made claims 

that their units are up to 95 percent efficient. However, review of the data suggests that 

heating efficiencies range from 28 to 55 percent12 (Section 2.1.4 contains a more detailed 

discussion of OWB efficiency). 

3.2. Sales Trend Analysis 

With the recent increase in the price of heating fuels, the use of wood for 

residential and commercial heating is on the rise.13 As part of this trend, there has been a 

rapid increase in the number of OWBs installed. Many of the OWB manufacturers began 

selling OWBs in the early 1980’s but sales began to rise substantially after 1999. Using 

sales data gathered by the New York Attorney General’s Office via subpoena of 21 

manufacturers and by EPA from nine manufacturers via a Clean Air Act Section 114 

request, NESCAUM estimates that over 155,000 OWBs have been sold nationwide since 

1990 (see Table 3-1). Appendix C contains estimates of sales by state. Of the estimated 

155,000 OWBs sold nationwide, 95% have been sold in nineteen states, as illustrated in 

Figure 3-1. These states include Connecticut, Indiana, Illinois, Iowa, Kentucky, Maine, 

Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New York, North 

Carolina, Ohio, Pennsylvania, Vermont, Virginia, West Virginia, and Wisconsin. 

Figure 3-1. States representing 95% of the OWBs sold 

National sales have been growing in the past five years at rates of 30 to 128 

percent. In October 2005, one manufacturer, Central Boiler, stated that its production 

 

12 Schreiber, Judith et al. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State. Office of 

the Attorney General; Albany, New York, 2005. Available at 

http://www.oag.state.ny.us/press/2005/aug/August%202005.pdf. 

13 http://www.usatoday.com/news/nation/2005-11-13-woodburning-pollution_x.htm. 
Assessment of Outdoor Wood-fired Boilers Page 3-3

was “double what it’s ever been before.”14 In December 2004, the same manufacturer 

stated its peak production at that time was 200 units a week, and it had the room to make 

1,000 a week, which would mean a capacity to produce over 50,000 annually.15 The 

number of OWBs sold in 2003 and 2004 equals the number OWBs sold in the previous 

fourteen years. Given the continued rise in the cost of natural gas and petroleum fuels, it 

is likely that an increasing sales trend will continue. EPA has estimated that it would 

take a minimum of five years for rules to be put in place, at which time over 500,000 

OWBs could be in place. 

New construction uses OWBs as primary furnaces and OWBs are also replacing 

conventional indoor wood stoves and oil or gas-fired furnaces. To promote the use of 

EPA certified wood stoves through the Wood Stove Change Out Campaign, EPA is 

educating the public about the dangers of using old non-certified wood stoves. Many 

homeowners, however, may be choosing to install OWBs instead of certified indoor 

wood stoves. One manufacturer, Central Boiler, estimated that about 50% of the OWBs 

sold replace indoor wood stoves.16 Therefore, while EPA’s change out effort is 

important, it may be resulting in increased installation of OWBs due to the absence of 

education about OWBs relative to certified wood stoves. This will lead to greater wood 

smoke pollution if consumers erroneously believe OWBs are a cleaner option than noncertified indoor wood stoves. 

Table 3-1. Estimated Sales of OWBs since 199017

Year US 

1990 195

1991 278

1992 398

1993 568

1994 811

1995 1,159

1996 1,656

1997 2,366

1998 3,380

1999 4,828

2000 6,865

2001 15,330

2002 10,552

2003 15,342

2004 24,560

2005 67,546

Total 155,834 

 

14 Rutledge, R. Milwaukee Journal Sentinel, Burning to Save a Buck, Oct. 7, 2005 

http://www.jsonline.com/news/gen/oct05/361678.asp

15 Bakken, R. Grand Forks Herald, Business Heats Up, December 6, 2004.

16 Ibid. 

17 Sales estimates derived from state specific sales data from nine manufacturers, national sales data from 

21 manufacturers, and trend analysis to forecast 1990-1998 and 2005 data. Appendix C provides state 

specific sales estimates. 
Assessment of Outdoor Wood-fired Boilers Page 4-1

4. PUBLIC HEALTH CONCERNS 

Wood smoke emissions represent a potential threat to the health of persons living 

in proximity to OWB devices. Scientific studies have found associations between the 

inhalation of wood smoke or wood smoke constituents and adverse health effects. These 

findings are of notable concern because a large fraction of the population is susceptible or 

at increased health risk from exposure to wood smoke. In addition, physical and 

operational factors unique to OWBs heighten the possibility that people are exposed to 

wood smoke in both outdoor and indoor environments.

4.1. Wood Smoke Composition 

Wood smoke contains a complex mixture of particles and gases, many of which 

have been shown to produce acute and chronic biological effects, as well as deleterious 

physiologic responses in exposed humans. The abundance of fine particulates in wood 

smoke presents perhaps the most serious health risk to exposed populations, and will be 

discussed in more detail in the following section. Wood smoke also contains numerous 

gases, including carbon monoxide, nitrogen and sulfur oxides, volatile organic 

compounds (VOCs), PAHs, and chlorinated dioxins. Carbon monoxide can cause 

respiratory and cardiac distress because it competes with oxygen on the hemoglobin 

molecule, forming carboxyhemoglobin. Studies have associated exposure to nitrogen 

oxides with toxicological effects including pulmonary edema, bronchoconstriction, and 

increased infection rates. Studies have also associated VOCs, such as aldehydes, with 

upper airway irritation, headaches, and other neurophysiologic dysfunctions, and possibly 

cancer. Studies have shown that polycyclic aromatic hydrocarbons, such as 

benzo(a)pyrene, are carcinogenic in animals and may cause cancer in humans. 

Toxicologic evidence also suggests that certain dioxin congeners commonly found in 

wood smoke are human carcinogens.18

 

4.2. Particulate Matter Health Effects and Populations at Risk 

The severity and variety of adverse health effects attributed to exposure to fine 

particulate matter suggests that the aerosol component of wood smoke plays a large role 

in the observed health effects associated with wood combustion. Because of its physical 

structure, PM2.5 can bypass conductive airways and deliver exogenous materials into the 

deep lung. These materials include reactive organic chemicals that adsorb onto the 

particle. Over the past 30 years, scientific evidence has found that short- (e.g., daily) and 

long-term (e.g., annual and multiyear) exposure to airborne PM is associated with 

 

18 Pierson WE, Koenig JQ, Bardana Jr EJ. Potential adverse health effects of wood smoke. West J Med

1989; 151:1-6. 

 Larson TV, Koenig JQ. A summary of the emissions characterization and noncancer respiratory effects 

of wood smoke. EPA-453/R-93-036. U.S. EPA, Office of Air Quality Planning and Standards: Research 

Triangle Park, NC, 1993. 

 Maynard RL, Waller R. Carbon Monoxide. In: Holgate, ST, Samet, JM, Koren, HS, Maynard, RL, eds. 

Air Pollution and Health. Academic Press: New York, NY 1999; pp. 749-796. 

 Zelikoff JT, Chen LC, Cohen MD, Schlesinger RB. The toxicology of inhaled woodsmoke. J Toxicol 

Envron Health 2002; Part B,5:269-282. 

 Steenland K, Bertazzi P, Baccarelli A, Kogevinas M. Dioxin revisited: developments since the 1997 

IARC classification of dioxin as a human carcinogen. Environ Health Perspect 2004; 112:1265–1268. 
Assessment of Outdoor Wood-fired Boilers Page 4-2

cardiopulmonary health effects, including increased respiratory and cardiac symptoms, 

hospital admissions and emergency room visits, and premature death. Other harmful 

health effects include aggravated asthma, decreased lung function, and chronic 

bronchitis. Recent studies suggest that chronic exposure to air pollution may actually 

result in the development of new cases of asthma and atopy.19 A review of adverse 

health effects of short-term exposure to particulate matter in study areas where residential 

wood combustion was considered a major source of ambient PM found higher health risk 

associations than those found in areas dominated by other sources of PM, especially for 

children.20 

Population subgroups susceptible or most affected by PM2.5 exposure comprise 

upwards of 50% of the general population, including children, asthmatics, persons with 

preexisting respiratory disease or cardiac problems, older adults, and healthy adults who 

work or exercise outdoors.21 Children’s exposure to air pollution is of special concern 

because their immune system and lungs are not fully developed when exposure begins. 

For example, the number of alveoli in the human lung increases from 24 million at birth 

to 257 million at age four. As the lung epithelium is not fully developed, there is greater 

permeability of the epithelial layer in young children. Also, under normal breathing, 

children breathe 50% more air per kilogram of body weight than adults. In addition, 

children’s high activity levels can result in increased ventilation, increasing exposure to 

air pollutants such as particulate matter. These factors suggest that there is a critical 

exposure time for children when air pollution may have long-term effects on respiratory 

health.22 However, PM exposure can adversely affect both susceptible and general 

populations, including healthy adults. The exact level where an individual might become 

ill or sensitized is unknown because of the inability of scientists to determine whether a 

threshold level exists or does not exist below which exposure to PM is safe.23

Even hourly exposures to fine particulate matter may result in acute health 

responses within susceptible subgroups. Clinical and epidemiological evidence now 

suggests cardiac health effects, including increased risk of myocardial infarction and 

decreases in heart rate variability, which may be associated with PM exposures with 

 

19 Pope CA, Burnett RT, Thurston GD, Thun MJ, Calle EE, Krewski D, Godleski JJ. Cardiovascular 

mortality and long-term exposure to particulate air pollution: epidemiological evidence of general 

pathophysiological pathways of disease. Circulation 2004;109:71-77. 

 Review of the National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of 

Scientific and Technical Information. EPA-452/R-05-005. U.S. EPA, Office of Air Quality Planning 

and Standards: Research Triangle Park, NC, 2005. 

 Peters JM, Avol E, Berhane K, Gauderman WJ, Gilliland F, Jerrett M, et al. 2004. Epidemiologic 

investigation to identify chronic effects of ambient air pollutants in southern California. Contract No. 94-

331. Los Angeles, CA:University of Southern California.

20 Boman BC, Forsberg AB, Järvholm BG. Adverse health effects from ambient air pollution in relation to 

residential wood combustion in modern society. Scand J Work Environ Health 2003;29:251-260. 

21 Johnson PRS, Graham JJ. Fine particulate matter National Ambient Air Quality Standards: public health 

impact on populations in the northeastern United States. Environ Health Persp 2005;113:1140–1147. 

22 Schwartz J. Air pollution and children’s health. Pediatrics 2004;113: 1037-1043. 

23 Daniels MJ, Dominici F, Zeger SL, Samet JM. 2004. The National Morbidity, Mortality, and Air 

Pollution Study, Part III: PM10 Concentration–Response Curves and Threshold for the 20 Largest US 

Cities. Research Report 94. Boston, MA:Health Effects Institute. 
Assessment of Outdoor Wood-fired Boilers Page 4-3

averaging times less than 24 hours (e.g., one to several hours).24 These findings, in 

combination with associations between daily changes in PM and cardiovascular deaths 

and hospitalizations, indicate that short-duration exposures to wood smoke emissions 

could have serious health effects, especially for susceptible members of the population, 

such as those with preexisting heart disease and older persons. 

While the above evidence regarding the severity of health effects and magnitude 

of populations affected by PM has led health scientists to conclude that exposure to wood 

smoke should be avoided,25 residential wood combustion (RWC) remains one of the 

largest sources of PM2.5 emissions to the atmosphere in North America.26 Studies in 

urban and rural areas have found that wintertime residential wood smoke contributes 

significantly to ambient concentrations of PM2.5 as well as VOCs.27 This can be 

aggravated in areas subject to persistent temperature inversions, located in valleys, or 

which have a high percentage of wood burning households.28 Wood smoke PM is 

dominated by particles with average mass diameters generally between 0.1 and 0.6 μm. 

Submicron particles readily penetrate residential structures as a result of the normal 

exchange of air.29 Particles in wood smoke emitted from chimneys have been found to be 

a major source of indoor particles and thus a source of exposure to residents, even in 

homes without wood stoves.30 Given the infiltration capacity of PM2.5, exposure to wood 

 

24 Brook RD, Franklin B, Cascio W, Hong Y, Howard G, Lipsett, M. Air pollution and cardiovascular 

disease: a statement for healthcare professionals from the expert panel on population and prevention 

science of the American Heart Association. Circulation 2004;109: 2655-2671. 

25 Pierson WE, Koenig JQ, Bardana Jr EJ. Potential adverse health effects of wood smoke. West J Med

1989; 151:1-6. 

 Koenig JQ, Larson TV, Hanley QS, Rebolledo V, Dumler K, Checkoway H, et al. Pulmonary function 

changes in children associated with particulate matter air pollution in a wood burning community. Environ 

Res 1993:63;26-38. 

 Larson TV, Koenig JQ. Wood smoke: emissions and noncancer respiratory effects. Annu Rev Public 

Health 1994; 15:133-156. 

 Zelikoff JT, Chen LC, Cohen MD, Schlesinger RB. The toxicology of inhaled woodsmoke. J Toxicol 

Envron Health 2002; Part B,5:269-282. 

 Bates DV, Koenig J, Brauer M. Health and Air Quality 2002 – Phase I: Methods for Estimating and 

Applying Relationships between Air Pollution and Health Effects. Final Report. British Columbia Lung 

Association. May 2003. 

26 Fine PM, Cass GR, Simoneit BRT. Organic compounds in biomass smoke from residential wood 

combustion: emissions characterization at a continental scale. J Geophys Res-Atmos 2002; 107(D21):8349.

