Outdoor hydronic wood boilers, commonly known as outdoor wood furnaces, have increased in popularity, and on-farm use has grown. As wood is a renewable resource, and many farmers already maintain forested acreage for firewood harvest, hydronic wood boilers seem to be a perfect fit. However, before making a decision, there are some things to consider.

An outdoor wood heater is located separately from the structure being heated, such as a residence, garage, barn, shed, greenhouse or outbuilding. Typically, heat energy from the burning wood is used to heat water, which is then transported via underground pipes to provide heat to a nearby building, greenhouse or even a swimming pool.

Many outdoor hydronic boiler manufacturers already offer models that meet the current voluntary EPA regulations, including Central Boiler.  Photo courtesy of Central Boiler.
Many outdoor hydronic boiler manufacturers already offer models that meet the current voluntary EPA regulations, including Central Boiler.
Photo courtesy of Central Boiler.


While indoor wood-burning furnaces have been regulated by the Environmental Protection Agency (EPA) since 1990, outdoor models have not. This has caused concerns about air pollution. Combined with nuisance complaints about smoke, the use of outdoor hydronic wood boilers has been controversial.

Unlike indoor units, outdoor furnaces are currently only under a voluntary EPA compliance standard. The EPA’s Hydronic Heaters Program encourages manufacturers to redesign models to decrease air pollutants. “Qualified” models burn 90 percent cleaner than nonqualified models. Proposed regulations would require that all new hydronic heaters be manufactured to the current voluntary standards for qualified models and would phase in more restrictive parameters over a period of several years.

The EPA released its proposed regulations for residential wood heaters in January 2014, and public comments were accepted until early May. Outdoor hydronic heaters are one of several types of wood heaters covered under the proposal ( The proposed standards would also set limits on particulate matter (PM) emissions for a variety of wood-burning heaters, including adjustable-rate woodstoves, pellet stoves, forced-air furnaces, masonry wood heaters, and a type of previously unregulated woodstove known as a single-burn-rate stove, in addition to both indoor and outdoor wood-fired hydronic heaters.

The proposed regulations for outdoor furnaces apply only to new furnaces and would phase in over a five-year period, beginning in 2015. The main purpose is to decrease emissions, primarily PM, but other types of emissions will likely be reduced as well. The allowed levels of PM being emitted from the wood combustion would be reduced from the current allowable level of 0.32 pound per million Btu heat output to a level of 0.06 pound per million Btu heat output. New units would carry a permanent label to designate that they meet all requirements. An alternative three-phase approach is also being considered to allow more time before the final, stricter PM emission standards are implemented.

This outdoor wood furnace heats a greenhouse and building at Cahill's Farm in Sussex County, New Jersey.  Photo by Tamara Scully.
This outdoor wood furnace heats a greenhouse and building at Cahill’s Farm in Sussex County, New Jersey.
Photo by Tamara Scully.

Beyond the federal

In addition to these proposed federal regulations, state and local ordinances exist to control the use of outdoor wood furnaces, again due primarily to the adverse effects of smoke and emissions. The Air Quality & Climate Division of the Vermont Department of Environmental Conservation maintains a list of regulations in Vermont and other states (

Vermont’s current standards already require that any outdoor hydronic wood boilers offered for sale or purchased for installation in Vermont must not exceed the PM limit of 0.32 pound per million Btu heat output – the same level established as the EPA’s current voluntary standard. A list of boilers that meet the standards, as well as an in-depth discussion of terms, is available at

Renewable energy sources are being promoted through many state and federal initiatives. The Biomass Energy Resource Center (BERC) is a program of the Vermont Energy Investment Corp., a nonprofit organization dedicated to minimizing the economic and environmental impacts of energy use. One of the center’s objectives is to advance community-scale biomass energy throughout North America.

Wood is a renewable biomass fuel, and it’s one option for increasing energy independence and reducing fossil fuel use. At the same time, the need to address pollution concerns such as the PM emissions from outdoor wood boiler use is evident.

According to a 2013 report by BERC: “One portion of the residential biomass thermal market that is experiencing state and even municipal regulations regarding air quality is the outdoor wood boiler (OWB) market. Regulations for OWBs vary from state to state. Currently, all five states reviewed allow OWBs provided they meet EPA certification standards and follow specific guidelines in terms of stack heights and setbacks to neighbors, etc. However, several municipalities in Massachusetts currently ban the use of OWBs. Several states have programs to fund the change-out/replacement of inefficient, polluting OWBs with more modern, efficient options.”The complete report can be found at

Another player in air quality regulations that’s involved in various initiatives to decrease air pollution concerns from outdoor hydronic heaters is the Northeast States for Coordinated Air Use Management ( This nonprofit consists of the New England states, plus New Jersey and New York.

