Over the past few weeks, we’ve been looking at wood burning—a popular and affordable heating option in rural New England. Ten or 15 years ago, a new option started showing up. Driving along country roads, we began to see shed-like structures with smoke billowing from smokestacks. These are outdoor wood boilers (sometimes called outdoor wood furnaces), and they have been the focus of considerable attention and debate in recent years, mostly over the pollution they generate.

A traditional outdoor wood boiler has a large firebox surrounded by a water jacket in which up to a few hundred gallons of water are heated. This hot water is piped into the house through a buried insulated pipe, and it heats the house either with a fairly conventional baseboard hot water (hydronic) system or a “fan coil” and forced-air distribution system.

There are a number of advantages to outdoor wood boilers. One is that they can accept far more cordwood than even the largest wood stove, offering longer burn times. Though manual stoking is required, an air inlet damper on the wood boiler is thermostatically controlled so that the delivery of heat is automated. When the house isn’t calling for heat, the outdoor wood boiler is damped down, which reduces the output and keeps the wood burning longer. Because the outdoor wood boiler heats water, the heat can be distributed to buildings that are some distance away—up to a couple of hundred feet. Finally, for people in rural areas, these systems can heat a large home, and even outbuildings, very affordably—especially for those who cut their own firewood.

Some of the advantages of outdoor wood boilers are also their problems. A number of the operational features of outdoor wood boiler operation make them very polluting. In standard outdoor wood boilers, the water jacket keeps the fire temperature low. We learned in last week’s column that low combustion temperatures result in significantly more pollution—because the carbon in the smoke is not fully combusted. If the water jacket in an outdoor wood boiler is separated by just an eighth-inch layer of steel, the water in the jacket (usually at a temperature of no more than 180 degrees Fahrenheit), prevents the fire from getting hot enough to ensure full combustion.

Second, most standard outdoor wood boilers are designed to damp down when heat is not called for by the house. This damping limits the air inlet into the firebox, so the wood smolders—producing a lot of smoke and little heat. While an advanced wood stove may operate at up to 80% efficiency, the efficiency of outdoor wood boilers can drop below 25% when damped down. Some outdoor wood boiler manufacturers boast that they can operate for two or even three days on a single stoking, but this is only by operating them in this oxygen-starved condition. When an outdoor wood boiler smolders like this, large amounts of creosote condense on the inside surface of the firebox, and when the damper reopens this creosote burns, generating a surge of smoke and odor.

Third, many operators of outdoor wood boilers burn green (improperly seasoned) or wet wood, exacerbating the low-temperature combustion and smolder problem.

The pollution problem from outdoor wood boilers is so bad that neighbors of homeowners using this type of heating system often complain of headaches and illnesses, and those who want to hang laundry on a clothesline can’t do so because the laundry gets covered with soot. According to the Vermont Department of Environmental Conservation, Air Pollution Control Division, old-style outdoor wood boilers emit an average of 71.6 grams per hour of particulate pollution—roughly ten times the emission limit of EPA-compliant wood stoves and more than twenty times the pollution of a state-of-the-art wood stove.

These complaints have led to regulatory action by a number of states, led by Vermont, starting in 2005. State regulations are needed because the 1988 EPA regulations that established emission limits for wood stoves specifically exempted wood-fired central heating systems, including outdoor wood boilers. Since March 31, 2008, outdoor wood boilers sold in Vermont can emit no more than 0.44 pounds of particulate matter per million Btus of heat input. (Regulations for wood stoves, by comparison, have just a single level of permissible emissions, no matter what the size of the wood stove.) A phase two standard, already adopted by Maine, New Hampshire, and Massachusetts, will reduce the allowable emissions to 0.32 pounds of particulate emissions per million Btus of heat OUTPUT (i.e. delivered heat).

While most outdoor wood boilers today are very significant sources of air pollution, there are cleaner alternatives. A number of outdoor wood boilers have been certified by the Vermont Agency of Natural Resources as meeting Vermont’s Phase I emission limit; these are listed on an Air Pollution Control Division website. Manufacturers are working on cleaner and more efficient outdoor wood boiler designs based on advanced technologies, such as downdraft gasification. “Changes are happening rapidly in this highly competitive market,” Christian Jones, the air pollution compliance chief at the Vermont DEC, told me. Vermont has already certified two cordwood models that comply with the presumed Phase II standard, according to Jones, and others are in the pipeline to be certified.

Locally, Landmark Trust USA has installed two Garn 2000 wood boilers (one of nine Phase I-compliant boilers). The Garn 2000 heats an 1,825-gallon reservoir of water and has a burn rate of 425,000 Btus per hour. A Garn boiler is burned hot and heat is transferred to the water. One of these boilers heats Rudyard Kipling’s home, Naulakha, and the carriage house; the other is installed at Scott Farm and heats the apple packing facility and will also heat other buildings there. David Tansey, of Landmark Trust, tells me that there is some visible smoke when the Garn boiler is first started up, but “virtually none when it is up to temperature, which happens quickly with the draft assist.”

The Garn 2000 boiler, which is manufactured in Minnesota, costs approximately $12,000, plus installation, which the manufacturer (Dectra Corporation) estimates will add 50-100% to the total cost.

The Garn boiler serving Naulakha, which was installed first, burns 20-22 cords per year (all harvested on the farm), replacing the 3,750 gallons of heating oil they previously had used annually, according to Tansey. Other manufacturers with Vermont-compliant outdoor wood boilers include Greenwood Technologies and Central Boiler, both of which offer products meeting the Phase II standards.