27 Johansson LS, Tullin C, Leckner B, Sjövall P. Particle emissions from biomass combustion in small 

combustors. Biomass Bioenerg 2003; 25:435-446. 

 McDonald JD, Zielinska B, Fujita EM, Sagebiel JC, Chow JC, Watson JG. Fine particle and gaseous 

emission rates from residential wood combustion. Environ Sci Technol 2000; 34:2080-2091. 

 Polissar AV, Hopke PK, Poirot RL. Atmospheric aerosol over Vermont: chemical composition and 

sources. Environ Sci Technol 2001; 35:4604-4621. 

28 Sexton K, Spengler JD, Treitman RD, Turner WA. Winter air quality in a wood-burning community: a 

case study in Waterbury, Vermont. Atmos Environ 1984; 18:1357-1370.

29 Boman BC, Forsberg AB, Järvholm BG. Adverse health effects from ambient air pollution in relation to 

residential wood combustion in modern society. Scand J Work Environ Health 2003; 29:251-260. 

 Abt E, Suh HH, Catalano P, Koutrakis P. Relative contribution of outdoor and indoor particle sources to 

indoor concentrations. Environ Sci Technol 2000; 34:3579-3587. 

30 Anuszewski J, Larson TV, Koenig JQ. Simultaneous indoor and outdoor particle light-scattering 

measurements at nine homes using a portable nephelometer. J Exposure Anal Environ Epidemiol 1998; 

8:483-493.
Assessment of Outdoor Wood-fired Boilers Page 4-4

smoke indoors—where individuals typically spend the majority of their time—can occur 

across large residential areas. 

4.3. Unique Factors Relating to OWB Emissions and Potential Public 

Health Threats 

Outdoor wood boilers are a unique emission source with operational and design 

factors not typically found in other residential wood burning appliances. First, the design 

of OWBs is to operate intermittently following the heat load of a building. This design 

enables boilers to burn wood in low temperature and oxygen-starved conditions to 

prolong the fuel source. In general, PM2.5 mass concentration increases during 

unsatisfactory operating conditions, especially where lower excess air ratios or lowquality fuel can yield more condensable gas and organic condensation nuclei.31 Reduced 

combustion air supply favors the emission and formation of especially harmful pollutants, 

as inefficient smoldering conditions can result in high emissions of particles and 

unoxidized gaseous compounds, leading to the formation of particle-bound PAHs.32

 

Unlike EPA-certified residential wood stoves, the design of many OWBs do not provide 

for the oxidation of incomplete combustion vapors. As a result, these are available for 

formation of PM2.5 rich in relatively high molecular weight organic compounds. 

Second, in addition to poor combustion properties, the relatively short stack 

height of OWBs creates dangerous dispersion conditions to nearby buildings, including 

an increased likelihood of worst-case emission scenarios such as fumigation and 

impingement. In contrast to indoor wood stove stacks that extend through the roof of a 

home to heights of 20 to 30 feet, OWBs come with short stacks typically between 8 to 10 

feet tall. In addition, the regular use of OWBs further exacerbate the potential impact of 

their emissions on nearby buildings because the appliances supply hot water for domestic 

consumption and heating every day, all year long—not just during wintertime. In-field 

ambient PM2.5 monitoring recently conducted by NESCAUM illustrate the potential for 

OWBs to affect nearby ambient air quality as discussed in Section 5.2 of this report. 

Finally, the use of OWBs for trash burning increases potential emission and 

public health problems related to these devices. The sizeable firebox capacity and large 

loading door dimensions characteristic of OWBs facilitate the loading and combustion of 

non-wood materials, such as household waste (e.g., paper, plastic, and packaging). The 

combustion of these materials in devices that have low stacks, lack emissions control 

systems, and operate under low temperature conditions creates the potential for 

generating hazardous air pollutants in close proximity to homes, schools, businesses and 

other areas where people spend significant amounts of time. Using OWBs without 

emission controls to burn trash is analogous to the use of burn barrels and burn piles, 

 

31 Barrefors G, Petersson G. Volatile hydrocarbons from domestic wood burning. Chemosphere 1995; 

30:1551-1556. 

 Johansson LS, Tullin C, Leckner B, Sjövall P. Particle emissions from biomass combustion in small 

combustors. Biomass Bioenerg 2003; 25:435-446.

32 Hueglin C, Gaegauf C, Kunzler S, Burtscher H. Characterization of wood combustion particles: 

morphology, mobility and photoelectric activity. Environ Sci Technol 1997; 31:3439-3447. 

 McDonald JD, Zielinska B, Fujita EM, Sagebiel JC, Chow JC, Watson JG. Fine particle and gaseous 

emission rates from residential wood combustion. Environ Sci Technol 2000; 34:2080-2091. 
Assessment of Outdoor Wood-fired Boilers Page 4-5

which many states and local governments have banned because they emit a variety of 

pollutants, including acidic gases, heavy metals, and dioxin. Inhaling these substances 

has the potential to cause health problems including eye and throat irritation, respiratory 

problems, and an increased risk of cancer.33

4.4. Cause for Concern 

Because OWBs are conducive to the formation of high PM emissions relative to 

background levels during routine operating conditions, OWBs may present a potential 

health risk to nearby populations. Should the use of OWBs become more prevalent in 

populated areas, OWB wood smoke particulate emissions could result in short- and longterm ambient and indoor air quality impacts on nearby neighbors, in light of the ability of 

fine aerosols to permeate readily into dwellings. These impacts likely would also affect 

populated areas subject to pollution loading arising from terrain and meteorological 

conditions favorable to inversion formation. This raises public health concerns because 

of the known health effects associated with exposure to PM, including a suite of 

respiratory and cardiac morbidity outcomes as well as premature mortality. Susceptible 

populations, such as the elderly, children, and persons with preexisting cardiopulmonary 

disease, may be at higher health risk and therefore disproportionately affected. 

Consideration of operating and design features typical to OWBs that can 

influence emissions supports these public health concerns, including poor combustion 

design, low stack height and poor dispersion, four-season utility, large firebox chamber 

capacity, and the potential to burn trash. Currently, few field assessments of OWB 

ambient emissions have been conducted, thereby limiting regulatory efforts to evaluate 

this potential public health problem, especially within the context of whether 24-hr and 

annual PM standards are suitably protective in areas with heavy wood burning. It is 

reasonable and prudent to assume that OWBs can present a public health risk to 

populations in proximity to these devices.

 

33 Lemieux PM. Evaluation of Emissions from the Open Burning of Household Waste in Barrels. EPA 

Project Summary. EPA/600/SR-97/134. U.S. Environmental Protection Agency, National Risk 

Management Research Laboratory: Cincinnati, OH, 1998. 
Assessment of Outdoor Wood-fired Boilers Page 5-1

5. EMISSIONS INFORMATION 

This section reviews previous studies of OWB emissions and describes the results 

of two studies conducted by NESCAUM. 

5.1. Previous Test Data 

A search of available data revealed limited information on OWB emission 

characterizations. Below are the results of this review. 

• EPA funded an OWB study conducted by Valenti and Clayton in 1998.34 The 

comparisons detailed within this study primarily provide data based upon a heat 

input basis. For reasons cited earlier in this report, the data have been converted 

to grams per hour measurements where possible. The testing was performed 

using a modified Method 28 fueling protocol and a modified Method 5G for 

measuring particulate matter, PAHs, and POMs. The OWBs tested under this 

effort exhibited PM emission levels as high as 143.2 g/hr for high heat removal 

firing and 55.4 g/hr at low heat removal. The application of a prototype catalyst 

on one device lowered PM emissions to 53.8 and 37.8 g/hr respectively. Table 4-5 

of the study compared PM emissions in milligrams PM per megajoules heat 

output for two OWB units; Furnace A produced 1,048 mg/MJ and Furnace B35

produced 681 mg/MJ. Table 4-5 indicated that non-catalytic wood stoves built in 

the 1990’s tested at 383 mg/MJ, catalytic stoves at 425 mg/MJ and pellet stoves at 

110 mg/MJ. 

Testing for total chromatographable organics (TCOs) showed emission rates as 

high as 5.4 g/hr at high heat removal to 8.31g/hr for low heat removal. The PAH 

emissions were as high as 2.8 g/hr for high heat removal and 0.64 g/hr at low heat 

removal. Surprisingly, the catalyst runs generated substantially higher levels of 

TCO and PAH compounds. The study did not provide comparison data on a 

grams per hour basis for TCOs or PAHs, however, it provided comparison data in 

micrograms per megajoules input basis. This data showed that PAH emissions 

for Furnace A were 15.6 mg/MJ, Furnace B 16.1 mg/MJ, non-catalytic wood 

stoves 28 mg/MJ, catalytic stoves 24 mg/MJ and certified pellet stoves 

0.082 mg/MJ. However, EPA caveated these data due to the different test 

methods, stating that only differences of an order of magnitude should be 

considered significant. The only publicly available data to compare PAH 

emissions on a grams per hour basis (total stack emissions) between OWBs and 

wood stoves is from an EPA wood stove field study on Phase 2 units in Klamath 

Falls and Portland, Oregon.36 This study found that the average “in-use” PAH 

 

34 Valenti JC, Clayton RK. Emissions from Outdoor Wood-Burning Residential Hot Water Furnaces. EPA 

Project Summary, EPA/600/SR-98/017. U.S. EPA, National Risk Management Research Laboratory: 

Cincinnati, OH 1998. 

35 Furnace B is a Central Boiler Model CL 17 (letter from Robert C. McCrillis, EPA, to Rodney Tollefson, 

Central Boiler, November 23, 1998). 

36 Fisher, L, Houck, J, Tiegs, P. Long Term Performance of EPA Phase 2 Woodstoves, Klamath Falls and 

Portland, Oregon, 1998/1999. EPA NRMRL RTP-195 (R3-27-00). 
Assessment of Outdoor Wood-fired Boilers Page 5-2

emissions for Phase 2 wood stoves, which had been in-use for over five years, 

was 0.149 g/hr for non-catalytic stoves and 0.165 g/hr for catalytic stoves. Based 

upon these data, OWBs may emit 4.3 to 18.8 times more PAHs than non-catalytic 

wood stoves and 3.9 to 16.9 times more than catalytic wood stoves. 

• Vermont Department of Environmental Conservation (VT DEC) determined, 

pursuant to litigation, that a unit produced by Central Boiler (Model CL-7260) 

had an “adjusted emission rate” of 93.76 g/hr for PM. This number was derived 

by state review of laboratory testing conducted by the manufacturer. Central 

Boiler claims in their submission of the tests that the actual rate was 3.6 g/hr. 

However, after thorough review, the Vermont DEC Air Pollution Control Agency 

concluded that Central Boiler incorrectly interpreted the data and believes that the 

state’s calculations of 93.76 g/hr are accurate. 

• US EPA and the New York Attorney General’s Office have obtained sales and 

emissions data from manufacturers. Table 5-1 contains emissions data made 

public by the New York Attorney General’s Office. OWB manufacturers have 

claimed that these data are Confidential Business Information and therefore 

NESCAUM has not been able to review the test reports or underlying test 

procedures37 and therefore cannot confirm the test results nor determine the 

amount of emissions that may not have been measured in the condensable portion 

of the emissions. However, the data do show high emission rates and indicate that 

manufacturers are aware of these high emission rates from their OWBs. 

Table 5-1. Unevaluated OWB Emissions Data38

Unit PM Emissions

g/hr 

PAH Emissions 

g/hr 

Heating 

Efficiency

Number of 

Test Runs 

OWB 1* 84 Not Available 30% 5 

OWB 2* 60 Not Available 37% 4 

OWB 3* 108 Not Available 28% 2 

OWB 4* 18 Not Available 31% 2 

OWB 5* 49 Not Available 55% 7 

OWB 6* 33 Not Available 37% 2 

OWB 7** 147 Not Available 55% 2 

OWB 8** 118 Not Available 53% 2 

OWB 9*** 179 Not Available 45% 

OWB 10*** 269 Not Available 46% 

* Intertek Laboratories 2004 

** Omni Laboratories 2004 

*** Intertek Laboratories 2004 from ASTM subcommittee, older model not currently being sold. 

 

37 Correspondence with EPA Region 1 in response to NESCAUM’s FOIA request of data submitted to 

EPA as a result of a Section 114 action. 

38 Schreiber, Judith et al. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State. Office of 

the Attorney General; Albany, New York, 2005. Available at 

http://www.oag.state.ny.us/press/2005/aug/August%202005.pdf 
Assessment of Outdoor Wood-fired Boilers Page 5-3

While there is only limited data at this time, the data show that PM emissions 

from OWBs are very high. One study concluded that OWB emissions are 10 to 20 times 

higher than certified indoor wood stoves.39 Such high emissions result from low 

efficiency and incomplete combustion of fuel. The Connecticut Department of 

Environmental Protection completed a comparison of homes heated with natural gas, oil, 

and OWBs, and concluded that emissions from one OWB are equivalent to emissions 

from four non-certified wood stoves, 18 certified wood stoves, 205 oil furnaces or 3,000 

to 8,000 natural gas furnaces.40 To put these emissions into perspective, one OWB can 

emit as much PM as four heavy duty diesel trucks. Figure 5-1 provides a chart 

comparing particulate emissions from various sources.

41

Figure 5-1. PM Emission Comparison (g/hr) 

 

39 Schreiber, Judith et al. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State. Office of 

the Attorney General; Albany, New York, 2005. Available at 

http://www.oag.state.ny.us/press/2005/aug/August%202005.pdf. 

