My neighbor just installed an outside wood boiler and it is putting out a lot of smoke into my yard. Can you tell me what I can do?


Here is a collection of information and articles that we have found that may give you the information you need on Outside wood boilers. For further information contact the Maine DEP at:



How Outdoor Wood Boilers "stack up"

June 13, 2003

Is the rising price of fuel getting to you? Are you looking for alternatives to oil that don’t involve the mess and hassle handing wood inside?

A new heating technology hitting the market might seem to offer a solution. It’s the option of burning wood in a outdoor wood boiler. Outdoor wood boilers heat water outside of your building in an insulated shed. Underground pipes carry water for both heat and hot water.

Sounds like the answer, but there is a problem—smoke. These burners smoke a lot!

Let’s examine why. For one thing, the firebox of most of the units available today is surrounded by a large water jacket. This is good for heating the water, but it acts to cool the escaping gases before combustion is complete. Incomplete combustion causes smoke.

Another reason that these units tend to be smoky is that the heat demand is very sporadic. There are long periods of time when the wood just smolders in these units. During these periods of low airflow, creosote collects on the water jacket walls. When heat is needed, air is re-introduced and the fire is rekindled. The creosote is then burned off, creating black soot. To make matters worse, the stacks of these units are very short, and the smoke and soot are released close to the ground where it can drift into your neighbor’s yard.

Why is this a concern? Wood smoke contains large amounts of particulate (“soot”), carbon monoxide, and other toxic pollutants. All of these are legitimate health concerns and can justify nuisance complaints. For exactly these reasons, some state and local municipalities have banned the use of outdoor wood boilers.

If you’re thinking about one of these units for your backyard, consider these tips from the Wood Heat Organization:

? Stick your head in the firebox and if you don’t see a lot of firebrick, there is almost no chance the unit can burn clean.

? A good outdoor boiler will be quite a lot more expensive than the smokers, so if the one you’re looking at is comparatively cheap, look elsewhere.

? Performance claims that seem extravagant are probably false: ask for proof.

As more manufacturers enter the market with new technology, the outdoor wood boiler will likely become cleaner and more efficient. For right now, if you have a modest house, it might be a better idea to get a wood stove, heating fireplace or small basement wood furnace. You’ll burn a lot less wood, make a lot less smoke, and save yourself a lot of money in the long run.

It’s clear that there is a growing need for alternative heating sources to fuel oil. While researching and purchasing these alternatives keep in mind the impact of your choice on the environment and remember to be a good neighbor in our back yard.

This column was submitted by Jim Gramlich, an Environmental Specialist with the Maine DEP's Bureau of Air Quality. In Our Back Yard is a weekly column of the Maine Department of Environmental Protection. E-mail your environmental questions to or send them to In Our Back Yard, Maine DEP, 17 State House Station, Augusta, ME 04333.


