UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

REGION III

1650 Arch Street

Philadelphia, Pennsylvania 19103

DATE: May 26, 2011

SUBJECT: Technical Support Document – Commonwealth of Pennsylvania – Control of 

Emissions of Particular Matter from the Operation of Outdoor Wood-fired Boilers

/s/

FROM: Rose Quinto, Environmental Engineer

Office of Air Program Planning

TO: File

/s/

THRU: Cristina Fernandez, Associate Director

Office of Air Program Planning

A. BACKGROUND

On July 18, 1997 (62 FR 38652), the Environmental Protection Agency (EPA) amended the 

National Ambient Air Quality Standard (NAAQS) for particular matter (PM) to add a new 

standard for fine particles, using fine particulates equal to or less than 2.5 micrometers in 

diameter (PM2.5) as the indicator. EPA set the health-based (primary) and welfare-based 

(secondary) PM2.5 annual standard at a level of 15 micrograms per cubic meter (μg/m3) and the 

24-hour standard at a level of 65 μg/m3. The health-based primary standard is designed to 

protect human health from elevated levels of PM2.5, which have been linked to premature 

mortality and other health effects. The secondary standard is designed to protect against major 

environmental effects of PM2.5 such as visibility impairments, soiling, and materials damage. 

On October 17, 2006 (71 FR 61236), EPA revised the primary and secondary 24-hour NAAQS 

for PM2.5 to 35 μg/m3from 65 μg/m3.

A significant and growing source of PM2.5 emissions in the Commonwealth of Pennsylvania is 

from outdoor wood-fired boilers (OWBs). OWBs, also referred to as outdoor wood-fired 

furnaces, outdoor wood-burning appliances, or outdoor hydronic heaters, are free-standing fuel 

burning devices designed: (1) to burn clean wood or other approved solid fuels; (2) specifically

for outdoor installation or installation in structures not normally intended for habitation by

humans or domestic animals, such as garages; and (3) to heat building space or water by means 

of distribution, typically through pipes, of a fluid heated in the device, typically water, or a water 

and antifreeze mixture. They resemble a small shed or mini-barn with a short smokestack on top. 

OWBs are being sold to heat homes and buildings; produce domestic hot water; heat swimming 

pools or hot tubs; and provide heat to agricultural operations such as greenhouses and dairies.
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A concern associated with certain OWBs is the air pollution they may produce. Smoldering fires 

and short smokestacks may create heavy smoke to the ground that sometimes causes a 

neighborhood nuisance or an adverse impact on public health and the environment. Smoke from 

OWBs which form from incomplete combustion, contain emissions from fine particle pollution, 

carbon monoxide, and other organic products, such as formaldehyde, benzene and aromatic 

hydrocarbons, all of which can cause cancer. When inhaled, fine particles from smoke emissions 

are carried deep into the lungs and can impair lung function and aggregate existing medical 

conditions such as asthma, lung or heart disease. Individuals particularly sensitive to PM2.5 

exposure include older adults, people with lung and heart disease, and children.

Unlike indoor wood stoves that are regulated by EPA, OWBs are not required to meet a Federal 

emission standard, and the majority of them are not equipped with pollution controls. EPA 

initiated a voluntary program that encourages manufacturers of OWBs to improve air quality

through developing and distributing cleaner-burning, more efficient OWBs. Through this 

voluntary effort, OWBs are certified and labeled to meet EPA emissions performance levels in 

two phases. Phase 1 of the program was in place from January 2007 through October 15, 2008. 

To qualify for Phase 1, manufacturers were required to develop an OWB model that was 70% 

cleaner-burning than unqualified models by meeting the EPA air emission standard of 0.6 pound 

PM per million British thermal unit (Btu) heat input as tested by an independent accredited 

laboratory. Phase 1 OWB models are labeled with an orange tag. Phase 1 Partnership 

Agreements ended when Phase 2 Partnership Agreements were initiated on October 16, 2008. 

To qualify for Phase 2, manufacturers must develop an OWB model that is 90% cleaner-burning 

than the Phase 1 OWBs and meet the EPA air emissions standard of 0.32 pound PM per million 

Btu heat output. The Phase 2 OWB models, just like the Phase 1 OWB models are tested by an 

independent accredited laboratory. Phase 2 OWB models are labeled with a white tag. 

