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Air: Outdoor Wood Boilers

Garage with an outside wood burner and stack of logs

Outdoor wood boilers (OWBs), or water stoves, are a cost-effective way to help take the chill out of the air in cooler months. Wood smoke from OWBs contributes to tiny particle air pollution, which can lead to health problems for people with heart and lung diseases. When they’re placed and operated correctly, OWBs can be a safer source of heat and hot water. Federal tax credits (30%) are available to offset the costs of certified OWBs purchased in 2023-2032.

Local health departments have the authority to address health concerns related to OWBs under Wis. Stat. ch. 254. On this page, learn more about recommended best practices for OWB use to reduce harmful smoke so everyone can breathe easier.

An OWB is a furnace, stove, or boiler designed to burn wood for heat. It's placed outside of the structure that’s being heated. Also known as water stoves, OWBs look like a small utility building with a smokestack.

OWBs provide heat and/or hot water to a home. Their design includes a firebox enclosed in a water jacket, surrounded by insulation, and vented through a chimney stack. The combustion of wood heats the water in a reservoir, then the heated water is carried through underground pipes to heat the home, outbuilding, pool, or hot tub—or to produce hot water.

OWBs are more popular in rural areas than urban and cost between $3,000 and $10,000. The Environmental Protection Agency (EPA) now requires people to purchase EPA-certified OWBs that are significantly cleaner, improving air quality and reducing health harms in communities where people burn wood for heat and hot water.

Illustration of how an OWB warms a house

Hydronic heater system cross section

OWB cross-section

Cross-section of a hydronic heater

Wood smoke contains a mix of at least 100 different harmful chemicals in the form of water, organic vapors, gases, and tiny particles. The chemicals in wood smoke can cause air quality and health problems. Four of the chemical compounds in wood smoke are on EPA’s list of six serious “criteria pollutants” in the National Ambient Air Quality Standards, including:

  • Carbon monoxide.
  • Nitrogen oxides.
  • Particulate matter.
  • Sulfur dioxide.

Wood smoke and creosote buildup from OWBs can also cause unhealthy levels of toxic air pollutants. Some of the pollutants are known to cause cancer, including:

  • Acrolein.
  • Benzene.
  • Formaldehyde.
  • Polycyclic aromatic hydrocarbons.

If you’re exposed to wood smoke, particulate matter can cause heart and lung problems. The tiny particles can lodge deep inside your lungs and even get into your bloodstream. Particulate matter is so tiny it behaves much like a gas with the ability to get into homes even when windows and doors are closed.

Health impacts and symptoms from wood smoke exposure include:

  • Aggravated angina.
  • Aggravated asthma, emphysema, bronchitis, and other lung diseases.
  • Cancers.
  • Coughing.
  • Chest pain and tightness.
  • Confusion.
  • Damaged lungs in children.
  • Decreased alertness.
  • Difficult or rapid breathing.
  • Eye, throat, and nose irritation.
  • Fatigue.
  • Headaches.
  • Impaired judgement.
  • Increased risk of heart attack, irregular heartbeat, heart failure, and stroke.
  • Increased risk of respiratory illnesses like pneumonia or bronchitis.
  • Nausea.
  • Premature death.

Who’s at risk?

Wood smoke exposure can affect everyone, but it bothers some people more than others. It can be particularly harmful to certain groups, including:

  • Babies, children, and teens.
  • Older adults.
  • Pregnant people.
  • People with heart or lung diseases.

People with heart and lung diseases are especially sensitive to wood smoke exposure, including those with:

  • Asthma.
  • COPD (chronic obstructive pulmonary disease).
  • Emphysema.
  • Heart disease.

People’s health risks from wood smoke exposure depend on the concentration of air pollutants and how long they’re exposed. Because most OWBs have short stacks and are located close to homes, people living near them, including neighbors, are at a greater risk of health harms.

Health risks from burning non-wood materials

Burning certain materials can release highly toxic chemicals. Never burn the following materials in OWBs:

  • Particleboard.
  • Wood that is treated, stained, painted, wet, or freshly cut.

Burning trash also is harmful because it releases chemicals that pollute the air, soil, food, lakes, and streams.

Burning plastic and treated wood also releases heavy metals and toxic chemicals such as dioxins. If you’re exposed to dioxins, they can cause skin problems, reproductive or developmental problems, and may even increase your risk of cancer.

Although the EPA has regulations for reducing pollution from residential stoves and fireplace inserts, there are currently no federal or state standards regulating the use of OWBs–other than EPA’s 2015 manufacturing requirements for cleaner OWBs. That’s why regulation at the local level is the best way to address public health concerns. Several Wisconsin communities have taken steps to create ordinances that ban or regulate the use of OWBs.

The EPA hosts a Burn Wise webpage with links to state and local agencies that work to reduce emissions from OWBs. The webpage also includes current regulations about OWB use.

In addition, the Wisconsin Department of Health Services (DHS) compiled a List of Local Ordinances Regulating Outdoor Wood Boilers and Residential Wood Smoke (PDF), though it’s not a complete list nor is it updated regularly. Instead, local communities should use it as a starting point when considering enacting their own ordinances.

Finally, the Wisconsin Department of Natural Resources (DNR) developed a guidance document and model ordinance for local communities interested in regulating OWBs and wood smoke to protect public health.

In some communities, the best approach to managing nuisance complaints and public health dangers is a local ordinance that restricts or bans the use of OWBs. Although OWBs are typically used in rural settings, an increasing number are being installed in subdivisions and small towns.

If your local government receives complaints about OWBs, consider the following best practices for managing their placement and use:

  • Ensure that OWBs are installed where they don’t create an air pollution health hazard. Local officials should carefully consider the influence that changes in land use can have on where OWBs are installed. This is especially important when agriculturally zoned land is changed to residential. This change frequently results in homes being built too close to OWBs.
  • Limit what can be burned in an OWB to clean, dry firewood.
  • Place OWBs at least 300 to 500 feet from the nearest building on neighboring properties.
  • Require OWB chimneys be 15 feet high, or higher than the roofs of nearby buildings.
  • Require annual permitting of OWBs by the local fire chief.

For more guidance, use the DNR’s guidance document and model ordinance for local communities interested in regulating OWBs and wood smoke.

For more information, contact:

DHS Division of Public Health
Bureau of Environmental and Occupational Health
PO Box 2659
Madison, WI 53701-2659

You also can use these helpful resources:

Five tips to safely operating OWBs

Follow these tips for keeping the air clean when using an outdoor wood boiler:

  • Use an EPA-certified, cleaner burning OWB.
  • Only burn dry (less than 20% moisture), untreated wood
  • Provide sufficient air to the fire–never let it smolder.
  • Close your windows and doors to keep smoke out of your home.
  • Make sure your chimney is at least 15 feet high, or higher than any nearby structures.
Outside wood burner installation diagram detailing chimney height

Source: University of Wisconsin-Madison

Last revised April 9, 2024