Outdoor Wood Boilers

outdoor wood boiler and adjacent wood shedWith the high cost of heating oil, more Vermonters are looking to wood as a source of heat and hot water, but not all wood heat is the same. While indoor wood stoves have been tested and certified by the EPA for emissions since 1990, testing and certification of more efficient outdoor wood boilers (OWBs) is a more recent development. Older uncertified OWBs can cause dense smoke, as many are equipped with limited exhaust systems that do not disperse smoke efficiently. The smoke emitted can negatively affect the health of the homeowner, nearby neighbors, and the environment. The newer certified "Phase II" OWBs are a great improvement. The claims from some OWB manufacturers about old style OWBs did not prove to be accurate with time and testing. The AQCD wants prospective OWB owners to have the facts before making an investment. With the adoption of new federal regulations effective May 15, 2015, OWBs are regulated nationwide as well in Vermont and may also be regulated (or banned) by your town. 

What are Outdoor Wood Boilers?

OWBs are residential or small commercial wood-fired water heaters that are located outdoors or are separated from the space being heated. The wood burned 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.

How are OWBs Regulated?

Vermont Agency of Natural Resources: In response to complaints about OWBs in the early 1990s the Vermont ANR adopted regulations (Section 5-204) that apply to OWBs installed after October 1, 1997. This regulation established set-back and stack height requirements for OWB installations but did not set an emission standard that would reduce the air pollution emissions from these units. Because complaints continued and OWBs became increasingly popular, causing greater impacts on Vermont's air quality, the ANR adopted amendments to the original regulation that established first a "Phase I" and then a "Phase II" particulate emissions standard for OWBs sold in Vermont. The current regulation also applies to outdoor pellet-fired boilers. The Phase II particulate standard, effective on March 31, 2010, promotes the sale and installation of much more efficient and cleaner wood burning technologies in Vermont.

Vermont OWB Rule - Section 5-204

U.S. Environmental Protection Agency: Federal regulations that became effective on May 15, 2015 regulate the manufacture and sale of nearly all wood-fired residential heating appliances. For the first time there is a particulate emissions standard that applies to OWBs manufactured and sold across the U.S. Any OWB sold in Vermont must be certified by the U.S. EPA.

Why are the New Certified OWBs Better?

Uncertified OWBs tend to cause dense smoke that can cause a nuisance and impact neighbors. 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?

outdoor boiler and woodpile

Most old style OWBs employ very primitive combustion technology. When the water circulating through the boiler 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 boiler 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 Stoves?

Yes. Indoor woodstoves manufactured since 1990 are required to meet strict US EPA particulate emissions standards (7.5 g/hr for non-catalytic stoves). New federal regulations require all woodstoves manufactured after May 15, 2015 to meet an emission standard of 4.5 g/hr. 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 uncertified 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?

  1. Consider your neighbors. Burning wood in some dense residential neighborhoods is not always a good idea.
  2. 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.
  3. If you have an older uncertified wood stove, consider purchasing a cleaner, more efficient EPA certified woodstove or pellet stove.
  4. 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.
  5. If you like the idea of an OWB, purchase one of the new units certified to meet Vermont’s emissions standard. After March 31, 2008, dealers were prohibited from selling uncertified OWBs in Vermont or for installation in Vermont. Purchasing an OWB is a big investment; make it wisely.

For more information please check out the related wood-burning literature on our publications page. If you have questions, please contact John Wakefield at john.wakefield@vermont.gov or call (802) 279-5674.