40 CTDEP. Connecticut Department of Environmental Protection. Connecticut DEP Fact Sheet: Outdoor 

Wood Burning Furnaces. State of Connecticut, Connecticut Department of Environmental Protection. July 

2004; available at: http://www.dep.state.ct.us/air2/consumer/index.htm; accessed June 14, 2005.

41 Based upon an average emission rate of 36 g/hr for diesel trucks; http://www.burningissues.org/compemmis-part-sources.htm. 

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Assessment of Outdoor Wood-fired Boilers Page 5-4

5.2. Near-Source Ambient Emission Field Monitoring 

Currently, few assessments exist of OWB impacts to ambient air quality, limiting 

regulatory efforts to evaluate exposure conditions and the potential health risks posed to 

nearby populations.42 In order to characterize ambient PM2.5 emissions near an OWB, 

NESCAUM performed a screening level evaluation in March 2005. 

Pilot field monitoring was conducted in Central New York State at a site within 

50-150 feet of a Hardy H5-1-07 “Economy” boiler. 43 The device had a fire chamber 

22.6 cubic feet in size, maximum capacity of 180,000 Btu/hr, and stack height of about 

10 feet. Fuel usage during monitoring was a mix of green oak logged 11/01/04 and split 

12/04 (stored dry) and maple/cherry/other hardwood seasoned about one-year (stored 

dry). The PM2.5 monitoring interval was a 15-second averaging time in order to capture 

high temporal resolution during different boiler operating modes and fuel loads. A 

Thermo Electron DataRAM 4000 performed the monitoring of PM2.5. This is a portable 

nephelometric monitor that employs light scattering to measure the fine particle fraction 

of airborne pollutants. The DataRAM has the ability to estimate particle size below 

PM2.5 and is an ideal instrument for portable and highly time-resolved applications. 

Results indicate that areas within 150 feet of an OWB can experience high PM2.5

concentrations relative to background levels. Continuous sampling recorded periodic 

values >1,000 μg/m3

 and frequent values >400 μg/m3 throughout the course of routine 

OWB operating conditions, including damper open (oxygen rich) and damper closed 

(oxygen starved) modes and within about 1 hour and 24 hours after fuel loading. The 

monitor found high PM2.5 levels at all sampled distances, recording values upwards of 

4,000 μg/m3 over distances of 50, 100, and 150 ft. The monitor observed a peak value of 

8,880 μg/m3 at 50 ft. 

The time-series plotted in Figures 5-2 and 5-3 show selected results. Figure 5-2 

displays fine particle values that the monitor obtained about 24 hours after the OWB had 

been loaded with a wheelbarrow of wood fuel. (Wind was calm and variable/southerly, 

temperature about 8 °C, and relative humidity about 44% with light snow.) As shown in 

Figure 5-3, the study also conducted PM2.5 monitoring before and within 1 hour after 

OWB loading with 1⁄2 wheelbarrow of wood fuel. (Wind was calm and 

variable/southerly, temperature about 6 °C, and relative humidity about 50% with light 

snow.) In both figures, the monitor recorded high PM2.5 readings during both damper 

open and damper closed modes at all distances along the monitoring transect. It found 

the highest values within 1 hour after fuel loading, with damper open. General 

observations of wind direction and speed indicate that changes in monitored PM2.5

concentrations occurred when the measurement device was directly downwind or not 

directly downwind of the OWB plume. 

While this screening level pilot study was not intended to quantify 24-hour or 

longer-term average exposures (e.g., a complete heating season), such information would 

be useful to collect in order to compare typical ambient OWB PM levels to current 24-

 

42 The Michigan Department of Environmental Quality modeled the impacts of an OWB based on 

theoretical emissions (see Appendix D). 

43 A detailed presentation of findings has been submitted for peer-reviewed publication to the Journal of 

Health and Ecological Risk Assessment by Philip RS Johnson, NESCAUM. 
Assessment of Outdoor Wood-fired Boilers Page 5-5

hour or annual PM national ambient air quality standards. There are no health-based 

benchmarks for PM2.5 for 15-second averaging times. However, the high PM2.5 15-

second concentrations observed in this study could indicate the potential for elevated 24-

hour average concentrations relative to current health-based standards.44 EPA’s current 

national ambient air quality 24-hr and annual PM2.5 standards are 65 μg/m3 (98th

percentile form) and 15 μg/m3, respectively. EPA recently proposed a revised 24-hr 

PM2.5 standard of 35 μg/m3

 (98th percentile form) with no proposed revision of the annual 

standard.45 Canada’s more stringent PM2.5 24-hr objective is 30 μg/m3.

46 Because of this 

study’s findings of high PM2.5 concentrations in proximity to an OWB under routine 

operating conditions, NESCAUM recommends further research to quantify the nature 

and magnitude of OWB ambient emissions in populated areas.

Figure 5-2. Field measurements of PM2.5 near outdoor wood boiler 

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time (15 sec intervals)

PM2.5 (ug/m3)

DR 150' from OWB DR 100' from OWB

OWB damper open Damper open Damper closed Damper closed

 

44 Because the monitored data NESCAUM obtained was from an economy-sized OWB (180,000 Btu/hr), 

there is the potential that other OWB devices emit higher PM2.5 concentrations; OWB models can range 

from about 115,000 to 3,200,000 maximum Btu/hr output. 

45 US Environmental Protection Agency. Revisions to the National Ambient Air Quality Standards for 

particulate matter. Final rule. Fed. Reg. 1997;62:38652–38760. 

 Review of the National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of 

Scientific and Technical Information. EPA-452/R-05-005. Office of Air Quality Planning and Standards, 

US Environmental Protection Agency: Research Triangle Park, NC, 2005. 

 US Environmental Protection Agency. National Ambient Air Quality Standards for particulate matter. 

Proposed rule. Fed. Reg. 2006;71:2620–2708. 

46 Canadian Council of Ministers of the Environment. Human Health Effects of Fine Particulate Matter: 

Update in Support of the Canada-wide Standards for Particulate Matter and Ozone. Health Canada: Ottawa, 

Canada, 2004. URL for CWS is www.ccme.ca/publications/. 
Assessment of Outdoor Wood-fired Boilers Page 5-6

Figure 5-3. Field measurements of PM2.5 near outdoor wood boiler 

0

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time (15 sec intervals)

PM2.5 (ug/m3)

DR 100' from OWB

OWB damper 

closed

OWB loaded with wood

Damper open Damper closed 

~17:35

5.3. Results of NESCAUM OWB “In Use” Stack Test 

In June 2005, NESCAUM completed stack testing on a 250,000 Btu/hr Central 

Boiler unit to gain a better understanding of OWB operations. NESCAUM made pilot 

measurements on an "in-use" (field, not laboratory) residential OWB to determine 

emission rates for different burn scenarios using real-time measures and short duration 

filter samples. In addition, the test also obtained data on particle size, stack temperature 

and flue velocity. The results from these measurements are anecdotal in nature because 

NESCAUM tested only a single furnace and wood type. Nevertheless, combined with 

other laboratory emissions data, these tests provide a better understanding of the potential 

impact of these sources on air quality. The experience gained from these tests in how to 

better characterize the real-world emissions from OWBs will be useful in guiding the 

design of future testing programs. 

5.3.1. Testing Methods 

NESCAUM performed testing in June 2005 on an OWB, Central Boiler CL-17, 

rated at 250,000 Btu/hr that was installed in 2000 to heat a residence, provide hot water, 

and heat a swimming pool. For this test, the heat load was the swimming pool; the 

damper was manually controlled to simulate moderate heating loads observed in 

NESCAUM’s previous test. The wood used was from the furnace owner’s wood pile. 

The wood consisted of mixed hardwoods with moisture contents ranging from 20 to 40%. 

Appendix E contains information on the moisture content along with opacity readings 

taken by USEPA Method 9-certified staff from the VT DEC. On day one of testing, the 

OWB received an initial load of wood with no coal bed that was not disturbed until the 

end of the test runs that day. On day two, the OWB started with a full load of wood on a 

bed of coals. The loading door was opened once during the second day of testing to 
Assessment of Outdoor Wood-fired Boilers Page 5-7

check the load and rake the coals. Nothing else was done to the wood load in the OWB 

during the testing period. 

A Thermo Electron DataRAM 4000 made continuous stack PM concentration 

measurements using light scattering to estimate PM2.5 concentrations. The OWB stack 

sample was diluted with ambient air by a factor that varied between approximately ten to 

twenty times (the actual dilution ratio was repeatedly measured and applied to the 

reported data). The dilution air was added within the stack to minimize water vapor 

condensation problems. The sample train provided sufficient residence time (several 

seconds) and near-ambient temperatures to allow organic gases in the sample to condense 

to particle phase. Thus, this method should be considered to be measuring both the “hot” 

and “cold” (condensed organic vapor) fractions of the stack PM emissions. Observed 

stack PM concentrations were five to ten times higher than expected, so there were some 

problems with the continuous PM measurements. The inlet probe clogged (partially or 

completely) several times, and concentrations exceeded the DataRAM’s useful range a 

significant amount of the time. Data from these periods have been excluded from this 

report, with the exception of a few brief periods that are noted as “saturated” on the 

continuous data plots. The continuous PM data have been converted into emission rate 

data (grams per hour) based on the average dry flow for each of the two burn modes (idle 

fire or damper closed mode and full fire or damper open mode). For the scatter plot 

comparing data from the DataRAM continuous PM method with the filter gravimetric 

method (see below), mass concentrations of PM in the stack are used. The DataRAM 

also reports a semi-quantitative measurement of the volume (mass) median particle 

diameter (an approximate estimate of particle size). 

A modified EPA Method 17 sampling system (using 0.3 μm pore-size glass fiber 

filters) collected short-term (3 to 20 minute) filter gravimetric PM samples. The stainless 

steel filter holders were preheated to 275° F prior to sampling, but were outside the stack 

and unheated during sampling (no hot-box was used), so the actual temperature of the 

filter varied with the furnace burn mode resulting in a substantial variation in the amount 

of “condensable” (semi-volatile organic) PM collected on the filters. “Full fire” mode 

(damper open) stack temperatures were usually above 400°F; this resulted in substantial 

under-measurement of PM due to the inability to measure the condensable PM. “Idle 

fire” mode stack temperatures were as low as 130°F, and much of the condensable PM 

was collected under these conditions even though the filter holders were preheated. 

5.3.2. Results and Discussion 

The following sections provide specific details on the testing results using the 

DataRam and filter test methods. 

Results from DataRam Measurements 

Figures 5-4a through 5-4e are time series plots of continuous PM emission data, 

particle mass median diameter, and stack temperature. Times and values of gravimetric 

filter sampling are also indicated on these plots. Missing time periods are when the 

DataRAM data are invalid for various reasons as noted above. 
Assessment of Outdoor Wood-fired Boilers Page 5-8

The continuous PM emission data demonstrate the short-term dynamics of 

emission rates in more detail than filter sample data. In some cases, changes in emission 

rates can be related to changes in burn conditions (damper state changed), but often the 

rates change dramatically for no obvious reason. One possible explanation is settling of 

the wood load inside the furnace, but this could not be observed without interrupting the 

testing process. The wide range of PM emission rates even under similar conditions 

demonstrates the need for more extensive testing to properly characterize sources. The 

mean emission rate from the continuous monitor was 161 g/hr.47 This rate does not 

include start up emissions (worst case scenario) and represents mid-range emissions, 

operating through several operating cycles. This rate is higher than the mean of filter 

emission rates for either mode (93 g/hr for full fire mode and 64 g/hr for idle fire mode), 

which is likely due to the filter sampling method’s inability to measure condensibles. 

Results from filter samples 

Table 5-2 gives a summary of filter test results for damper open or full fire, and 

Table 5-3 gives results for damper closed or idle fire. The mean PM emission rate for all 

full fire filter runs is 93 grams per hour (g/hr) with a range of 13 to 237 g/hr. For idle fire 

conditions, the mean is 64 g/hr with a range of 13 to 148 g/hr. To put these emissions 

into a hypothetical ambient PM concentration context, if the maximum rate of 237 g/hr 

were emitted into a totally stagnant air mass of 100 cubic meters (for example, an area of 

100x100 meters and a height of 100 meters), the ambient PM concentration would be 237 

μg/m3

 after one hour. 

We must note that caution should be taken in making relative comparisons of 

emission rates from the two burn modes based on the filter data. These data imply that 

on average, full fire PM emission rates are approximately 50% higher than idle burn 

rates. As noted above, and discussed in detail below, the filter data from full fire samples 

is likely to be biased low by a large factor because of the loss of condensable PM from 

the hot filter. This implies that the actual full fire emission rate is much higher than the 

idle rate. An additional uncertainty in calculation of idle fire emission rates is the stack 

flow measurement; the flows were very low in that mode and thus difficult to measure 

accurately. 

Comparison of two testing methods 

To assess the comparability of the two PM measurement methods used 

(continuous and filter-based), Figure 5-5 shows scatter plots of matching time periods 

when collecting both filters and valid continuous PM data. Data from the two burn 

modes are compared separately because of the substantial difference in how much 

condensable PM was presumably collected with the filter method between the two 

modes. Table 5-4 presents the data used in Figure 5-5. Note that Table 5-4 presents 

these data as concentrations (grams per cubic meter) rather than emission rates. 

