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Outdoor wood boilers

Outdoor wood boilers (OWBs) have become more popular in the United States, particularly in rural areas, as the prices for heating oil, propane and natural gas have increased. A 2006 report by Northeast States for Coordinated Air Use Management (NESCAUM) [exit DNR] ranked Wisconsin as the second-highest state for OWB sales since 1990. This page contains information on the basics of these furnaces, potential health impacts and how to handle and reduce complaints.



An outdoor wood boiler is any furnace, stove or boiler designed to burn wood, where the unit is not located within a building intended for habitation by humans or domestic animals. They are also known as water stoves and wood-fired hydronic heaters and typically look like small utility buildings with a smokestack. Outdoor wood boilers provide heating and/or hot water to a single residence.

How OWBs work

The basic design of an OWB 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 the reservoir. Heated water is carried through underground pipes to heat a home, farm building, swimming pool, hot tub or to produce domestic hot water. Most OWBs cost between $3,000-$10,000 installed.

The typical design of the OWB encourages a slow, cooler fire to maximize the amount of heat transferred from the fire to the water. Slow, cooler fires, however, burn inefficiently and create more smoke and creosote than higher temperature fires. The most efficient wood-burning furnaces burn at very high temperatures, include a heat store of several hundred gallons of water and have refractory tunnels where high-temperature secondary combustion can take place. These units are typically installed inside the home, have very low emissions to meet U.S. EPA standards, and have a stack height of 20-30 feet. Outdoor models are often missing these important features.


Potential health impacts

Outdoor wood boilers (OWBs) have the potential to aggravate or cause health problems. Wood smoke contains a variety of pollutants that can affect nearby residents. When DNR air quality notices warn of unhealthy levels for particulate matter, wood burning can contribute to air pollution over a wide area. Emissions from burning trash and other inappropriate materials in OWBs are typically of a greater health concern than burning clean wood.

Wood smoke

At the relatively low temperatures that OWBs operate, the stoves can produce thick smoke and creosote. This smoke may contain unhealthy levels of toxic air pollutants, including known carcinogens.


People exposed to wood smoke from OWBs may experience:

  • eye and nose irritation;
  • breathing difficulty;
  • wheezing;
  • coughing; and
  • headaches.

People with heart disease, asthma, emphysema or other respiratory diseases are especially sensitive. In particular, wood smoke can be harmful to the elderly, babies, children and pregnant women.

The chance a person will experience health effects as a result of exposure to smoke depends on the concentration of air pollutants they breathe and the duration of their exposure. Because most OWBs have very short stacks and are located close to homes, there is a greater potential for emissions to create a health hazard for those living near the unit, including neighbors. In areas where homes are not close together, and where smoke is not trapped by surrounding hills, the use of an OWB may not be a health hazard for neighbors.

Pollutants in wood smoke

Wood smoke contains a mixture of at least 100 different compounds in the form of gases and fine sooty particulate matter. Fine particles (PM2.5) are so small that they behave much like gases. They can penetrate homes, even when windows and doors are closed. Fine particles can lodge deep in the lungs and are not easily expelled.

Some of the major components of wood smoke are on the U.S. Environmental Protection Agency's (EPA's) list of criteria pollutants regulated by the National Ambient Air Quality Standards, including:

  • carbon monoxide;
  • nitrous oxides;
  • sulfur dioxide; and
  • particulate matter.

Ozone, another criteria pollutant, is not directly emitted but the precursor chemical components of it are in wood smoke.

The criteria pollutants were singled out by the EPA because of their negative impacts on human health, which include:

  • coughing and difficult or painful breathing;
  • increased susceptibility to respiratory illnesses like pneumonia and bronchitis;
  • eye and nose irritation;
  • hospitalization for heart or lung diseases; and
  • premature death.

Burning garbage and other inappropriate materials

Particleboard, treated, stained, painted or wet wood and trash should never be burned in OWBs. The burning of most solid waste materials, including treated wood, plastics, rubber and asphalt, is prohibited in Wisconsin. The combustion of treated wood, agricultural plastics and other garbage releases heavy metals and toxic chemicals such as polycyclic aromatic hydrocarbons [exit DNR] and dioxins. Exposure to dioxins is linked to skin problems, reproductive or developmental problems and increased risk of cancer. Trash burning [exit DNR] is especially harmful because it releases chemicals that are persistent in the environment, polluting our air, food, lakes and streams.

