Outdoor Wood Boilers (Water Stoves)
Printable version of this factsheet
(PDF, 94 KB)
for local government and public health officials on the use
of outdoor wood boilers (OWBs)
Public health hazards can result from excessive
smoke produced when outdoor wood boilers are improperly
placed and operated. Local health departments have the authority to
address public health hazards and health nuisances under Wisconsin
State Statute 254. This fact sheet summarizes current
Wisconsin Department of Health Services recommendations
for best-management practices of outdoor wood boilers.
What is an Outdoor Wood
Health Hazards Associated with Outdoor Wood Boilers
Public Health Laws and Ordinances
Local Governments Do About Outdoor Wood Boiler Complaints?
an Outdoor Wood Boiler?
An outdoor wood boiler (OWB)
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. OWBs, also known as water stoves, typically look
like a small utility building with a smoke stack. OWBs provide
heating and/or hot water to a single residence. The basic design of an outdoor wood boiler (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. OWBs are more popular in rural areas than in other areas. Most OWBs cost between $3,000-$10,000 installed.
The basic 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, and
have a stack height of 20-30 feet. Outdoor models often are missing these
Public Health Hazards Associated with
Health officials worldwide have only recently begun to understand the
health problems seen in people who regularly cook or work around wood
fires. Wood smoke contains a mixture of at least 100 different compounds
in the form of gases and fine sooty particulate matter (PM). Some of the
major components of wood smoke are on EPA’s list of six "criteria
pollutants" in the National Ambient Air Quality Standard (NAAQS),
including ozone, carbon monoxide, nitrous oxides, particulate matter, and
sulfur dioxide. The six criteria pollutants were singled out by the United
States Environmental Protection Agency (EPA) because of the negative
impacts of these pollutants on human health, which include coughing and
difficult or painful breathing, increased susceptibility to respiratory
illness like pneumonia and bronchitis, eye and nose irritation,
hospitalization for heart or lung diseases, and premature death.
Smoke associated with OWBs
At the relatively low temperatures at which
OWBs are designed to burn wood, the stoves can produce thick smoke and
creosote. This smoke can contain unhealthy levels of toxic air
pollutants and known carcinogens, including significant amounts of
particulate matter (PM) of various polycyclic aromatic compounds (PAHs). Exposure
to PM can trigger or aggravate respiratory and cardiovascular problems.
PM are so small that they behave much like gases -- they can penetrate
homes, even when windows and doors are closed.
PM can lodge deep
in the lungs of those exposed to wood smoke, and are not easily expelled.
Symptoms of people exposed to wood smoke from OWBs include 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, the use
of an OWB may not be a health hazard for neighbors.
Hazards associated with burning garbage or inappropriate materials
Burning particleboard, treated, stained, painted, wet or freshly cut
wood can release very toxic chemicals. These materials should never be
burned in OWBs. Trash burning is especially harmful because it releases
chemicals that are persistent in the environment, polluting our air, food,
lakes and streams. Burning plastic and treated wood also releases heavy
metals and toxic chemicals such as dioxins. Exposure to dioxins can cause
skin problems, reproductive or developmental problems, and may even
increase the risk of cancer.
Existing Public Health Laws and Ordinances
Human health hazards can result from the use of outdoor wood boilers. Local health departments, which have the authority to address
health hazards, may be asked to respond to complaints from the public
regarding problems with water stove use. In addition, several communities
in Wisconsin have taken the step of creating ordinances that ban or
regulate the use of OWBs. The adoption of local ordinances regulating
outdoor wood stoves is currently the best way to address the issue
proactively. Although the US EPA has regulations for reducing pollution
from residential stoves and fireplace inserts, there are currently no
Federal or State standards regulating the use of outdoor wood
boilers. The US EPA does maintain a
Burn Wise website (exit
DHS) with links to state and local air agencies working to reduce
emissions from OWBs, as well as current regulations governing OWB use.
The Wisconsin Department of Health Services (DHS) compiled a
list of local ordinances
regulating OWBs and residential wood smoke in Wisconsin. The Wisconsin Department of Natural Resources (DNR) has developed a
document and model ordinance (exit DHS) for
local communities interested in regulating outdoor burning, burning of
refuse, and the installation and use of OWBs.
What Can Local Governments Do About Water Stove
In some communities, the best approach to managing nuisance complaints
and public health hazards is a local ordinance that restricts or bans the
use of OWBs. While water stoves are typically used in rural settings, an
increasing number are being installed in subdivisions and small towns. If
your municipality is receiving complaints about OWBs, you should consider
the following best-management practices for their placement and use:
- Ensure that OWBs are installed where they do not create an air
pollution health hazard. Local officials should give careful
consideration to 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 frequently
results in homes being built too close to OWBs.
- Restrict what can be burned in an OWB to clean dry firewood.
- Place OWBs at least 300-500 feet from the nearest building which is
not on the same property as the unit.
- Require that OWB chimneys be 15 feet high, or at least as high as
the roofs of nearby buildings.
- Require annual permitting of OWBs by the local fire chief.
For more guidance on establishing control over the installation and use
of OWBs please refer to the DNR
"Model Ordinance for Outdoor
Burning, Open Burning and Burning of Refuse – A Guide for Wisconsin
Counties, Cities, Villages and Towns." (exit
For More Information
Contact the Wisconsin Division of Public Health, Bureau of
Environmental and Occupational Health, PO Box 2659, Madison, WI
53701-2659, (608) 266-1120.
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August 21, 2014