About Outdoor Boilers

A Controversial Wood Heating Technology

“We found that while OWBs are advertised as a clean and economical way to heat one’s house and water, OWBs may be among the dirtiest and least economical modes of heating, especially when improperly used.”
NY Attorney General’s Report

What are Outdoor Wood-fired Boilers (OWBs)?

OWBs are residential or small commercial wood-fired water heaters that are located outdoors or are separated from the space being heated. The fires in the large fire boxes heat water that is circulated into the home through underground pipes. The energy may be used to heat houses, shops, domestic hot water, greenhouses, swimming pools and spas.

Why Are the New Certified OWBs Better?

Old Style OWBs tend to cause dense smoke that impacts neighbors who complain about nuisance and health problems. Most OWBs come equipped with very short stacks. The smoke from these low stacks disperses poorly. In addition, the owners often operate the OWBs to heat hot water or swimming pools during the summer when neighbors have their windows open and are trying to enjoy the outdoors.
New certified OWBs burn the wood gases more completely and extract the heat more efficiently causing less smoke. Owners will need to burn less wood in the new certified OWBs because they are so efficient.

What Causes Old Style OWBs To Smoke?

Most old style OWBs employ very primitive combustion technology. When the water circulating through the furnace reaches an upper set point (usually around 180°F) the air supply to the fire is cut-off, cooling the fire so the water will not overheat. The furnace operates in this “idle” mode until the water temperature hits a lower set point and the air supply is re-established.

Under some conditions, the OWB may be in idle mode far longer than in operating mode. This type of operating causes very poor combustion and heavy foul smoke. Most of the smoke emitted is fine condensed organic material that does not burn under cool, oxygen starved conditions. In addition, many owners burn green wood full of moisture which also causes poor combustion. Wood from the outdoor winter wood pile may be very cold when loaded into the OWB causing an even colder fire.

Are Old Style OWBs Worse Than Indoor Woodstoves?

Yes. Newly manufactured indoor woodstoves are required to meet strict US EPA particulate emissions standards, 4.1 grams per hour for catalytic stoves and 7.5 g/hr for noncatalytic stoves. Certification tests are conducted in EPA approved laboratories. As expected, emissions during actual use are somewhat higher.

In contrast, the New York State Attorney General’s office found that average emissions during laboratory testing of OWBs was 71.6 g/hr or roughly ten times the particulate emission rate from indoor woodstoves. Although older style indoor wood stoves emit more than new certified stoves, they are still several times less polluting than OWBs. Due to the poor combustion conditions, it is also probable that OWBs emit proportionately more benzene, polycyclic aromatic hydrocarbons, formaldehyde, and other toxic partial combustion products which have been linked to asthma, heart attacks, and cancer.

Is Natural Wood Smoke Harmful?

Yes, all wood smoke is harmful, but smoke from old OWBs is worse due to the poor combustion and large amounts of smoke emitted. While smoke and gases from burning fossil fuels, such as oil, contain air pollutants like sulfur dioxide, the smoke from wood burning contains much higher levels of small particles. In fact, the vast majority of particulate emitted by OWBs is very fine (less than 2.5 microns in size) and can become trapped in the delicate air exchange sacs deep in your lungs when inhaled. Numerous studies have found strong relationships between high fine particulate levels and chronic lung diseases, cardiovascular disease and premature death. According to the American Lung Association, the fine particulate found in woodsmoke can be linked to higher school absenteeism, emergency room visits and hospitalizations for cardiopulmonary conditions, respiratory infections and asthma.

I Want To Burn Wood-What Should I Do?

  • First, consider your neighbors. Burning wood in some dense residential neighborhoods may never be a good idea.
  • Have an energy expert inspect your home. You may find that more insulation or other energy-saving improvements may be a better investment than an expensive wood heating device.
  • If you have an older non-certified wood stove, consider purchasing a cleaner, more efficient EPA-certified woodstove or pellet stove.
  • Consider an efficient indoor wood boiler that may include a large hot water storage tank. Operated properly, these units cycle less and burn hotter and cleaner.
  • If you like the idea of an outdoor wood boiler, purchase one of the new units certified to meet Vermont’s emissions standard. After March 31, 2008, dealers are prohibited from selling uncertified OWBs in Vermont or for installation in Vermont. Purchasing an OWB is a big investment; make it wisely.

For additional information about OWBs, contact the Vermont Department of Environmental Conservation, Air Quality & Climate Division. Download Information in pdf files: Fact Sheet (2pgs).