Additional information about the EPA voluntary OWB program is available on EPA’s Web site 

at www.epa.gov/burnwise. Furthermore, the Northeast States for Coordinated Air Use 

Management (NESCAUM), which is a regional air pollution control organization, comprised of 

the air program directors of all the New England states, New York and New Jersey, in 

coordination with a number of states and EPA, developed a model rule for regulating OWBs 

(also known as outdoor hydronic heaters (OHHs)). The model rule was released in January 2007 

and is available at www.nescaum.org/topics/outdoor-hydronic-heaters. The purpose of the model 

rule is to assist state and local agencies in adopting requirements that will reduce air pollution 

from OWBs. The model rule establishes emission limits and labeling requirements for new 

OWBs and contains the following components for both new and existing OWBs: setback 

requirements from property lines, structures, and homes; stack height requirements; and 

distributor and buyer notification requirements.

On October 20, 2010, the Pennsylvania Department of Environmental Protection (PADEP) 

submitted a State Implementation Plan (SIP) revision pertaining to the control of emissions of 

particular matter (PM) from the operation of OWBs. This SIP revision is based on the 

NESCAUM model rule. The emission standard established in this SIP revision is the Phase 2 

emission standard described in the EPA voluntary OWB program. 
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B. STATE SUBMITTAL

The SIP revision adds definitions and terms to Title 25 of the Pennsylvania Code (25 Pa. Code) 

Chapter 121.1, relating to definitions, used in the substantive provisions of this SIP revision. The 

SIP revision also adds a new regulation pertaining to the standards for contaminants on 

particulate matter emissions in 25 Pa. Code Chapter 123 (Standards of Contaminants), Section 

123.14 – Outdoor Wood-Fired Boilers. 

The following definitions and terms were added in 25 Pa. Code 121.1: 

Btu – British thermal unit – The amount of thermal energy necessary to raise the temperature of 1 

pound of pure liquid water by 1° Fahrenheit (F) at the temperature at which water has its greatest 

density (39°F).

Clean wood – The term includes the following: (1) wood that contains no paint, stains, or other 

types of coatings; and (2) wood that has not been treated with preservatives or chemicals, 

including copper, chromium arsenate, creosote and pentachlorophenol.

New Phase 2 outdoor wood-fired boiler – A Phase 2 outdoor wood-fired boiler that is installed 

on or after October 2, 2010.

Non- Phase 2 outdoor wood-fired boiler – An outdoor wood-fired boiler that has not been 

certified or qualified by EPA as meeting a particulate matter emission limit of 0.32 pounds per 

million Btu output or lower and is labeled accordingly.

Outdoor wood-fired boiler – A fuel-burning device that: (1) is designed to burn, or is capable of 

burning, clean wood or other fuels listed under allowed fuels in this SIP revision; (2) has a rated 

thermal output of less than 350,000 Btu per hour; (3) the manufacturer designs or specifies for 

outdoor installation or installation in structures not normally intended for habitation by humans 

or domestic animals, including garages and sheds; and (4) heats building space or fluid, or both, 

through distribution, typically through pipes, of a fluid heated in the device, typically water or a 

mixture of water and antifreeze. The fuel-burning device may also be known as an: outdoor 

wood-fired furnace; outdoor wood-burning appliance; and outdoor hydronic heater.

Phase 2 outdoor wood-fired boiler – An outdoor wood-fired boiler that has been certified or 

qualified by EPA as meeting a particular matter emission limit of 0.32 pounds per million Btu 

output or lower and is labeled accordingly.

Summary of the SIP Revision

The SIP revision adds a new regulation in 25 Pa. Code Chapter 123 (Standards for Contaminants, 

Particulate Matter Emissions), Section 123.14 – Outdoor Wood-Fired Boilers. 
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The requirements of this SIP revision, beginning October 2, 2010 (effective date of the 

regulation), applies to the following: 

(1) A person, manufacturer, supplier or distributor who sells, offers for sale, leases or distributes 

an OWB for use in this Commonwealth.

(2) A person who installs an OWB in this Commonwealth.