The difference in both numerical agreement and correlation between the two PM 

measurement methods between the two burn modes is consistent with the differences in 

 

47 This measurement came from a mostly contiguous 3.5 hour period on day two of testing after two initial 

modulations of the unit running a period beginning 2.5 hours after the fuel charge and ending six hours 

after fuel charge. 
Assessment of Outdoor Wood-fired Boilers Page 5-9

filter temperature between the two modes. In the idle burn (damper closed) mode, stack 

(and therefore filter) temperatures are relatively low, allowing collection of much of the 

condensable organic PM. In this mode, the two methods correlate well and the mean PM 

is within 30%. In the full burn (damper open) mode, the stack (and filter) temperatures 

are much higher, so the filter is not collecting much of the condensable PM. In this 

mode, the two measurement methods do not correlate, and the filter PM data are much 

lower than the continuous PM data. In the damper open mode, the continuous data range 

is approximately a factor of 10; the filter data range for this mode is small, with five of 

the six samples between 0.13 and 0.21 g/m3

. The mean of the continuous DataRAM PM 

is nearly seven times higher than the mean of the filter PM for this sample subset period. 

These differences between the two PM methods are not unexpected, because the 

modified method 17 used for the filter measurements does not attempt to efficiently 

collect the condensable organic PM fraction. 

Stack Conditions 

Stack temperatures varied widely, from as low as 130°F (idle burn) to 600°F (full 

burn). Median particle size also varied widely, from 0.1 to over 1 μm in diameter. 

Smaller sizes were generally associated with full (hot) burn modes, with larger sizes 

during idle (cooler) burns. This is consistent with what could be expected; hotter burns 

would have less organic carbon material because the higher temperatures combust more 

of the organic carbon. 

Conclusions 

While it would be reasonable to assume that a “fresh” load of burning wood in the 

OWB would result in higher emission rates compared to an “aged” load that is more 

charcoal-like, this was not the case. The idle burn mode filter test results were 

insufficient to show this and the full burn mode tests would not be appropriate to use 

because the filter measurements did not capture the majority of PM emissions. Idle burn 

filter PM data from June 21 were too limited (2 samples, both 2-3 hours after loading) for 

this assessment. On June 22, six idle-fire filter measurements were made from about 1⁄2 

hour after the OWB was loaded to almost 6 hours after. The successive idle burn filter 

PM emission rates were 51, 83, 109, 148, 27, and 13 grams/hour. Although the two 

lowest PM emission rates were at the end of the test sequence, the highest value of 

148 g/hr occurred almost five hours after the wood was loaded. Thus, no clear 

conclusion from these limited data can be made regarding emissions and age of load. 
Assessment of Outdoor Wood-fired Boilers 

Table 5-2. Filter Measurements Idle Burn Mode 

Test Run 3 5 7 10 12 14 16 Date 6/21/2005 6/21/2005 6/22/2005 6/22/2005 6/22/2005 6/22/2005 6/22/2005 Clock Time (24 hour) 15:08 – 15:23 16:15 – 16:31 10:03 – 10:22 11:21 – 11:40 12:47 – 12:50 14:18 -14:21 15:08 – 15:11 Test Duration (minutes) 15 16 19 19 3 3 3 Sample Volume (dscf) 4.583 4.838 6.09 5.416 0.865 0.862 0.862 Test Measurements Isokinetics (%) 88.9 87.9 92.2 82.1 101.6 95.5 91.6 Moisture Content (%) 7.5 7.5 7.5 7.5 7.5 7.5 7.5 Temperature (F) 200 200 200 200 356 280 250 Gas Composition –  CO2 (%) 6.4 6.4 6.4 6.4 6.4 6.4 6.4  O2 (%) 13.3 13.3 13.3 13.3 13.3 13.3 13.3  CO (%) 0 0 0 0 0 0 0  N2 (%) 80.3 80.3 80.3 80.3 80.3 80.3 80.3 Gas Velocity (fps) 2.4 2.4 2.4 2.4 2.6 2.5 2.5 Gas Volumetric Flow (dscfm) 37 37 37 37 32 34 36  (acfm) 50 50 50 50 54 52 52 

PM Emission Determinations

Concentration (grains/dscf) 0.268 0.2705 0.354 0.5804 0.8778 1.1171 0.1933 Emission Rate (lbs/hr) 0.08 0.09 0.11 0.18 0.24 0.33 0.06 Concentration (grams/meter3) 0.614 0.619 0.811 1.329 2.010 2.558 0.443 Emission Rate (grams/hour) 38.56 38.92 50.93 83.50 109.22 147.69 27.06 
Assessment of Outdoor Wood-fired Boilers 

Table 5-3. Filter Measurement Full Burn Mode 

Test Run 1 2 4 6 8 9 11 13 Date 6/21/2005 6/21/2005 6/21/2005 6/22/2005 6/22/2005 6/22/2005 6/22/2005 6/22/20Clock Time (24 hour) 

14:11 – 

14:19 

14:38 – 

14:45 

15:37 – 

15:56 

09:44 – 

09:49 

10:43 – 

10:50 

10:59 -

11:00:30 

12:16 – 

12:31 

13:50 

14:06 

Test Duration (minutes) 8 7 19 5 7 1.5 15 16 Sample Volume (dscf) 2.583 3.701 4.662 1.586 1.946 0.36 4.06 4.223 Test Measurements Isokinetics (%) 134 95.3 99.3 133.4 116.7 193.4 102.3 101.6 Moisture Content (%) 21.7 21.7 21.7 21.7 21.7 21.7 21.7 21.7 Temperature (F) 450 590 630 360 455 455 478 500 Gas Composition –  CO2 (%) 8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3  O2 (%) 11.8 11.8 11.8 11.8 11.8 11.8 11.8 11.8  CO (%) 0 0 0 0 0 0 0 0  N2 (%) 79.9 79.9 79.9 79.9 79.9 79.9 79.9 79.9 Gas Velocity (fps) 6.4 6.9 7 6 6.4 6.4 6.5 6.5 Gas Volumetric Flow (dscfm) 60 56 55 63 60 60 60 58  (acfm) 134 144 147 126 130 134 136 136 

PM Emission Determinations 

Concentration (grains/dscf) 0.9965 0.0763 0.0755 0.9691 0.2244 0.9774 0.0908 0.077Emission Rate (lbs/hr) 0.51 0.04 0.04 0.52 0.12 0.06 0.05 0.04 Concentration (grams/meter3) 2.282 0.175 0.173 2.219 0.514 2.238 0.208 0.177 Emission Rate (grams/hour) 232.49 16.61 16.15 237.40 52.35 228.03 21.18 17.39 
Assessment of Outdoor Wood-fired Boilers 

Table 5-4. Comparison of Filter Data and DataRam 

Full Burn Data 

6/21/2005 6/21/2005 6/22/2005 6/22/2005 6/22/200

 14:38 – 14:45 15:37 – 15:56 10:43 – 10:50 12:16 – 12:31 13:50 – 14:06 

PM Concentration, g/m3

DR 3.070 0.340 1.355 1.174 1.900 

Idle Burn Data 

 6/21/2005 6/22/2005 6/22/2005 6/22/2005 6/22/2005  16:15 – 16:31 12:47 – 12:50 14:18 -14:21 15:08 – 15:11 15:18 – 15:21 

PM Concentration, g/m3

DR 0.661 2.586 2.956 1.269 0.501 Filter 0.619 2.010 2.558 0.443 0.195 
Assessment of Outdoor Wood-fired Boilers 

Figure 5-4a. VT Outdoor Wood Furnace Stack Sampling 

Continuous PM (Data RAM) 21-June 2005 

14:28 to 15:08

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--- filter = 16.6 g/h ---
Assessment of Outdoor Wood-fired Boilers 

Figure 5-4b. VT Outdoor Wood Furnace Stack Sampling 

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Assessment of Outdoor Wood-fired Boilers 

Figure 5-4c. VT Outdoor Wood Furnace Stack Sampling 

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filter end --- filter = 52.3 g/h ---
Assessment of Outdoor Wood-fired Boilers 

Figure 5-4d. VT Outdoor Wood Furnace Stack Sampling 

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(109 g/h) filter = 21.2 g/h
Assessment of Outdoor Wood-fired Boilers 

Figure 5-4e. VT Outdoor Wood Furnace Stack Sampling 

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--- filter = 17.4 g/h --- filter = 13.3 g/h
Assessment of Outdoor Wood-fired Boilers Page 5-18

5.4. ASTM Test Protocol 

In June 2004, the ASTM sub-committee was formed to develop a standardized, 

replicable laboratory method to measure particulate emissions from OWBs. The goal of 

this process is to develop a laboratory-based test method that evaluates emissions 

between different OWBs and measures particulate emissions and delivered heating 

efficiency at a minimum of four heat output rates. This will provide accurate and reliable 

data on the performance of OWBs as an aid in developing cleaner technology. There are 

currently thirty voting members on the committee; nineteen are manufacturers, three are 

from OWB testing companies, two are from state environmental agencies, one is from a 

state attorney general’s office, one is a consultant, one is a coordinator for the HPBA 

trade association, one is from a state economic development agency, one is from 

Environment Canada, and one is from EPA.48

 

5.5. Conclusions on Emissions Testing 

 Both the ambient and stack testing conducted by NESCAUM on smaller OWBs 

burning appropriately seasoned hardwood showed high PM2.5 emissions, suggesting that 

people living near OWBs may be affected by unhealthy levels of PM2.5. Because 

manufacturers are selling OWBs primarily for use in residential settings, NESCAUM 

believes a single PM emission limit expressed in grams per hour should be developed to 

reduce emissions and assure public health protections. 

 NESCAUM also concludes that it may be appropriate to use newer, continuous 

PM measurement methods to measure emissions from OWBs and other wood burning 

devices. In order to accurately quantify emissions for OWBs, the testing method must 

have the capability to provide accurate results given the likelihood of filter loading and 

excessive moisture content. In addition, it is critical that any robust measurement of 

OWB emissions be capable of measuring the condensable fraction of PM because the 

majority of wood smoke emissions are in the condensable fraction. Traditional “backend” techniques (such as method 202) are not designed for wood smoke and may not 

efficiently capture the condensable fraction. Unlike power plant stack aerosols, wood 

smoke is not water soluble, and the small particle sizes generated in full fire mode are 

difficult to collect using inertial techniques. Therefore, the use of alternative 

technologies and test methods such as continuous PM measurement may be more 

appropriate for OWBs. Such alternatives already exist and EPA has conditionally 

approved them as alternatives to traditional Method 5 testing. Continuous PM monitors 

allow for PM measurement throughout the burn cycle rather than averaging several runs 

in a burn cycle. These methods would allow for accurate, continuous measurements in 

both low and high firing modes throughout the entire fuel charge, resulting in an accurate 

average emission rate for an entire burn cycle. Second, these methods overcome many 

 

48 Information regarding the ASTM work can be found at http://www.astm.org/cgibin/SoftCart.exe/DATABASE.CART/WORKITEMS/WK5982.htm?L+mystore+kltm0898. 
Assessment of Outdoor Wood-fired Boilers Page 5-19

Damper closed (Idle burn):

Mean DR: 1.59 g/m3

Mean Filter: 1.17 g/m3

R

2

 = 0.93

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Mean DR: 1.56 g/m3

Mean Filter: 0.23 g/m3

R

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Both methods measuring most of

condensables (filter T is ~120 to 180 F)

Dataram measuring most of condensables;

filter is losing most (filter T is ~300 to 550 F)

of the issues associated with filter-based testing including filter loading, excessive 

moisture content and condensable losses. 

Figure 5-5. VT Outdoor Wood Furnace Stack Sampling Continuous PM 

(Data RAM) vs. Filter PM 21-22 June 2005 
Assessment of Outdoor Wood-fired Boilers Page 6-1

6. OWB CONTROL STRATEGIES 

Policy makers often face challenges in determining emission control and exposure 

reduction requirements with uncertain and often conflicting scientific and toxicological 

opinions before them. However, there is mounting evidence to support the need for 

action to reduce emissions from OWBs in order to maintain public health protections. 

The options government agencies should consider are to rely on existing regulations, 

adopt new requirements, or invest in voluntary initiatives. This section outlines these 

options and provides an overview of relevant judicial rulings and enforcement actions. 

6.1. Regulatory Action 

Regulatory agencies can institute one or more strategies to address emissions 

from OWBs. These strategies include setting emission standards for OWBs, requiring 

permits for OWB installation, requiring the removal of non-certified appliances, 

establishing no burn days, instituting and implementing nuisance rules and/or opacity 

rules, and developing standards for fuels. The following section provides a brief 

overview of the options available to regulatory agencies. 

6.1.1. Ban/Removal of OWBs 

One strategy would ban the sale of all new OWBs and establish a timeline for 

removal of all existing OWBs. This strategy would address the installation of new 

OWBs by placing an immediate ban on the sale of new OWBs, and would address 

existing OWBs by establishing a timeline to remove them. Several states and 

municipalities have attempted to put bans into effect but only a few municipalities have 

succeeded. In 2004, Connecticut considered legislation that would ban the sale of OWBs 

until a federal emission standard had been adopted but this legislation was defeated under 

strong lobbying efforts from OWB manufacturers. A similar strategy would prohibit the 

sale of OWBs until clean burning units come to the market. These strategies are the only 

avenue that would immediately address emissions from all OWBs. 

6.1.2. Emission Standards 

Another strategy would create a PM emission limit for OWBs and prohibit the 

sale or installation of any unit that did not meet the standard. There are three potential 

approaches to creating a standard for OWBs: 

• a simple mass emission rate limit (grams/hour) regardless of an OWB’s 

rated output or actual thermal output, 

• an emission rate limit that is tied to an OWB’s rated output or actual 

average thermal output (grams of PM/Btu/hr or mg PM/megajoule), or 

• a limit on the concentration of the PM in an OWB’s stack (grams/cubic 

meter or grains/cubic foot of exhaust gas). 