Complaints, ordinances and regulations

Complaints, ordinances and regulations

The DNR and Department of Health Services have limited ability to address outdoor wood boiler (OWB) related problems because residential OWBs are not regulated by the state.


Citizen complaints are handled on a case by case basis depending on the community where the OWB is located and the nature of the grievance.

As a first step, citizens should contact local government officials to see if there is an OWB ordinance for their area. Where there is no local ordinance, direct complaints about the burning of materials such as garbage, plastic and recyclables in OWBs to DNR. For health concerns due to smoke inhalation (for example, if a home is routinely overcome with smoke or if asthma is triggered by the use of an OWB), contact your local health department.

Government authorities may facilitate voluntary compliance by:

  • contacting the OWB operator about the complaint;
  • reviewing proper wood-burning practices with the operator; or
  • checking for proper stack height so that smoke will not impact neighbors.

In communities that have existing OWB ordinances, it is easier for officials to handle complaints because they can write enforcement letters by referring to the ordinance sections being violated.


By adopting ordinances, local authorities can proactively manage OWBs. The DNR has developed a guidance document and model ordinance for local communities interested in regulating the installation and use of OWBs, as well as outdoor burning and the burning of refuse.

Federal regulations

Effective May 15, 2015, new outdoor and indoor residential wood-fired boilers became subject to particle pollution limits under U.S. EPA's New Source Performance Standards (NSPS) for residential wood heaters. These standards regulate the manufacture, sale and operation of new wood heating devices. All devices manufactured after May 15, 2015, are required to meet the new pollution limits.

The standards do not affect devices already in use in homes. The EPA encourages homeowners to replace old wood heaters with newer, cleaner and more efficient heaters.

More information is available at EPA's Controlling Air Pollution from Residential Wood Heaters [exit DNR].


Recommendations for outdoor wood boiler operators

The state does not regulate outdoor wood boilers (OWBs). The suggestions below are based on information from a variety of sources including the Wisconsin Department of Health Services (DHS) and U.S. Environmental Protection Agency (EPA) and may help reduce pollution exposure complaints.

Before you buy an OWB

  • Check with your local government about any restrictions on the use of OWBs in your community. Some areas have ordinances that ban or limit their use.
  • Talk to your neighbors about your plans. This will reduce the likelihood of aggravating health problems or creating a nuisance.
  • Purchase a cleaner burning furnace by choosing one with the EPA's hang tags [exit DNR]. White hangtag furnaces are approximately 90% cleaner than unqualified units. Any wood- or pellet-burning stove that meets the EPA's 75% efficiency rating may qualify for a tax credit.
  • Ensure appropriate siting. Plan to place outdoor wood boilers at least 300 to 500 feet from the nearest building that is not on the same property as the unit. Use chimneys that are a minimum of 15 feet high, or preferably as high as the roofs of nearby buildings, to enable dispersion of the smoke.

Best burn practices

  • Only burn dry, untreated wood. Do not burn wet wood, treated wood or trash. Burning solid waste materials such as treated wood, plastic, rubber and asphalt is prohibited statewide. Burning treated wood and plastics releases toxic chemicals [exit DNR] such as heavy metals and dioxins that can cause a range of health issues including skin, reproductive and developmental problems and increase the risk of cancer.
  • Do not use lighter fluid, gasoline or other chemicals to start the fire. Their fumes contribute to the formation of ground-level ozone, an air pollutant that is particularly harmful for people with respiratory and heart diseases. Instead, use clean, dry kindling to start the fire.
  • Follow the manufacturer’s written instructions for wood loading. This will help the furnace burn efficiently, reducing both wood usage and air emissions. Remember, even dry, untreated wood releases some pollutants when it burns.

More info

DNR resources

Additional information

Various organizations offer information about outdoor wood boilers, their potential health impacts and ways to handle and reduce complaints.

Wis. Department of Health Services (DHS)


Other states and organizations