2009 March 23: IA Linn County: Overview of Outdoor Wood Boilers Draft Ordinance

Overview of Outdoor Wood Boilers Draft Ordinance

2009 March 23: Linn County regulations of outdoor wood boilers (modeling)


Linn County Regulation of Outdoor Wood Boilers

March 12, 2009
Updated March 23

*** Quick Links ***
Overview of Draft Ordinance
Current Regulatory Framework in Linn County
EPA’s Hydronic Heater Program
Local Comparative Data
Assessing Health Impacts
Additional Information

Linn County Public Health is drafting a proposal to amend the Linn County Code of Ordinance, Chapter 10 – Providing for Air Quality addressing the siting and operation of Outdoor Hydronic Heaters, a.k.a. Outdoor Wood Boilers (OWB) or Outdoor Wood Furnaces (OWF). This draft ordinance has been developed in response to the growing use of Outdoor Wood Boilers and the increasing concern to public health that emissions from these units create.
EPA and the OWB industry have collaborated to form a voluntary certification program.  The proposed Linn County Ordinance relies upon this certification program to determine setback distances from the property line that are necessary to protect air quality and public health . Typical residential outdoor wood boilers are rated at 350,000 Btu/hr heat input or less.  Under the current proposal, an uncertified 350,000 Btu/hr unit with a 20 foot stack would require an 850 foot setback and a 125,000 Btu/hr unit would require a 275 foot setback.   A Phase 1 certified unit would only require a 425 foot and 200 foot setback for respectively sized units.  A Phase 2 certified unit would only require a 100 foot setback for either unit.

Overview of Draft Ordinance
Based on the LCPH Air Quality Division analysis of OWB emissions, review of other state regulations on OWBs, EPA’s voluntary OWB certification program, citizen input, vendor input, and Board of Health direction, the following elements of the draft OWB ordinance are proposed.

  • Public health is to be maintained through protection of air quality and maintenance of National Ambient Air Quality Standards (NAAQS).
  • OWBs shall be sited at a minimum distance from the property line based on the size of the unit, manufacturers certified emission rate, and the height of the stack.
  • New units will be required to meet the setback requirements upon installation.  Existing units will have a grace period (the Linn County Board of Health currently proposes 3 years) to meet the setback distance requirements.
  • Emission rates relied upon in the ordinance are those established under EPA’s Hydronic Heater Voluntary Certification Program.
  • A local certification may be applied for to LCPH in lieu of reliance on EPA’s Phase 1 and Phase 2 OWB certification rates.  A local certification application must be accompanied by emission test data (likely provided by the manufacturer).  LCPH will issue a case-by-case determination for set back distances of locally certified units.
  • OWB fuels are limited to wood and other bio-mass.  Fuels stored on site shall be covered or kept in an enclosed structure to prevent excessive moisture.
  • New and existing OWBs will require a registration permit, the fee for which will be set by the Board of Supervisors in accordance with current code.  New units will require a permit prior to installation; existing units must apply for a permit one year prior to the effective date for installed units.
  • Persons failing to comply with these requirements will be subject to the same fines and penalties currently in place for open burning.

Current Regulatory Framework in Linn County

Linn County Code of Ordinance (LCCO), Chapter 10, establishes regulations for ambient air quality.  These regulations must be as stringent as State and Federal regulations.  OWBs are currently exempt from local industrial air quality permitting.  LCCO Chapter 10 Section 8, Emissions from Fuel-Burning Equipment establishes a performance standard that states:

  • This section applies to installations in which fuel is burned for the primary purpose of producing steam, hot water, hot air or other liquids, gases or solids and in the course of doing so, the products of combustion do not come into direct contact with process materials.
  • Emission Limitation: No person shall cause or permit the emission of particulate matter caused by combustion of fuel in fuel-burning equipment, from any stack or chimney in excess of the quantities set forth…For heat inputs less than 10 million BTU, 0.6 lb/million BTU shall apply.

These rules were developed shortly after the passage of the Clean Air Act in the 1970s.  Historically, Iowa DNR has chosen not to enforce these rules on residential units including OWBs.  However, citizens retain the right under the Clean Air Act to seek an enforcement action.

EPA’s Hydronic Heaters Program


The hydronic heaters program encourages manufacturers to improve air quality through developing and distributing cleaner, more efficient hydronic heaters. EPA has chosen to develop this voluntary program with the belief that such an approach will achieve emission reductions and protect public health sooner than a federal rule.


  • An EPA Phase 1 Certified Unit has been tested to demonstrate that it produces no more than 0.6 pounds of fine particulate matter for every million BTUs of wood fuel that is fired in the unit.
  • An EPA Phase 2 Certified Unit has been tested to demonstrate that it produces no more than 0.32 pounds of particulate matter for every million BTUs of wood fuel that is fired in the unit and emits no more than 18 grams of fine particulate matter an hour.
  • EPA states that an unqualified unit emits about 2.00 pounds of fine particulate matter per million BTUs.  A one million BTU/hr boiler would emit 2.00 lbs/hr.

EPA has prepared this FACT SHEET regardings Phase 2 qualified boilers and emissions from Phase 1, Phase 2, and uncertified outdoor wood boilers.

Local Comparative Data

The Air Quality Division of Linn County Public Health requires periodic testing of industrial coal boilers.  Results of some of recent tests on industrial boilers include:

  • ADM, 4 coal fired boilers, rated capacity of 551.5 mmBTU/hr, each; Tested emission rates varied from 5.63 to 10.64 lbs/hr.
  • Cargill, 1 coal fired boiler; 1 Boiler, rated capacity of 241 mmBTU/hr; Tested emission rate of 2.49 lbs/hr .

Assessing Health Impacts
The Air Quality Division reviews the potential health impacts for all industrial emission sources.  The Air Quality Division performs computer modeling to predict how pollutants disperse into the air.  This dispersion model allows us to estimate the concentrations of a pollutant in the ambient air resulting from a new installation.  This process is applied to all industrial sources in Linn County. The predicted concentrations from the installation are added to the pollutant concentration currently measured in the environment.  Added together, the predicted concentrations of the pollutant in the ambient air cannot exceed the National Ambient Air Quality Standards (NAAQS) established by EPA.  Linn County Public Health continuously monitors fine particulate matter.  Our historic levels have been a little more than 80% of the NAAQS.  Should Linn County exceed the NAAQS, restrictions on construction of new industrial sources and requirements to retrofit existing industrial source would be required under the Clean Air Act.  So, not only does the exceedance of the NAAQS endanger public health, it also has economic impacts to local business and industry.  
The setback distances proposed in this draft ordinance at set at the point where an OWB can operate without exceeding the National Ambient Air Quality Standards.


Although not originally intended for OWBs, current Linn County Code of Ordinance establishes a maximum emission rate for any boilers at 0.60 lb/mmbtu.  Based on EPA data, it is unlikely that all currently installed uncertified units would meet this regulatory standard.  A setback distance for these higher emitting units is appropriate to protect ambient air quality, public health, and industrial growth in Linn County.  The Air Quality Division continues to seek input from the public on this proposal.  After a draft is finalized and approved by the Board of Health, it will go before the Board of Supervisors for review, public comment, and vote on adoption.

Additional Information

Much of the analysis performed by Linn County Public Health is based on testing and analysis performed by the U.S. EPA and a consortium of Air Quality Agencies representing states in the northeast U.S. known as NESCAUM.  The following links will take you to their websites were information on testing, analysis, regulation, and voluntary programs are posted.

Linn County Public Health Air Quality Assessment of OWBs
US EPA Hydronic Heater Website
NESCAUM Outdoor Hydronic Heater Website


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