Although all three of these approaches could reduce PM emissions from OWBs, 

only the first approach (grams/hour) specifically addresses both the regional and local 

aspect of OWB smoke. If the goal of a standard is in part to protect against local (50 to 

1000 feet from source) high PM concentrations, then the total PM emitted from a given 
Assessment of Outdoor Wood-fired Boilers Page 6-2

stack must be controlled without regard to OWB size. The second and third approaches 

above would not accomplish this because they allow a unit’s overall emissions to 

increase as the unit’s output increases. 

Both the second and third approaches address PM emissions from a relative 

efficiency perspective (PM emissions per unit heat produced). The second approach 

creates additional hurdles when developing and implementing a test protocol. First, it 

requires a clear definition on how to define Btu/hr – heat input, heat output as it leaves 

the OWB or heat output as it reaches the area to be heated. In addition, it will require 

that the testing precisely measure this additional feature. Another disadvantage to this 

method is that measurement would allow for dilution to minimize emissions when OWBs 

are tested in a laboratory setting. In addition, this method does not normalize with 

different fuel sources. The third approach, a grain loading standard, is similar to 

measurement based on heat ratings. However, a grain loading standard with a correction 

for carbon dioxide would ensure that the measurements in the testing are not diluted to 

reduce emissions. Furthermore the grain loading measurement that mandates a correction 

for carbon dioxide levels minimizes variability that may arise due to inconsistency in the 

fuel source. 

The state of Washington has adopted a mass emission rate regulation that requires 

all solid fuel burning devices, including OWBs, to meet an emission limit of 4.5 g/hr PM 

for non-catalytic appliances and 2.5 g/hr for catalytic appliances. The state of Vermont 

has proposed a regulation based on the third approach. The proposed Vermont standard 

would limit OWB emissions to 0.20 grains per dry standard cubic foot of exhaust gas 

corrected to 12% CO2. Calculations based on testing data estimate that the mass 

emission limit for smaller OWBs would approximate 22-36 g/hr PM, assuming a 50% 

duty cycle. Because the Vermont standard is a concentration-based standard, a 

residential wood boiler larger than the 250,000 Btu/hr used for these tests could have 

higher PM emission rates (assuming similar duty cycles). Appendix F includes copies of 

the Vermont and Washington regulations. 

Establishing an OWB emission standard would reduce PM emissions from new 

OWBs and place the burden for reductions on manufacturers rather than homeowners. 

This option would allow the sale of clean burning OWBs while prohibiting the sale of 

dirty units. This strategy, however, would not address existing installations, but when 

combined with another strategy to address existing OWBs, it could present the best 

option to address OWB emissions. 

6.1.3. Fuel Requirements 

Another strategy would require that all wood burning devices burn seasoned 

wood and eliminate any state loopholes that would allow the burning of trash, yard waste 

or other waste materials. Several municipalities in Colorado, California, Washington, 

and Canada have this type of regulation. However, this strategy has several 

disadvantages. First, even under ideal conditions, burning seasoned cordwood, OWBs 

emit extremely high levels of particulate matter and air toxics. Second, enforcement of 

this rule would be a challenge. Finally, using wood that is too dry, such as pallets, may 

also cause excessive smoke and air emissions due to the increased likelihood of explosive 

incineration. 
Assessment of Outdoor Wood-fired Boilers Page 6-3

6.1.4. Mandatory Removal 

Mandatory removal requires removal of any wood burning appliance that is not 

certified by a certain date and/or rendered permanently inoperable by a certain date. This 

strategy would reduce emissions from the older, dirtier OWBs and significantly reduce 

particulate emissions from these devices. However, it also would encumber the 

homeowner with substantial costs and require extensive enforcement.

6.1.5. No Burn Days 

No Burn Days would prohibit the use of any wood burning device during 

episodes when the likelihood exists for high PM levels. New Mexico and municipalities 

in Alaska, California, Colorado, Oregon, South Dakota, Utah, and Washington have 

instituted a no burn day program. This strategy would address high emission events but 

would not address the day to day contribution of pollution from OWBs to ambient air 

quality. In addition, it does not address the severe local impacts that may occur. It also 

could be difficult to implement and enforce statewide no burn days. 

6.1.6. Nuisance Rules 

Nuisance rules would establish new regulations or more aggressively enforce 

existing regulations that limit or prohibit actions from causing or contributing to a 

nuisance condition. Many states have such regulations, however, using these rules to 

protect neighboring properties have proven difficult. Connecticut’s rule calls any visible 

emission crossing the property line at ground level a nuisance violation. Once the agency 

documents a violation, it issues a notice of violation (NOV) giving the OWB owner thirty 

days to remedy the violation. In the past, OWB owners have attempted to burn smaller 

loads, use dry wood, raise the stack height of the unit and install baffles without 

resolution of the underlying problem. Connecticut DEP has issued several NOVs for 

nuisance situations where the conditions causing the nuisance were not mitigated but 

none has resulted in the elimination of the issue. After many attempts to modify fuel use 

(using seasoned wood), add-on equipment (catalysts or baffles) and limit use, 

enforcement programs have indicated that shutting down or limiting OWB operation are 

the only viable remedies for nuisance and odor issues. 

6.1.7. Opacity Rules 

Many states, including Massachusetts, have requirements establishing smoke 

opacity limits. There is mounting evidence that suggests that OWBs cannot meet current 

opacity standards.49 A strategy to ensure that OWB’s comply with opacity regulations 

could require a permit to install and/or operate an OWB and require a demonstration that 

the unit meets the opacity limit before it can commence operation. This strategy would 

ensure that only clean OWBs are installed, but the burden of proof would be placed on 

the homeowner rather than the manufacturer. Furthermore, testing could only commence 

once the OWB owner had already invested in the purchase and installation of a unit. 

Experiences within the NESCAUM states have shown that once a unit is installed, it is 

very difficult for the state to remove the unit or prohibit operation. Therefore, even with 

 

49 During NESCAUM’s stack testing, a certified smoke reader recorded opacity readings. Appendix E 

includes a copy of the field report. 
Assessment of Outdoor Wood-fired Boilers Page 6-4

opacity regulations in place, states still face a high hurdle to enforce against OWBs based 

on opacity regulations. 

The following lists are the opacity regulations for the NESCAUM states: 

• Connecticut’s air regulations (RCSA Section 22a-174-18) limit opacity 

(except for periods of startup, shutdown, and malfunction) to 20% during 

any six-minute block average or to 40% during any one-minute block 

average. 

• Maine’s regulations (ME DEP Chapter 101) limit visible emissions from 

any wood-waste or biomass unit to 30% opacity on a six-minute block 

average basis, except for no more than two six-minute block averages in a 

three-hour period. 

• Massachusetts’ regulations [310 CMR 7.06(1)] limit opacity to 20% for a 

period or aggregate period of time in excess of two minutes during any 

one hour, with a maximum of 40% at any time. 

• New Hampshire’s air regulations (Env-A 2002.02) limit average opacity 

from fuel-burning devices installed after May 13, 1970 to 20% for any 

continuous 6-minute period, except for periods of startup, shutdown, 

malfunction, soot blowing, grate cleaning, and cleaning of fires. 

• New York’s air regulations (NYCRR Section 211.3) require that no 

person shall cause or allow any air contamination source to emit any 

material having an opacity equal to or greater than 20% (six minute 

average) except for one continuous six minute period per hour of not more 

than 57% opacity. 

• Rhode Island DEM’s Air Pollution Control Regulation No. 1 states that no 

person shall emit into the atmosphere from any source any air contaminant 

for a period or periods aggregating more than three minutes in any one 

hour which is greater than or equal to 20% opacity.

• Vermont’s regulation (Section 5-211) limits opacity to 20% for an 

aggregate period of six minutes during any one hour with a maximum of 

60% opacity for any two minute average; however the Vermont regulation 

exempts any wood-fuel burning equipment with a rated output of less than 

40 horsepower (1 boiler hp = 34,500 Btu/hr). 

6.1.8. Removal Prior to Sale or Transfer of Property 

Another strategy would require a certification prior to the completion of a sale or 

transfer of any real property on or after a certain date, that all wood burning appliances 

that are not certified would be required to be replaced, removed or rendered permanently 

inoperable. This would require an investment from the homeowner as would the 

mandatory removal. This strategy relies on the strength of the real estate market and the 

thoroughness of the home inspector. Enforcement of the program could be implemented 

via the building inspection process. 
Assessment of Outdoor Wood-fired Boilers Page 6-5

6.1.9. Zoning Regulations 

Another strategy would establish property line setbacks for OWBs to protect 

neighboring properties from nuisance smoke and odors. Vermont and Connecticut both 

have such regulations. Connecticut’s rule requires a 200 foot setback and stack heights 

greater than the roofline of the neighboring properties. This rule was adopted in spring 

2005 and therefore there is no experience to estimate the efficacy of this rule. Vermont 

adopted a “zoning” type regulation in 1997 that sets minimum setbacks, stack heights, 

and requires that purchasers of OWBs be notified about the Vermont rule requirements. 

Vermont has experienced many problems with this rule and has found that it has not 

addressed the underlying public health, nuisance and odor conditions created by OWBs. 

Setbacks and stack height requirements do not address the overall emissions from OWBs. 

In August 2005, Vermont proposed a new regulation that would establish an OWB 

emission standard. 

6.2. Voluntary Programs 

Voluntary programs rely on the consumer to purchase cleaner burning units or the 

manufacturer to make cleaner units. The section below overviews two voluntary 

strategies. 

6.2.1. Change-out Programs 

This strategy eliminates the use of old, dirty burning appliances by providing 

incentives for the public to purchase new, clean burning equipment. This strategy 

addresses PM2.5 emissions generated by wood burning appliances and eliminates older, 

dirtier OWBs. Because the program is voluntary in nature, the program needs significant 

participation and a large amount of resources to be successful. It is likely that a changeout program will have a better chance of success if it is supported by a rule prohibiting 

the installation of wood-burning appliances that do not meet specified standards for low 

emissions. 

6.2.2. Voluntary Industry Actions 

Another strategy would rely on manufacturers to voluntarily commit to 

developing cleaner OWBs. In 2001, New York held a meeting where manufacturers 

stated that they had plans to redesign their OWBs. Manufacturers once again made this 

claim to the Connecticut legislature during their regulatory process in June 2004. In June 

2005, EPA unsuccessfully asked for a meeting with manufacturers. To date none of these 

efforts have led to cleaner burning OWBs. Appendix A includes a copy of EPA’s request 

and a response from one of the largest OWB manufacturers. 

6.3. Court Actions 

NESCAUM has identified six lawsuits in the Northeast relating to OWBs. In 

every case, the end result has been the removal of, or prohibition to operate, the offending 

OWBs. In many cases, this has been the only option for neighboring properties to find 

adequate resolution to the nuisances created by OWBs. 

In Connecticut, five civil suits have been brought under Connecticut’s General 

Nuisance Laws. In every case, the outcome has been in the complainant’s favor. In at 
Assessment of Outdoor Wood-fired Boilers Page 6-6

least three of the Connecticut cases, the OWB manufacturer provided an attorney at no 

cost to the OWB owner. In only one case did the OWB owner go to trial; the other cases 

were settled out of court, and required confidentiality of the settlement. In all cases the 

OWB was either removed or prohibited from operating so long as the complainant 

resides in the neighborhood. 

In Stamford, Vermont, a landscape company that had improperly installed an 

OWB filed suit against the distributor and manufacturer (Central Boiler) of the OWB. In 

this case, the owner placed the unit within 200 feet of a neighboring residence, in 

violation of the Vermont OWB regulation. The OWB owner claimed that he had not 

been informed about the notification and setback requirements. The unit was eventually 

removed, and the details are not available because the case was settled out of court. 

In Grafton, Massachusetts, a resident made numerous complaints to the 

Massachusetts DEP about his neighbor’s OWB. The smoke from the OWB was coming 

directly into the complainant’s house causing him to have chronic bronchitis for the first 

time in his life. The Town of Grafton Board of Health issued a cease and desist order 

until the unit had been inspected and had the proper permits issued. The complainant 

ultimately took his neighbor to Housing Court and won. The owner of the OWB is no 

longer allowed to use it in the summer, had to increase the stack height, adjust the air 

flow, and change burning practices. 

While individuals have been successful in bringing these suits under nuisance 

laws, it seems unreasonable to expect that all affected individuals have the financial 

resources and other wherewithal to pursue such actions for the vast majority of potential 

OWB nuisances. In addition, public health harm can arise from the collective impact of 

emissions from many OWBs even if no individual OWB can by identified as causing 

unique harm to a specific person, as may be required under nuisance law. Therefore, it 

may not be an option available to all members of the public nor broadly covering OWBs. 

6.4. Enforcement Actions 

Several states have issued Notices of Violations (NOVs) for OWBs, however, 

enforcement actions against residential OWBs tend to be resource intensive and difficult 

to resolve. The following provides an overview of the available data. 

6.4.1. Connecticut 

Connecticut has issued approximately fifteen NOVs; none of these have resulted 

in assessed penalties, OWBs removed, or consent orders. OWBs complaints continue to 

increase. Implementation of Connecticut’s new rule requiring setback distances and 

stack heights has not eliminated complaints but has increased enforcement efforts and 

resources to address improperly installed units. 

6.4.2. Massachusetts 

An overview of fourteen actions taken by Massachusetts against OWBs: 

• A residential property owner installed an OWB in the spring of 2005 for heat 

and hot water. The smoke from the unit impacted the nearest residential 

neighbor. The neighbor complained of smoke infiltrating the house and 
Assessment of Outdoor Wood-fired Boilers Page 6-7

resulting health issues. The complainant also alleged that the smoke in their 

yard was so thick that it is like fog and even filled up the garage and car. The 

Board of Health told the owner of the OWB to shut the unit down for the 

summer. Before the unit can be restarted, the owner must change the air 

mixture, increase the stack height, and change his wood burning practices. If 

these measures do not improve the excessive smoke, the unit will have to be 

closed down. 

• Massachusetts DEP staff observed large amounts of smoke from the highway 

in Auburn, MA. They traced the smoke to an OWB operating at Brady 

Sunroom. The facility was burning glued boards in the unit for heat recovery. 

The facility entered into a consent order with penalty and removed the unit. 

• A company in Auburn, MA uses an OWB to heat their facility. When the unit 

was first put in operation, the fire department received many complaints about 

the unit. A trailer home park is located directly behind the company. The 

Massachusetts DEP staff have driven by this company on numerous occasions 

to try to verify excess smoke from this unit. The fire department has agreed to 

call the DEP this upcoming heating season if the unit is still causing a 

problem. 

• Massachusetts DEP received anonymous complaints about smoke and odors 

from an OWB. A daytime drive by on a cold day showed an OWB operating 

on a small farm but no opacity was observed. The Fire Department worries 

that the unit is burning large amounts of cardboard. The Fire Department will 

continue to monitor the OWB and wait for further complaints. 

• A residential property owner installed an OWB approximately six years ago 

and the neighbor has been having problems with smoke from that unit since 

the date of installation. The unit releases a great deal of smoke that goes all 

over the property and into their home. The homeowners have expressed 

concern about the health effects, difficulty in breathing by the neighbor, the 

inability of their daughter to visit them due to the smoke, and their inability to 

use their yard. The homeowner worked with the Board of Health to try to 

resolve the situation but was informed that he would need to hire an attorney. 

• A residential property owner installed an OWB to heat their home, pool and 

Jacuzzi. The large amounts of smoke from the unit were impacting the 

nearest neighbor who has two daughters with asthma. The smoke from the 

OWB was coming directly into their home and yard allegedly causing the 

children to have increased asthma attacks. The Board of Health required the 

owner of the OWB to cease using the unit during the summer. 

• A residential property owner installed an OWB to heat their home and provide 

hot water. Residents several miles downwind were getting smoke coming into 

their homes in the middle of the night, thus waking them up and giving them 
Assessment of Outdoor Wood-fired Boilers Page 6-8

headaches. The neighbors traced the source of smoke and odor to the OWB. 

The Board of Health and the Groton Fire Department investigated the matter 

and have met with the owner of the OWB several times but no formal action 

to date has been taken. 

• In summer of 2003, a resident began complaining about smoke from an OWB 

unit. A member of the town Board of Selectman was in the process of 

building a new home adjacent to the resident’s house and installed an OWB. 

The resident claimed that the owner of the OWB was also using the unit as an 

incinerator to burn construction debris. The resident complained about the 

large amounts of smoke coming from the OWB, the inability to use his 

lakeside property, and the inability to feed the numerous song birds that no 

longer come to his house. The resident was 84 years old, and reported that he 

had severe health issues. The resident has many photos of the OWB smoking. 

Visits by the Massachusetts DEP confirmed the presence of the OWB but did 

not observe it operating. The Board of Health took the lead in the matter. 

• One municipality received numerous residential complaints about an OWB 

located at a manufacturing facility. The facility purchased the OWB to burn 

pallets in order to reduce disposal costs and provide building heat. The Board 

of Health and fire department made many visits to the facility and witnessed 

excessive smoke from the OWB. The town eventually requested that the 

Massachusetts DEP provide some assistance in shutting down the unit. The 

DEP conducted an unannounced inspection and found the unit to be operating 

with excessive smoke while burning pallets. It issued a notice of 

noncompliance to the company, which removed the unit. 

• In another municipality, residents contacted the Massachusetts DEP and the 

town’s Board of Health regarding smoke complaints from their neighbor’s 

residential OWB. The unit was located in a historical district and operated 

without any town permit. The unit smoked so much that it obscured visibility 

several times on Main Street. The local paper even documented and 

published one of these instances. The complainant’s wife has had numerous 

respiratory problems due to the operation of the OWB. The town’s historical 

commission had the owner remove the unit. It is currently in the owner’s 

backyard and not hooked up. The town’s Board of Health contacted the 

Massachusetts DEP about guidance on regulations to restrict the location and 

operation of OWBs. The town has held two of three public meetings on a 

proposed regulation. 

• A residential property owner installed an OWB to heat the home and provide 

hot water. Five neighbors have complained to the Board of Health and fire 

department that the smell from the OWB forces them to keep their windows 

shut at all times. Even so, the odor still seems to seep through into their 

homes. The owner of the OWB was burning green pine in the unit. The 

Board of Health and fire department worked with the owner to improve the 
Assessment of Outdoor Wood-fired Boilers Page 6-9

smoke from this unit. Massachusetts DEP worked with the town in an 

advisory capacity. 

• While visiting a facility, a Massachusetts DEP inspector observed large 

amounts of smoke coming out of an OWB located at a land clearing company. 

The company burns waste wood to heat their building. At the time of the 

inspection, a 2.5 diameter by 3 foot long unsplit log was being burned in the 

unit. The Department issued a Notice of Non-Compliance (NON) requiring 

the source to stop the excessive smoke. The company could not meet the 

requirements in the NON and agreed to cease use of the unit. 

• A residential property owner installed an OWB to heat his home and provide 

hot water. The smoke from the unit impacted several of his neighbors. The 

neighbors called the Board of Health and Massachusetts DEP to complain 

about how the smoke was harming them (i.e., could not use yards, had to keep 

windows closed on beautiful days, etc.). The Board of Health required the 

owner of the OWB to cease using it for the summer. 

6.4.3. Vermont 

Vermont has taken action against one unit not related to their zoning regulation in 

Brandon, VT. In this instance, the Vermont Department of Environmental Conservation 

(VT DEC) determined that a nuisance condition existed and required the owner to not to 

operate the OWB during the summer months and to increase the OWB stack height. 

Vermont has taken enforcement actions against dealers and owners who have 

violated their setback and notification regulation. These actions have led to the shutdown 

or removal of six OWBs. In other instances, it required the unit owner to raise the stack 

height of the OWB. Currently, Vermont has four pending OWB violations. 
Assessment of Outdoor Wood-fired Boilers Page 7-1

7. CONCLUSIONS/RECOMMENDATIONS 

There is mounting evidence that supports the need for action to reduce emissions from 

OWBs. Based upon the data, NESCAUM has reached the following conclusions: 

• Sales of OWBs continue to increase by 25-50 percent annually. 

NESCAUM estimates that there are over 155,000 OWBs nationally. 

Recent manufacturer statements indicate that sales in 2005 increased by 

200 to 350 percent. If sales trends continue, there is the possibility that 

there will be almost 5,000 OWBs in Massachusetts and 500,000 OWBs 

nationally by 2010. 

• State experience suggests that manufacturers are unlikely to voluntarily 

improve OWB performance. 

• EPA is unlikely to develop a federal standard and even if it were to act 

immediately, it would take a minimum of five to seven years to enact a 

standard. 

• EPA’s focus on wood stove change-outs could result in an increase in 

installation rates for OWBs. 

• Without aggressive public policy to limit fine particle and air toxic 

emissions from OWB sources, the number of potentially exposed at-risk 

individuals will continue to grow. 

• Emissions from OWBs are 22 to 40 times greater than EPA certified 

indoor wood stoves. 

• Near-source ambient monitoring indicates that OWB emissions have the 

potential to create significant public health concerns. 

• Existing state regulations are inadequate to address OWBs and state 

environmental agencies lack the tools to effectively enforce against 

residential OWBs. Currently, the only viable route in some states to 

address OWB complaints is via state opacity requirements that are 

resource intensive to investigate and civil legal action, but this may not be 

equitable or broadly applicable. 

• Complaints and enforcement actions against OWBs continue to increase, 

draining both state and local agency resources. 

Based on these conclusions, NESCAUM recommends the following: 

• Given that federal action is unlikely to take effect in the near term, states 

should move quickly to address OWB emissions. 

• States should adopt regulations that establish emission limits for OWBs, 

similar to those of other residential woodburning units, because this is the 

best strategy for addressing the elevated emissions of fine particles (and 

the cancer-causing substances associated with particles) from OWBs. 
Assessment of Outdoor Wood-fired Boilers Page 7-2

• Because OWBs are most often used in residential settings and vary greatly 

in size, states should base an emission standard on a grams per hour basis 

to adequately protect public health. 

• Additional studies should be conducted to assess impacts and determine 

acceptable exposure levels. 

• EPA should support additional testing to gain a better understanding of the 

overall emissions profile of OWBs, specifically particulate matter and 

toxics. 

• EPA should develop tools to assist states in addressing OWBs. Tools 

could include model regulations, outreach materials, SIP credit, and 

research data. 
Assessment of Outdoor Wood-fired Boilers Page A-1

Appendix A: Outdoor Wood-fired Boiler 

Correspondence 
July 29, 2004 

Jeffrey R. Holmstead 

Assistant Administrator for Air and Radiation 

U.S. Environmental Protection Agency 

Ariel Rios Building 

1200 Pennsylvania Avenue, N. W. 

Washington, DC 20460

 Re: Outdoor Wood Boilers

Dear Mr. Holmstead: 

Over the past several years, a number of state air pollution control programs, as well as 

municipalities, have experienced a marked increase in the number of complaints related 

to Outdoor Wood Boilers (OWBs). The Northeast States for Coordinated Air Use 

Management (NESCAUM) believe the growing use of OWBs is a serious problem and 

are therefore exploring ways to take immediate action to prevent the continued 

proliferation of these sources until the units can be re-designed to ensure they meet 

stringent emissions standards. Concurrently, we would like to work with EPA because 

OWBs are sold nationwide and numerous states are facing similar problems with them. 

We believe a national regulatory approach is warranted, and we respectfully request that 

your office consider adopting a national strategy to regulate OWBs. Inasmuch as we 

have been collaborating with other states on this issue, you may receive similar 

correspondence from them. 

With recent increases in the price of heating fuels, the use of wood for residential and 

commercial heating is on the rise and the number of OWBs has increased to the point that 

they constitute a significant compliance issue. While the nuisance smoke and odor 

problem associated with OWBs is well documented, limited emission testing 

demonstrates that OWBs are also large generators of fine particulate matter. Exposure to 

particles can lead to a variety of adverse respiratory and cardiac health effects, especially 

among people with heart or lung diseases who comprise a substantial percentage of the 

population. Moreover, testing for Hazardous Air Pollutants may demonstrate that OWBs 

pose an additional threat to human health, suggesting a need to regulate these sources 

under an area source MACT. 

For a number of reasons, OWBs present problems not normally observed with indoor 

wood stoves, wood furnaces, or fireplaces. An OWB has a very short stack which emits 
Mr. Jeffrey R. Holmstead Page 2 

Outdoor Wood Boilers July 29, 2004

smoke near the ground and allows for little dispersion. The majority of units are 

designed to provide long burn times and be loaded once a day, or less frequently. The 

large fuel capacities and automatic damper controls, typically combined with primitive 

combustion design, frequently result in poor combustion, heavy smoke, noxious odors, 

and high concentrations of fine particulate and other air pollutants associated with low 

temperature combustion of wood fuel. Because they are also used to provide hot water 

for heating swimming pools, many OWBs are operated during summer months, when 

windows in nearby residences are open.

Emission testing performed under the direction of the U.S. Environmental Protection 

Agency in 1997 found particulate emissions from two common OWBs ranged from 1.5 – 

3.1 pounds/mmBTU heat input. These tests were conducted on new units operating 

under controlled conditions, at a fraction of rated capacity, while burning seasoned 

firewood. In practical application, particulate emissions are likely to be much higher. 

However, OWBs were exempted from compliance with the national “Standards of 

Performance for New Residential Wood Heaters” (40 CFR Part 60, Subpart AAA) and 

are currently regulated by only a few state and local governments. Therefore, we feel 

that it is critical that EPA take immediate action to regulate these sources. 

Thank you for your consideration of this timely and important concern. 

Sincerely, 

Kenneth A. Colburn 

Executive Director 

cc: John DuPree, EPA Headquarters 

Fred Weeks, EPA Region 1 

Denny Dart, EPA Region 1 

Gil Wood, EPA OAQPS 

Vinson Hellwig, Michigan DEQ 
The Capitol, Albany, NY 12224 q (518) 474-8096 q Fax (518) 473-2534

STATE OF NEW YORK

OFFICE OF THE ATTORNEY GENERAL

ELIOT SPITZER DIVISION OF PUBLIC ADVOCACY

Attorney General ENVIRONMENTAL PROTECTION BUREAU

August 11, 2005 

VIA OVERNIGHT MAIL

The Honorable Stephen L. Johnson

Administrator, Environmental Protection Agency

Ariel Rios Building

1200 Pennsylvania Avenue, N.W.

Washington, DC 20460

Re: Petition for rulemaking under 42 U.S.C. § 7411(b)(1) 

Regarding Outdoor Wood Boilers

Dear Administrator Johnson:

The States of New York, Connecticut, Maryland, Massachusetts, Michigan, New Jersey

and Vermont, and the Northeast States for Coordinated Air Use Management (NESCAUM)

hereby petition the U.S. Environmental Protection Agency (EPA) to use its authority under

section 111(b)(1) of the Clean Air Act (the “Act”), 42 U.S.C. § 7411(b)(1), to list outdoor wood

boilers (OWBs) as a category of stationary sources under section 111(b)(1)(A) and to

promulgate standards of performance for OWBs under 42 U.S.C. § 7411(b)(1)(B). In the

alternative, after listing OWBs as a category of stationary sources under section 111(b)(1)(A),

EPA could revise the existing standards for residential wood heaters, at 40 CFR §§ 60.530-

60.539b, to include standards for OWBs.

