Outdoor wood boilers (OWBs), also known as outdoor wood furnaces or hydronic heaters, resemble outdoor sheds with a short smoke stack. They heat water, or a combination of water and antifreeze, that is then pumped to the house through underground pipes. The water is then circulated around the home to provide heat.
A thermostat inside the house varies the amount of air supplied to the firebox for combustion. When the desired temperature is reached, the oxygen is cut. The wood then smolders until more heat is desired. This smoldering design leads to large emissions of toxic particulate pollution.
Northeastern States for Coordinated Air Use Management (NESCAUM), the average OWB emits as much particulate pollution as 8,000 gas furnaces. They report that there is so much smoke seepage into nearby homes that neighbors’ indoor smoke and carbon monoxide alarms have been known to activate.
that OWBs are designed to provide heat and hot water year round, so they pollute all year. They also note that the oversized fire boxes of OWBs allow users to burn inappropriate materials that would never fit in a wood stove or fireplace. “Enforcement programs have discovered OWBs burning tires, large bags of refuse and railroad ties.” The temptation to burn inappropriate materials is high, since OWBs require large amounts of wood, and supplying them with actual firewood year-round can become unaffordable.
At 150 feet away, OWBs to produce particulate level peaks of over 1,000 μg/m3, with frequent values over 400 μg/m3 during routine usage. At 50 feet away, a shocking 8,800 μg/m3 peak measurement was recorded.
To put this in perspective, when China declared red alert for dangerous air quality, levels had reached 291 μg/m3 — a mere fraction of the hazardous particulate levels to which neighbors of OWBs are regularly exposed.
As of 2015, new emissions standards for OWBs were put into effect by the U.S. EPA. Some sellers of outdoor wood boilers have been touting their boilers as “clean burning” because they meet these new standards. However, by nature of their design, these boilers will never be a “clean burning” source of heat.
Environment and Human Health, Inc (EHHI) has noted that some states in the US have enacted setback regulations for OWBs of 100 or 200 feet from the nearest neighbor. Their research has shown that a house even 850 feet away from an OWB had PM2.5 levels “six times that of the background houses, and four times higher than the EPA air standards, showing that a 200-foot setback regulation in no way protects property values or human health.”