Photo courtesy of Central Boiler.
Photo courtesy of Central Boiler.

Cause for concern

Smoking issues have tended to cause the most uproar and concern on the local level. Not only can the odor be unpleasant, but the very small PM that is released when wood is burned is considered detrimental to health; it can become embedded in the air sacs in the lungs.

Short smokestacks, found on outdoor wood furnace models that aren’t compliant with the EPA’s voluntary standards, cause increased smoking, as the smoke cannot readily disperse. Outdoor wood boilers that do not meet the current voluntary EPA standards have been found to have 10 times the particulate emission rate of indoor woodstoves, which are regulated by the EPA.

Another issue with boilers that don’t meet the current voluntary standards is partial burn. This incomplete combustion causes the release of benzene, polycyclic aromatic hydrocarbons, formaldehyde and other toxic partial combustion products, all of which have been shown to be associated with negative health impacts.

Outdoor wood furnaces that aren’t up to the EPA’s voluntary standards tend to operate by shutting off the oxygen to the fire when a set temperature for the water is reached. When the water temperature drops below a certain point, the air supply is re-established. This idle time causes incomplete combustion and lots of dense smoke. Loading cold wood, burning green wood or operating an oversized unit exacerbates these problems.

Best practices

Many outdoor hydronic boiler manufacturers already offer models that meet the current voluntary EPA regulations. One example is Central Boiler’s E-Classic furnaces, which are designed to heat multiple buildings. Central Boiler ( also offers systems geared to greenhouse heating efficiency.

A list of the manufacturers with models that meet the current voluntary standards can be found at Some of these models are batch-loaded, while others offer continual feed options. Some burn wood or other biomass pellets; some burn cordwood.

According to the EPA’s list, WoodMaster ( offers the most efficient models, with an annual PM emissions rating of 0.04 pound per million Btu heat output. That’s below the suggested five-year mark of 0.06 pound per million Btu heat output in the recently proposed regulations. Wood-Master’s two flex-fuel boilers, which burn wood pellets or cordwood, are at the top of the EPA’s list of the cleanest-burning hydronic boilers and are over 92 percent efficient.

No matter which model of outdoor wood furnace is being used, there are – as in all areas of agriculture – best management practices that make the operation of these units safe, more efficient and less controversial. The Vermont Air Quality & Climate Division offers the following guidelines (

  • Burn only seasoned wood, as green wood’s high moisture content severely reduces combustion efficiency and can even harm boilers.
  • Burn only hardwood, as softwood may burn too quickly for proper unit operation.
  • Don’t burn treated, painted or artificial wood products.
  • Follow the manufacturer’s directions for operating at peak efficiency.
  • Don’t overload the boiler, but match the wood load to the day’s demand for heat.
  • Consider installing a heat storage system, which allows an entire load of wood to burn without shutting down the boiler and stores the heated water in bulk tanks, which can provide heat for several days without additional burning.

While the EPA regulations would grandfather in existing outdoor furnaces that don’t meet the regulations, a better option may be to purchase units that already meet the current voluntary standards or the more stringent standards being proposed for future models.

Emissions-Compliant Boiler Manufacturers

The Air Quality & Climate Division of the Vermont Department of Environmental Conservation lists the following manufacturers that offer outdoor wood boilers that meet the particulate matter emissions standard of 0.32 pound per million Btu heat output:

Central Boiler…… 800-248-4681

Dectra Corp…… 612-781-3585

Earth Outdoor Wood Furnaces…… 417-746-0043

Hardy Mfg…… 601-656-5866

Hawken Energy…… 888-LOG-BURN

HeatMasterSS…… 877-325-9792

Heatmor…… 800-834-7552

HeatSource1…… 620-763-2500

Interphase Energy (Kedel)…… 207-370-2690

Maine Energy Systems…… 207-824-NRGY

Northwest Mfg. (WoodMaster)…… 800-932-3629

Polar Furnace…… 855-582-2233

Portage & Main…… 800-561-0700

Pro-Fab Industries…… 888-933-4440

Wood Boiler LLC…… 888-786-8703

Wood Doctor…… 877-770-8778

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