As explained in the attached report of the New York Attorney General’s Office,

Environmental Protection Bureau, entitled, Smoke Gets in Your Lungs: Outdoor Wood Boilers in

New York State (the “New York Report”), OWBs are becoming increasingly common in rural

and suburban towns and villages throughout much of the nation. Emissions of fine particulate

matter (particulate matter with a diameter less than 2.5 microns [PM 2.5]) and toxic materials

from OWBs exceed those from indoor wood stoves (called wood heaters by EPA), both on a perdevice basis and in proportion to the energy created. Despite polluting at a significantly higher

rate than residential wood heaters, OWBs are exempt from the standard applicable to residential

wood heaters and are not required to meet any testing, performance, or emission standards. 
Petition to EPA by NY, CT, MD, MA, MI, NJ, VT and NESCAUM

August 11, 2005

Notable findings of the New York Report include:

• While advertised as a clean economical way to heat one’s house and water, OWBs may

be among the dirtiest and least economical modes of residential heating, especially when

improperly used;

• Even when used properly, OWBs emit, on an average per hour basis, about 4 times as

much PM 2.5 as conventional wood stoves, about 12 times as much PM 2.5 as EPAcertified wood stoves, 1000 times more PM 2.5 than oil furnaces, and 1800 times more

PM 2.5 than gas furnaces;

• When OWBs are used improperly to burn wet or treated wood, scrap, or garbage, they

generate even more smoke and emit additional toxic chemicals;

• The pollutants emitted by OWBs can cause or contribute to short-term health harms such

as eye, nose, throat, and lung irritation, coughing and shortness of breath, and long-term

health effects such as asthma, heart and lung disease, and cancer;

• The generally short chimneys and reduced draft of OWBs fail to disperse emissions

adequately and can cause smoky conditions at or near ground level;

• OWBs are generally more expensive to install than comparable heating sources using oil,

or gas, or indoor wood stoves, and may be more expensive to operate depending on the

availability and price of dry seasoned wood;

• OWBs do not currently have to meet federal or state performance emission standards; 

• The absence of any federal regulations has led to various state and local efforts to

regulate OWBs. 

Since the problems associated with OWBs are widespread and exist across much of the

northern U.S., it is sensible for the federal government to enact federal standards of 

performance, as it has with respect to indoor wood heaters, so as to avoid the development of a

patchwork of state and local regulations.

Section 111(b)(1)(A) requires EPA to include in the listing of categories of stationary

sources under section 111 a category that “causes, or contributes significantly to, air pollution

which may reasonably be anticipated to endanger public health and welfare.” The findings in the

New York Report establish that OWBs should be listed. Accordingly, the EPA should

promulgate regulations for OWBs under section 111(b)(1)(B), establishing standards of

performance that reflect the degree of emission limitation achievable through the best system of

emission reduction that has been adequately demonstrated. 42 U.S.C. § 7411(a)(1). Consistent

with the general framework of the Act, such federal regulations should serve as a “floor,”

allowing states or municipalities to enact more stringent regulations as necessary to combat

particularized local air quality problems. 
Petition to EPA by NY, CT, MD, MA, MI, NJ, VT and NESCAUM

August 11, 2005

The time has come for EPA to regulate emissions from OWBs in order to protect public

health and the environment. Therefore, please consider this letter to be a formal request pursuant

to the Administrative Procedure Act, 5 U.S.C. § 553(e), for a rulemaking to list OWBs as a

category of stationary sources and to establish standards for emissions from new OWBs. 

Sincerely,

ELIOT SPITZER

Attorney General

State of New York

On behalf of:

RICHARD BLUMENTHAL

Attorney General

State of Connecticut

THOMAS F. REILLY

Attorney General

 Commonwealth of Massachusetts

STEVEN E. CHESTER

Director

Michigan Department of Environmental Quality

JOHN J. FARMER, JR.

Attorney General

State of New Jersey

WILLIAM H. SORRELL

Attorney General

State of Vermont

ARTHUR N. MARIN

Executive Director

Northeast States for Coordinated Air Use

Management (NESCAUM)









Assessment of Outdoor Wood-fired Boilers Page B-1

Appendix B: Outdoor Wood-fired Boiler 

Manufacturers 
Assessment of Outdoor Wood-fired Boilers Page B-2

Alternate Heating Systems, Inc 

2395 Little Egypt Road 

Harrisonville, PA 17228 

http://www.alternateheatingsystems.com/

Aqua-Therm LLC 

48301 State Hwy 55 

Brooten, MN 56316 

www.aqua-therm.com

Alpha American 

10 Industrial Blvd 

Palisade, MN 56469 

http://www.yukon-eagle.com

Central Boiler, Inc. 

20502 160th Street 

Greenbush, MN 56726 

www.centralboiler.com

Charmaster Products, Inc. 

2307 Highway 2 West 

Grand Rapids, MN 55744 

www.charmaster.com

Dectra Corporation 

3425 33rd Ave NE 

St Anthony, MN 55418 

www.dectra.net/garn

Freedom Outdoor Furnace 

7958 Curwensville Tyrone Hwy 

Olanta, PA 16863 

www.freedomoutdoorfurnace.com

Global Hydronics 

Box 717 

Winkler, Manitoba, CANADA R6W 4A1 

www.globalhydronics.com

Hardy Manufacturing 

12345 Road 505 

Philadelphia, MS 39350 

www.hardyheater.com

Heatmor Inc. 

105 Industrial Park Court NE 

Warroad, MN 56763 

www.heatmor.com

Heat Innovations 

499 Manitoba Road 

PO Box 989 

Winkler, Manitoba, R6W 4B1 

CANADA 

Heatsource1 

2201 Ridgeview Drive 

Beatrice, NE 68310 

www.heatsource1.com

Hicks Waterstoves and Solar Systems 

2541 South Main Street 

Mount Airy, NC 27030 

Horstmann Industries, Inc. 

301 Second Street 

Elroy, WI 53929 

www.royalfurnace.com

Innotech Developments 

2015 James Street South 

Thunder Bay, Ontario P7J1G6 

CANADA 

www.outdoorfurnaces.com

Johnson Manufacturing 

PO Box 345, 8187 State Rte 12 

Barneveld, NY 13304 

www.hud-son.com/woodfurnaces.htm
Assessment of Outdoor Wood-fired Boilers Page B-3

Mahoning Outdoor Furnace 

RD #1 Box 250 

Mahaffey, PA 15754 

www.shol.com/mahoning

Noonan’s Welding and Heating 

105 1st Street South 

Keewatin, MN 55753 

www.northlandoutdoorwoodfurnace.com

Northwest Manufacturing 

600 Polk Ave SW 

Red Lake Falls, MN 56750 

www.woodmaster.com

Outside Heating Systems – Wood Doctor 

Box 567 

Stewiacke, Nova Scotia B0N2J0 

CANADA 

www.wooddoctorfurnace.com

Pacific Western 

Box 267 

Atikokan, Ontario P0T 1C0 CANADA 

Pro-Fab Industries Inc./Empyre/Cozeburn 

Box 112 

Arborg, Manitoba R0C 0A0 CANADA 

http://www.burnsbest.com or 

www.profab.org

SFC Industries 

2219 County Highway G 

Rhinelander, WI 54501 

TARM USA, Inc. 

Main Street Box 285 

Lyme, NH 03768 

www.woodboilers.com

Taylor Manufacturing, Inc. 

1585 US HWY 701 South 

Elizabethtown, NC 28337 

www.taylormfg.com

Timber Ridge, Inc. 

2020 Highway 11-E 

Jonesborough, TN 37659 

www.freeheatmachine.com

Turbo Burn, Inc. 

4225 E Joseph 

Spokane, WA 99217 

www.turboburn.net
Assessment of Outdoor Wood-fired Boilers Page C-1

Appendix C: Estimated National Sales of OWBs 
Assessment of Outdoor Wood-fired Boilers Page C-2

State Estimated number of OWBs 

sold since 1990 

Alabama 74 

Alaska 185 

Arizona 24 

Arkansas 574 

California 182 

Colorado 271 

Connecticut 1,538 

Delaware 77 

District of Columbia 13 

Florida 32 

Georgia 50 

Hawaii 0 

Idaho 401 

Illinois 4.798 

Indiana 7,518 

Iowa 2,762 

Kansas 515 

Kentucky 1,148 

Louisiana 3 

Maine 1,968 

Maryland 872 

Massachusetts 1,308 

Michigan 29,568 

Minnesota 13,936 

Mississippi 0 

Missouri 4041 

Montana 350 

Nebraska 190 

Nevada 1 

New Hampshire 1,981 

New Jersey 215 

New Mexico 12 

New York 13,182 

North Carolina 2,561 

North Dakota 87 

Ohio 13,605 

Oklahoma 76 

Oregon 555 

Pennsylvania 11,836 

Rhode Island 206 

South Carolina 124 

South Dakota 40 

Tennessee 573 

Texas 16 

Utah 38 

Vermont 2.033 

Virginia 4.658 

Washington 393 

West Virginia 3,725 

Wisconsin 27,452 

Wyoming 66 

US Total 155,834 
Assessment of Outdoor Wood-fired Boilers Page D-1

Appendix D: Michigan DEQ Modeling 
Residential Wood Boiler Study

MAXIMUM PREDICTED 1-HR AVG CONCENTRATIONS 

Four contour plots of the max 1-hr concentration at an emission rate of 1lb/hr. Since its 1 stack, the impacts can be scaled 

(linear relationship) to a different emission rate. Modeling is based on the following: 

stack height = 8' 

temperature = 250F 

vel=1.5 m/s 

diameter = 6" 

50' x 100' bldg 22' high 

Ran 25', 50', 75', and 100' set back distances from the bldg due north using 1983 KIS met data. 



Assessment of Outdoor Wood-fired Boilers Page E-1

Appendix E: Field Report on Moisture Readings 

and Opacity 
Assessment of Outdoor Wood-fired Boilers Page E-2

Vermont Department of Environmental Conservation 

Air Pollution Control Division 

AIR POLLUTION TRIP REPORT 

PROPERTY: Confidential DATES OF SITE VISIT: June 21 & 22, 2005 

PURPOSE OF VISIT: To observe/assist with testing of an outdoor wood boiler. 

LOCATION OF SITE: Vermont 

PEOPLE PRESENT: Lisa Rector, NESCAUM 

 George Allen, NESCAUM 

 John Jasko, Air Quality Technical Services 

 Chris Jones, Vermont DEC (6/21/05) 

 Philip Etter, Vermont DEC 

WEATHER: Temperature: Highs generally 80s°F 

 Wind: 0-15 mph, Direction variable 

 Cloud Cover: Clear to Partly Cloudy 

 Precipitation: None 

 

INVESTIGATOR: Philip L. Etter, Environmental Analyst 

We attended the emissions testing of an outdoor wood boiler (OWB) to assist with the 

testing and observe the operation of the OWB unit. While on site I made some 

measurements of the moisture content of the wood fuel being used during the test using a 

Delmhorst moisture meter and conducted some visible emissions observations for general 

informational purposes. I also took numerous digital photos during the testing on 

6/22/05. 

Wood Moisture Measurements 

All wood moisture measurements were done on 6/22/05. The wood supply 

consisted of mixed hardwoods but mainly maple and red oak in two to three foot lengths. 

Most of the wood had been split, but some of the wood was in the round. The wood was 

uncovered and stacked in a semi sunny location. I checked the moisture content of three 

pieces of maple and two of red oak. One of the maple pieces was round but all the others 

had been split. I tried to choose pieces of larger size from various places in the small pile 

that had been designated as the fuel for the test. Using wedges and a sledge hammer, I 
Assessment of Outdoor Wood-fired Boilers Page E-3

split each piece roughly in the center just before taking measurements. I then took three 

measurements on the inside split surface, one in the center and one on either side of 

center roughly halfway from the center to the ends of the piece. The upper moisture 

content limit of the meter is 40%. Moisture readings above this limit were recorded as 

greater than 40%, but were treated as 40% when calculating averages. Readings are 

documented in the following table. 

PERCENT MOISTURE IN WOOD FUEL 

Species Rough Size 

(inches) 

Reading 1 

% 

Reading 2 

% 

Reading 3 

% 

Average 

%M 

Maple 5” split 26.8 29.3 24.3 26.8 

Maple 5” round 28.5 28.5 25.6 27.5 

Maple 7” x 4” split 30.5 27.7 32.6 30.3 

Red Oak 5” split >40 37.5 >40 >39.2 

Red Oak 8” x 3” split >40 >40 >40 >40 

Note that the moisture content measurements were done for general information 

only. No attempt was made to weigh the fuel charged to the unit or to count the chunks 

of various species of wood being charged. Some of the wood charged on the second day 

was taken from another part of the pile that had been recently rained on. Overall, the 

moisture content appeared to be fairly high. Oak generally takes longer to dry than many 

other hardwood species but the history of particular pieces of wood is unknown so it may 

be that the oak was cut later than the maple. 