Regulation of Outdoor Wood Boilers
Effective Date: November 9, 2007
Contact: 1-800-452-1942 or 207-287-2437
Outdoor wood boilers (OWBs) are heating appliances that burn firewood and are located outside of the building which they are heating. The formal industry name for an OWB is “Outdoor Wood Fired Hydronic Heating Appliance.” They are also known as outdoor wood furnaces and water stoves. Conventional OWBs consist of a firebox surrounded by a water jacket, a weatherproof cabinet and a short smokestack. Some outdoor wood boilers do not have a weatherproof cabinet and must be installed in an outbuilding such as a garage or shed. OWBs with a heat input of less than 3 million
Btus per hour (MMBtu/Hr) are subject to regulation by the Maine Department of Environmental Protection (DEP). These regulations were effective November 9, 2007. Larger OWBs have to comply with different DEP regulations.
A standard method to test and evaluate the emissions and efficiency of OWBs was established nationally in 2007 by the U.S. EPA. New boilers are being designed and tested to determine how clean and efficient they are. When boiler tests are published, they will appear at the federal EPA web site Outdoor wood boilers with emissions equal to or less than 0.60 pounds of particulate per 1 million Btu of fuel burned (lbs/MMBtu) will qualify for the EPA Outdoor Wood-Fired Heater program. Look for the "orange label" to check for the EPA certified emission rating.
As of April 1, 2008, Maine retailers must phase out high emitting units over the following year, and only restock with units that have an EPA certified particulate emission rating of no more than 0.60 lbs/MMBtu. By April 1, 2009, the sale of units with emissions higher than 0.60 lbs/MMBtu will be phased out completely. Starting April 1, 2010, further reductions to a 0.32 lbs/MMBtu emission limit will be required for all new units sold under Maine law. Retailers must provide buyers with a copy of the DEP “Control of Emissions from Outdoor Wood Boilers” rules that contain the emission limits and setback requirements along with the owner’s manual and additional written information.
OWB installations need to meet minimum setback requirements designed to protect public health. The setback distance required depends on the unit’s emission rating, with reduced setbacks allowed for cleaner-burning OWB models. The setback table below lists the minimum distance an OWB unit needs to be from any neighboring property line. Buyers should carefully consider whether their property configuration provides the necessary space to meet the setback requirements before purchasing a boiler unit.
OWB Emission Rating (in pounds per million BTUs or lbs/MMBtu)
Minimum Setback Distance from
Property Line (in feet) 0.32 lbs/MMBtu 50 feet
0.60 lbs/MMBtu 100 feet
>0.60 lbs/MMBtu (including uncertified OWBs) 250 feet
Special Cases* 500 feet
*Special Cases apply to any State licensed school, daycare or health facility.
If terrain conditions could complicate air flow patterns on a parcel of land (e.g. in a valley, hilly, or tall trees nearby), it may be necessary to install the OWB even farther away than the minimum setback distances to avoid costly changes that could be required later if a nuisance condition occurs when the boiler is operated.
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The minimum stack height for all OWB units is at least 10’ from the ground. However, the stack height needs to be extended at least 2’ higher than the peak of the building served by the OWB (or the nearest building in the case of a pool heater) if either of the following are met:
1) If an OWB, installed after 11/9/07, with a particulate emission rating greater than 0.60 lbs/MMBtu (or an uncertified OWB), is within 500’ of any abutting residence; or
2) If an OWB, installed after 11/9/07, with a particulate emission rating of 0.60 lbs/MMBtu or less, is within 300’ of any abutting residence.
If a residence is built on abutting property after the OWB is installed, the stack height will have to be increased to meet the criteria described above. Additional stack height requirements may be necessary in conditions where topography or other buildings restrict the dispersion of smoke and create a nuisance condition.
The sale of used OWBs is allowed but the buyer must comply with the setback distances and stack height listed for the emission rating of the boiler. If the emission rate is not documented, the boiler must meet the criteria listed for an uncertified OWB.
OWBs that are larger than 350,000 Btu/hr, and all OWBs for commercial applications must have an engineering analysis to determine a number of factors that will determine the proper boiler size, stack height and other items specific to the particular installation and the particular site.
No rain caps are allowed unless required by manufacturer specifications. Rain caps can restrict the flow of air and help to create a nuisance condition.
Allowed Fuels: Only CLEAN WOOD or wood pellets from clean wood can be burned in OWBs. (Clean wood has no paint, stain or other types of coating or treatments with preservatives of any type.) Home heating oil, propane or natural gas may be used as auxiliary fuel in dual-fired OWBs designed to burn those types of fuel.
Prohibited Fuels: any wood that is not clean; ?garbage; tires; lawn clipping or yard waste; ?materials containing plastic; materials containing rubber; waste petroleum products; paints and paint thinners; chemicals; glossy or colored papers; construction & demolition debris; plywood; particleboard; salt water driftwood and other salt-water saturated materials; manure; animal carcasses; asphalt products; materials containing asbestos; materials containing lead, mercury, or other heavy or toxic metals; and coal (unless the OWB is specifically designed to burn coal).
No OWB, regardless of the date of installation, can cause or allow a smoke plume of 30% or greater opacity for more than two six-minute periods in any 3-hour period of time. Opacity is a visual measurement by an EPA- or DEP-certified smoke reader. It is a measure of the "thickness or density of the smoke" emitted from a stack. OWBs producing 30% or greater opacity need to modify operating practices to reduce air pollution problems. Ask for DEP’s Operating Tips for OWBs by calling the number above.
No OWB, regardless of the date of installation, is allowed to operate when conditions cause any visible smoke plume to cross onto adjacent owner’s land and buildings for 12 minutes or more in any hour. Sending smoke on adjacent land or buildings for 12 minutes or more is a nuisance and a violation of the regulation.
For the full rules on controlling emissions from outdoor wood boilers, visit
For more information on outdoor wood boiler emissions, visit

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