Purchasing an Outdoor Wood Boiler? Items to Consider

picture showing a typical outdoor wood boiler

Typical Outdoor Wood Boiler


picture showing smoke plume

Smoke Plume from an Outdoor Wood Boiler

Outdoor wood boilers (OWBs) are a type of wood-fired hydronic heater. Hydronic heaters heat water that is circulated through pipes to heat exchangers in the home such as radiators, baseboard units and radiant floor tubes to provide heat when needed. Outdoor wood boilers can produce a lot of smoke that contains high levels of particulate matter, carbon monoxide and other chemicals that can cause health problems. Smoke can affect your family's health and the health of your neighbors. If you are thinking about purchasing an outdoor wood boiler, consider the information provided below.

There are two types of wood-fired hydronic heaters available, outdoor wood boilers and dual-stage wood gasification boilers. Both types of units can also provide domestic hot water. Gasification boilers will operate more efficiently, especially when used with a heat storage water tank. Before making a purchase, consider your particular site and whether central wood heating is right for your setting.

OWBs may cost less initially, but they produce much higher levels of smoke pollution than a wood gasification boiler. This is an important issue in areas where there are neighboring homes and can be especially important in settings such as valleys where smoke can affect you and homes over a quarter mile away because during calm weather conditions, smoke can travel downhill and collect in narrow valleys.

Smoke from OWBs has caused neighbor complaints. Some OWBs have been shut down by regulatory agencies and others have been the cause for private legal actions. OWBs smolder and dense visible smoke is often present. When operated during the non-heating months, OWBs smolder most of the time due to the low heat demand and the smoldering smoke odor can deteriorate outdoor air quality. Along with the smoke, OWBs use considerably more fuel than dual-stage wood gasification boilers for heating similar space because they burn less efficiently.

Dual-stage wood gasification boilers are the most efficient firewood burning units available on the market today. They can be installed indoors or in a structure close to the home. They are called dual-stage because the smoke and gasses are burned in a hot secondary chamber with temperatures much hotter than an outdoor wood boiler. This high efficiency design results in much less smoke, however, they also have more exacting fuel wood requirements. They work best with very dry wood and will not operate with poorly seasoned wood. They also work best when operated with an auxiliary heat storage water tank since this allows the entire charge of wood to be burned continuously, further improving efficiency. The stored heat is then used over time depending on demand. When operated during the non-heating months with an auxiliary heat storage water tank, gasification units only require a fire about every four to seven days depending on how much hot water you use.

Heat storage water tanks are insulated water storage tanks used to store heat, allowing a boiler to operate most efficiently by burning the firewood hot and fast. The stored heat can then be used for heating your home as needed. Heat storage water tanks work well with gasification boilers, and can also be used for solar heat storage. They are rarely used with OWBs.

If you decide to buy an OWB, you should follow the manufacturers guidelines and/or EPA's best burn practices.

To summarize, if you are in an area where others might be affected by your woodsmoke, stick with your oil or gas heating system or get a high efficiency dual-stage wood gasification unit. Wood gasification boilers may cost more initially, and may have more exacting firewood fuel requirements, but they do not smoke like outdoor wood boilers and are much more efficient requiring less wood.

Additional Information

  • The New York State Department of Environmental Conservation provides information on outdoor wood boilers, including regulations for OWBs in New York State.
  • Renewable Heat NY is a NYS program that provides incentives toward removal of older, polluting wood burning appliances and the installation of high-efficiency, low-emission wood heating technologies for qualifying homeowners and businesses.
  • EPA’s “Burn Wise” website can help people burn wood the right way to reduce air pollution.
  • Cornell Cooperative Extension provides up to date information on wood burning technology.