(3) A person who purchases, receives, leases, owns, uses or operates an OWB in this 

Commonwealth.

The SIP revision does not apply to a person, manufacturer, supplier, or distributor who sells, 

offers for sale, leases or distributes in this Commonwealth a non-Phase 2 OWB, if the person, 

manufacturer, supplier or distributor demonstrates the non-Phase 2 OWB is intended for 

shipment and use outside of this Commonwealth. The SIP revision also does not apply to a 

permanently installed OWB that was installed prior to October 2, 2010, and is transferred to a 

new owner as a result of a real estate transaction.

Another exemption in this SIP revision states that a person may not sell, offer for sale, distribute 

or lease a non-Phase 2 OWB in this Commonwealth, unless the OWB was manufactured, 

distributed, purchased or leased, and received in this Commonwealth before May 31, 2011. This 

exemption shall remain in effect until May 31, 2011. 

A non-Phase 2 OWB purchased during the sell-through period shall meet the following 

requirements: 

(1) The non-Phase 2 OWB shall be installed a minimum of 150 feet from the nearest property

line.

(2) Have a permanently attached stack that extends a minimum of 10 feet above the ground, and 

is installed according to the manufacturer’s specifications.

Furthermore, a person may not sell, offer for sale, distribute or install an OWB for use in this 

Commonwealth, unless it is a Phase 2 OWB. A person may also not purchase, lease or receive 

an OWB for use in this Commonwealth unless it is a Phase 2 OWB.

The requirements for a new Phase 2 OWB in this Commonwealth are the following:

(1) A setback requirement for a new Phase 2 OWB is that a person may not install a new Phase 2 

OWB unless the boiler is installed a minimum of 50 feet from the nearest property line. 

(2) The stack height requirement for a new Phase 2 OWB is that a person may not install, use or 
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operate a new Phase 2 OWB, unless the boiler has a permanently attached stack that extends a 

minimum of 10 feet above the ground and be installed according to the manufacturer’s 

specifications.

Furthermore, a person that owns, leases, uses, or operates an OWB in this Commonwealth shall 

use only one or more of the following fuels: 

(1) Clean wood.

(2) Wood pellets made from clean wood.

(3) Home heating oil, natural gas or propane that complies with all applicable sulfur limits and is 

used as a starter or supplemental fuel for dual-fired OWBs.

(4) Other types of fuel approved in writing by PADEP upon receipt of a written request. 

In addition, a person that owns, leases, uses, or operates an OWB in this Commonwealth shall 

not burn a fuel or material in that OWB other than those fuels listed above.

Lastly, a person shall not use or operate an OWB in this Commonwealth unless it complies with 

applicable Commonwealth, county and local laws. 

C. EPA EVALUATION

The emission standard established in the SIP revision is the Phase 2 emission standard described 

in the EPA voluntary program. This SIP revision is also based on the NESCUAM model rule 

that assisted PADEP in adopting requirements that will reduce air pollution from OWBs. The 

PADEP’s SIP revision establishes emission limits and labeling requirements for new OWBs and 

contains the following components for both new and existing OWBs: setback requirements from 

property lines, structures, and homes; stack height requirements; and distributor and buyer 

notification requirements.

The Commonwealth will benefit from reduced emissions of fine particulate matter (PM2.5) from 

OWBs by following the requirements of the Phase 2 emission standards in the EPA voluntary

program, and the NESCUAM model rule. Attaining and maintaining levels of PM2.5 below the 

health-based NAAQS will reduce premature mortality and other health effects associated with 

PM2.5 exposure. This SIP revision will reduce the problems associated with the operation of 

OWBs, including smoke, odors and burning prohibited fuels, including garbage, tires, and 

hazardous waste. Reductions in ambient levels of PM2.5 will promote improved visibility, 

decreased soiling and materials damage, and decreased damage to plants and trees.
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D. CONCLUSIONS AND RECOMMENDED AGENCY ACTION

The regulation to control PM2.5 emissions from OWBs adopted by the Commonwealth of 

Pennsylvania will result in the reduction of PM2.5 emissions from the operation of OWBs and

will help the Commonwealth in attaining compliance with PM2.5 NAAQS. EPA approval of the 

SIP revision is recommended.