Which Way and How Much Does the Smoke Blow?

A look at outdoor wood boiler regulations
By Kathleen Hatt

Outdoor wood boilers (OWBs) can be an efficient way of heating farm buildings, greenhouses and homes. If you are considering one for your farm, the regulations that govern their operation could affect your decision.

Outdoor wood boilers can provide efficient heat for farm buildings.
Photos by Kathleen Hatt.

However, unraveling the regulations that may or may not apply to outdoor wood boilers where you live can be complicated. Some states and/or localities regulate OWB installation and operation, others do not. Although it has been widely anticipated that the Environmental Protection Agency (EPA) will issue regulations governing outdoor wood boilers, no such rules currently apply. However, the EPA has adopted voluntary standards for outdoor wood boilers. Consider one designed to reduce atmospheric pollution. In states where OWBs are regulated, these so-called "White Tag" models are generally the only ones available. But first ...

What is an OWB and how does it work?

Outdoor wood boilers generally heat and/or provide hot water for a single building. Units are designed to burn wood in a firebox surrounded by an insulated water jacket. They are vented through a smokestack. OWBs, also known as hydronic heaters, are the approximate size and shape of an outhouse or small utility building. Water heated in an insulated water jacket flows through underground pipes to a farm building, house or swimming pool. The unit may also produce domestic hot water. Most OWBs burn wood slowly. Current models, those marketed since 2008, burn only dry wood and will plug up if logs containing more than about 15 to 20 percent moisture are used. Ideally, the wood does not exceed 2 feet in length. Prices for OWBs generally run from $10,000 to $15,000 installed, according to Wes Smith of New England Outdoor Furnaces in Newport, N.H. Prior to 2008, when OWBs were generally unregulated and could burn wet wood, unit prices ranged from $7,000 to $10,000, according to Smith.

So, what's the problem?

Because OWBs are designed to burn wood slowly, they create more smoke and creosote than fires burning at higher temperatures. Burning green or wet wood or trash increases smoke and smoldering. OWBs can emit thick, black smoke, and those with short chimneys emit smoke closer to the ground than those with higher chimneys. Wood smoke contains more than 100 gaseous compounds as well as particulate matter. Wood smoke can negatively affect human health and the health of the planet. For that reason, some governmental entities regulate their placement and operation.

What's the significance of a White Tag?

In 2008, the U.S. EPA issued Phase 2 Voluntary Standards for outdoor wood boilers. Many local agencies have adopted Phase 2 standards in whole or in part, banning unqualified hydronic heaters and establishing minimum distances to neighbors as well as minimum chimney heights. A model rule has been developed by the Northeast States for Coordinated Air Use Management (NESCAUM) with technical and financial support provided by the U.S. EPA. The New England states as well as at least 14 more states throughout the country have incorporated voluntary standards into law. It is anticipated that at some point there will be nationwide standards.

Wood boilers that are designed and operate within the voluntary EPA Phase 2 emission levels bear a white hang tag. The tag signifies that that model hydronic heater has been tested by an EPA-accredited laboratory and is cleaner than other, earlier models.

The following elements of the White Tag are often reflected in the regulations of states and municipalities that have adopted EPA Phase 2 standards:

Outdoor wood boilers manufactured after 2008 are generally compliant with EPA voluntary Phase 2 standards.

Blowing your stack

It's what happens when smoke from your neighbor's outdoor wood boiler blows into your space. Although the amount of particulate matter being dispersed to the atmosphere impacts life on the entire planet, it's the smoke blowing toward neighbors that brings citizen complaints to regulatory agencies. Although farmers are less likely to have nearby neighbors than city folk, it's still wise to take neighbors, as well as the creatures on your own farm, into consideration when siting an outdoor wood boiler. Factors that contribute to the direction and elevation at which smoke and particulate matter is dispersed include:

Height and width of the chimney pipe

Building downwash - downward movement of the smoke plume from relatively short smoke stacks due to disturbances in flow attributable to the unit itself. Nearby structures can also contribute to downwash.

Terrain - mountains or valleys in which air stagnates

Weather and climate - particularly wind

Some state regulations

A sample model code designed to regulate outdoor wood boilers begins by pointing out that while outdoor wood boilers are attractive to those looking for an economic alternative to conventional heating systems, outdoor furnaces may not be so attractive to neighbors (see In which ways and how much various factors are perceived as potentially annoying or detrimental can be quite different from one city or state to another. Regulations generally address setback restrictions, stack location and height, limits on months of operation and permitting requirements. To illustrate the range of regulations, here are some examples gleaned from some state and local regulations.


Minimum lot size

Permitted fuel

Stack location

Months of operation

Before you buy

Contact your local and state governments for a copy of the regulations that govern installation and operation of OWBs in your area. Start with local code enforcement officers and your state's environmental services department. Learn before you burn.

When you buy, be sure your boiler is EPA Phase 2 compliant (as most OWBs manufactured after 2008 are). Tempting as it may be to try to burn green or treated wood or other fuel, remember that in the long run atmospheric pollution hurts us all.

Kathleen Hatt is a freelance writer and editor and has been a frequent contributor to Farming since 1998. She resides in Henniker, N.H.

For Further Information

The EPA's Phase 2 list of cleaner-burning hydronic heaters listed by manufacturer, model name, heat output rating, annual emission rate, heat output and fuel type can be found at: