FARMINGTON -- The popularity of outdoor wood furnaces has increased as the costs of oil, natural gas and electricity have risen.

Stove owners are happy with the lower costs, but sometimes their neighbors aren't. A typical outdoor, wood-fired heater looks like a small shed with a smokestack. The furnace transfers heat by circulating a fluid, usually water, through a closed system of pipes. The shorter metal chimneys can keep the heavy smoke closer to the ground, and health concerns and public nuisance complaints have forced some communities to rethink and even rewrite their ordinances.

According to the American Lung Association, smoke from older technology like outdoor wood boilers can contain pollutants that are harmful to breathe. The residue can be so fine that it can go into a person's lungs and directly into the bloodstream. Exposure to particles 2.5 micrometers in diameter and smaller is linked to a variety of health problems.

Several manufacturers of outdoor boilers are making cleaner models available to consumers. To comply with new U. S. Environmental Protection Agency standards, outdoor furnaces may emit no more than 0.6 pounds of particle pollution per million British thermal units of heat from the wood burned. Maine's Bureau of Air Quality compliance manager, Louis Fontaine, is happy about that improvement.

"The cleaner furnaces are more efficient and deliver about 80 percent of the heat from the wood burned in it, which is about twice the efficiency of the older outdoor boilers of 20 years ago," Fontaine said.

He estimates that newer technology means the heaters will pollute about 70 percent less than current models. The EPA model for regulating these outdoor wood-fired heaters can be adapted to the requirements of local ordinances, according to Fontaine. The model rule establishes emission limits and labeling requirements, and it suggests revised standards for new and existing outdoor wood-fired boilers.

Todd Richard and Marty Farnum own Northern Lights Hearth and Sports in Farmington and Rangeley, and they have switched to the cleaner-burning furnaces.

"We carry the Greenwood line, and their Aspen furnace meets the new regulations without sacrificing efficiency," Richard said. "The stove burns the wood completely and combusts the gas instead of sending it out into the air as smoke or creosote."

Richard offered words of caution to the public about their expectations of outdoor wood furnaces.

"They have to be installed correctly by a professional," he said.

Farmington's code enforcement officer, Steven Kaiser, agreed.

"When they're installed, the stack needs to be high enough to make sure the smoke doesn't blow into your neighbors' yards," he said. "We've had complaints, and these furnaces can be a real problem in the middle of town."

Maine has laws that prohibit burning of household garbage; cardboard; plastics; pressure-treated, stained or painted wood; plywood; particle board; or any wood containing glue that releases toxic chemicals when burned.

Kaiser said he always is willing to review an installation or answer questions about the town's requirements for setback allowance from neighboring property lines, requirements for chimney heights or other safety issues. He advises homeowners to check with their insurance companies to be certain they have proper coverage.