Visible Emissions 

In an attempt to determine how the visible emissions vary over time and the burn 

mode cycling of the OWB, I evaluated visible emissions for 24 minutes on 6/21/05 and 

for nearly two hours on 6/22/05 (see attached VE evaluation forms). These observations 

were purely informational. Currently, OWBs are exempt from Vermont’s visible 

emissions standards due to their small size. The green foliage of the woods behind the 

OWB provided a good contrasting background for the light colored smoke. The sun was 

always well within appropriate angles for Method 9 observations. I ceased making 

observations on 6/22/05 at about 1140 hours as the sun’s vertical angle was becoming 

potentially inappropriate. During midday hours in late June, the vertical angle of the sun 

is inappropriate for observations in most situations. Observations were made difficult by 

the highly variable wind speed and direction, the plume often down washing, looping or 

blowing away from the observer making proper readings impossible. 

The nature of the plumes also made VE evaluation more difficult. Much of the 

time, especially during the open damper mode, the plume appeared to be largely 

condensed organics; with the densest part of the plume a few to several feet beyond the 

top of the stack. On 6/21 I tended to read the plume a couple feet above the stack before 

the densest smoke formed. On 6/22/05 I was reading the opacity in the densest part of 

the plume, a few to several feet above the stack. I did not notice any indications of a 
Assessment of Outdoor Wood-fired Boilers Page E-4

condensed water vapor component in the plume although I can’t rule-out with absolute 

certainty that some moisture condensation was occurring. Meteorological conditions 

would have caused any condensed water vapor in the plume to rapidly dissipate leaving a 

less dense trailing particulate plume. This was not apparent during the test burns as the 

dense plumes carried for a considerable distance, gradually dispersing. 

I also found evaluating the opacity during the idle mode to be difficult due to the 

very low gas flows emanating from the stack. Often the plume seemed to narrow beyond 

the top of the stack perhaps causing the plume to appear denser than if it were spread 

over the full width of the stack. The idle mode plumes appeared to be fully condensed as 

they exited the stack, which isn’t surprising given the very low stack gas temperatures 

recorded during the idle mode. 

The observations on the morning of 6/22/05 represent the worst case, as they were 

made during the beginning of the burn cycle. The OWB was loaded before the beginning 

of the test in the morning but not later. Although I didn’t conduct formal observations 

later in the day during the charcoal stage of the burn cycle, informal observations 

indicated that visible emissions were greatly reduced with opacities more in the 40-50% 

range during the charcoal stage operating modes rather than the 90-100% opacities near 

the beginning of the burn cycle. The observations done on 6/21/05 were later in the day 

but I am uncertain how long it had been since the OWB was charged. 

Summary 

These visible emissions observations indicate that the smoke from this OWB was 

densest during the first hours of the burn cycle and less dense during the latter charcoal 

phase burning. The opacities and total volume of visible smoke (i.e. the overall size or 

volume of the plumes) were also heaviest following the opening of the damper. 

Opacities and total smoke volume were much reduced during the idle mode and were 

greatly reduced as the duration of the idle mode increased probably due to the cooling of 

the fire greatly reducing combustion gas production. Opacities also seemed to decrease 

somewhat as the stack temperatures increased with the length of the open damper 

operating mode. 



Assessment of Outdoor Wood-fired Boilers Page F-1

Appendix F: Outdoor Wood-fired Boiler 

Regulations 
PROPOSED RULE (ver.7/28/2005) Page 1

AGENCY OF NATURAL RESOURCES 

Waterbury, Vermont 

ENVIRONMENTAL PROTECTION REGULATIONS 

CHAPTER 5 

AIR POLLUTION CONTROL 

Subchapter II. Prohibitions 

5-204 SITING AND STACK HEIGHT STANDARDS FOR OUTDOOR WATERSTOVES WOOD FIRED 

BOILERS; NOTIFICATION TO PURCHASERS

(a) Applicability. 

(1)This section shall apply to all outdoor waterstoves installed after

October 1, 1997 except outdoor waterstoves with weighted average

particulate matter emissions of less than 4.1 grams per hour.

Compliance with this standard shall be determined in accordance with

federal test method 28 for establishing test conditions and weighted

emission values and federal test methods 5G or 5H for determining

particulate emission concentrations (40 CFR Part 60, Appendix A), or

other methods approved by the Air Pollution Control Officer.

This section shall apply to each outdoor wood-fired boiler installed 

after October 1, 1997, except those outdoor wood-fired boilers that are 

subject to and compliant with Section 5-205 herein.

(b) Definitions. For the purpose of this section and Section 5-205 herein,

the following definitions apply, in addition to those of Section 5-101 

of this chapter. 

“Outdoor Waterstove” means any individual hand-fed furnace designed to

burn wood and used for the purpose of heating water where the furnace 

is located outside the structure into which the hot water produced 

thereby is piped.

“Outdoor Wood-Fired Boiler” means a fuel burning device designed: (1) 

to burn primarily wood by hand-firing; (2) not to be located inside 

structures ordinarily occupied by humans; and, (3) to heat spaces or 

water by the distribution through pipes of a fluid heated in the 

device, typically water. Examples of common uses of outdoor wood-fired 

boilers include: residential or commercial space heating, heating of 

domestic hot water, and heating of water for swimming pools, hot tubs 

or whirlpool baths.

(c) Prohibition.

No person shall install or allow the installation of an outdoor

waterstove wood-fired boiler subject to the requirements of this 

section unless the outdoor waterstove wood-fired boiler:
PROPOSED RULE (ver.7/28/2005) Page 2

(1) Is located more than 200 feet from any residence other than a 

residence served by the outdoor waterstove wood-fired boiler or 

owned by the owner or lessee of the outdoor waterstove wood-fired

boiler;

(2) Has an attached permanent stack extending higher than the roof

line peak of the roof of the structure(s) being served by the 

outdoor waterstove wood-fired boiler, if any residence is located 

more than 200 but less than 500 feet from the outdoor waterstove

wood-fired boiler other than a residence owned by the owner or

lessee of such outdoor waterstove wood-fired boiler; and, 

(3) Complies with all applicable laws, including but not limited to 

local ordinances, but excluding Section 5-205 of this chapter,

and its operation does not create a public nuisance. 

(d) Notice to Buyers.

(1) No outdoor waterstove wood-fired boiler subject to the 

requirements of this section shall be sold or offered for retail 

sale or lease within the State unless prior to any sales or lease 

agreement, the seller or dealer provides the prospective buyer or 

lessee with written notice stating that: 

(i) Only untreated natural wood may be burned in an outdoor 

waterstove wood-fired boiler;

(ii) Installation of the outdoor waterstove wood-fired boiler is 

subject to the distance and stack height requirements 

provided in this section. [Each notice shall expressly 

disclose each such requirement]; 

(iii) Use of an outdoor waterstove wood-fired boiler that meets 

the distance and stack height requirements provided in this 

section is not appropriate in some areas due to terrain 

that could render the operation of an outdoor waterstove

wood-fired boiler to be a nuisance or a public health 

hazard.

(2) The written notice specified above shall be signed by the 

prospective buyer or lessee to indicate receipt of notification 

of the requirements of this section. Prior to making delivery of 

an outdoor waterstove wood-fired boiler into the possession of 

any buyer or lessee, the seller or dealer shall mail or otherwise 

provide a copy of the signed notice specified above to the: 

Air Pollution Control Division 

103 South Main Street 

Building 3 South 

Waterbury, Vermont 05671-0402. 

Said notice shall contain the name, address and telephone number 

of both the seller or dealer and the buyer or lessee, the 

location where the outdoor waterstove wood-fired boiler will be 

installed and the make and model of the outdoor waterstove woodfired boiler.
PROPOSED RULE (ver.7/28/2005) Page 3

5-205 CONTROL OF PARTICULATE MATTER FROM NEW OUTDOOR WOOD-FIRED BOILERS

(a) Applicability

This section shall apply to any outdoor wood-fired boiler that is 

distributed or sold in Vermont or for installation in Vermont on or 

after January 1, 2006, except that this section does not apply to any 

outdoor wood-fired boiler that: (1) is or has been owned by an 

individual for his or her own personal use and is distributed or sold 

to another for his or her own personal use; or (2) was purchased and 

received by any person other than the manufacturer before January 1, 

2006.

(b) Definitions. For the purposes of this section, the following 

definitions apply, in addition to those of Sections 5-101 and 5-204 of 

this chapter.

“Distribute or Sell” means to distribute, sell, advertise for sale, 

offer for sale, hold for sale, ship, deliver for shipment, release for 

shipment, or receive and (having so received) deliver or offer to 

deliver. This term also includes conditional sales and long-term 

leases. This term does not include the distribution or sale by a 

manufacturer of an outdoor wood-fired boiler that is installed outside 

Vermont.

“Manufacturer” means any person who constructs or imports an outdoor 

wood-fired boiler.

“Model line” means all outdoor wood-fired boilers offered for 

distribution or sale by a single manufacturer that, in the judgment of 

the Air Pollution Control Officer, are similar in all material 

respects.

(c) Standard for Particulate Matter; Certification

(1) No person shall distribute or sell an outdoor wood-fired boiler 

in Vermont or for installation in Vermont unless the Air 

Pollution Control Officer has issued a certification to the 

manufacturer that the boiler, or the boiler model line to which 

it belongs, complies with the following particulate matter 

emission limit: An outdoor wood-fired boiler shall not emit, or 

cause or allow to be emitted, any gases that contain particulate 

matter in excess of 0.20 grains per dry standard cubic foot of 

exhaust gas corrected to 12% CO2, as determined in accordance with 

the test methods and procedures in subsection (d) of this 

section.

(2) Unless revoked sooner by the Air Pollution Control Officer, a 

certification issued under this subsection shall be valid for 

five years from the date of issuance.

(3) The distribution or sale of each outdoor wood-fired boiler 

subject to this section that has not been certified by the Air 

Pollution Control Officer as meeting the particulate matter 

emission limit in this subsection shall constitute a separate 

violation and be subject to civil or criminal penalties as 

provided in 10 V.S.A. Chapters 201 and 211, or 10 V.S.A. §568.
PROPOSED RULE (ver.7/28/2005) Page 4

(d) Emission Test Methods and Procedures

(1) In order to obtain certification of an outdoor wood-fired boiler 

under subsection (c) of this section, the manufacturer of any 

such boiler shall have emission test(s) conducted to determine 

compliance with the particulate matter emission limit under 

subsection (c) of this section and furnish the Air Pollution 

Control Officer a written report of the results of such tests, 

including a detailed description of the operating conditions of 

the boiler during the tests. Said written report shall contain 

such documentation and other information and follow such format 

as may be specified by the Air Pollution Control Officer. In the 

discretion of the Air Pollution Control Officer, a manufacturer 

of an outdoor wood-fired boiler subject to this section may have 

emission testing conducted of a representative boiler within a 

model line of outdoor wood-fired boilers and may use those tests 

to demonstrate compliance of all units manufactured in that model 

line.

(2) All emission testing required under this section shall be 

conducted by independent testing consultants who have no conflict 

of interest and receive no financial benefit from the outcome of 

the testing. Manufacturers of outdoor wood-fired boilers shall 

not involve themselves in the conduct of any emission testing 

under this section nor in the operation of the unit being tested, 

once actual sampling has begun.

(3) Emission tests shall be conducted and data reduced in accordance 

with 40 CFR Part 60, Appendix A, Test Methods 1 through 5, and 40 

CFR Part 51, Appendix M, Test Method 202, or alternative methods 

approved by the Air Pollution Control Officer. All tests shall 

be conducted in accordance with Vermont’s “Source Emission 

Testing Guidelines” (January 2002 revision, as amended) and under 

a test protocol which has received the prior approval of the Air 

Pollution Control Officer. Emission tests shall be conducted 

under such conditions as the Air Pollution Control Officer shall 

specify, based on representative performance of the outdoor woodfired boiler under actual field operating conditions.

(4) The manufacturer of the outdoor wood-fired boiler shall provide 

the Air Pollution Control Officer with at least 30 days prior 

notice of any emission test to afford the Air Pollution Control 

Officer the opportunity to have an observer present. The 

manufacturer of an outdoor wood-fired boiler(s) being tested as 

required by this section shall reimburse the state of Vermont for 

the reasonable expenses incurred by any such Agency observer for 

out-of-state travel to observe such testing, including among 

other items the costs of transportation, lodging and meals.

(e) Notification by Manufacturers 

(1) By March 1st of each year and as necessary when an outdoor woodfired boiler is certified, each outdoor wood-fired boiler 

manufacturer shall provide the following information in writing 

to any person to whom the manufacturer has distributed or sold, 

intends to distribute or sell, or actually distributes or sells 
PROPOSED RULE (ver.7/28/2005) Page 5

outdoor wood-fired boilers in Vermont or for installation in 

Vermont:

(i) A list of all the models of outdoor wood-fired boilers it 

manufactures; and

(ii) An identification of which, if any, of said models or 

boilers has received a certification of compliance under 

subsection (c) of this section and thus may be distributed 

or sold in Vermont or for installation in Vermont. 

(2) By March 15th of each year, a copy of all written information 

provided to comply with paragraph (1) of this subsection and a 

list of persons to whom it was provided shall be submitted to the 

Air Pollution Control Officer.