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Hearth.com LLC
Staff member
Oct 18, 2013
Written by Eric Johnson and Craig Issod

Outdoor Wood Boilers, also called OWBs or Outdoor Wood Burners, consist of a firebox surrounded by a water jacket and a control system all housed in an insulated, steel-sided enclosure with a roof. Hot water is circulated through insulated, underground piping between the boiler and the house. OWBs have become very popular in many rural areas of the United States, with substantial sales in the Northeast and Midwest, where hardwood is plentiful and winters are long and cold.


OWB Typical Setup

Source: HPBA

Although typically referred to as “boilers,” OWBs are not boilers in the traditional sense. In a conventional indoor boiler (oil, gas, wood or coal hydronic heater), water is heated in a pressurized vessel and then circulated through the heating system. In an OWB, the water is open to the atmosphere and thus, not pressurized. This allows the OWB to be built to different standards than a traditional indoor boiler, and makes it exempt from some of the regulations that apply to pressurized heating appliances.

There are pros and cons to consider when shopping for an OWB. One of the advantages of open systems is that they are safer than pressurized boilers. And because they’re usually sited away from the house and other buildings on the property, they dont affect your homeowners insurance rates. All the creosote, smoke, ash, wood storage, etc. are outside of the house as well, which is appealing to many people familiar with wood stoves. And they can burn much bigger pieces of wood than a stove.


improper burning in an OWB

On the down side, open systems are prone to corrosion and the accumulation of debris in the system, which can be a maintenance concern. Overall system efficiency is a problem with OWBs as well, since it is virtually impossible to efficiently burn wood in a firebox surrounded by water. The result is smoke, which is basically unused potential heat going up the stack and polluting the air. Lower efficiency translates into more wood burned - roughly twice as much as a modern wood gasification boiler.

An OWB can be tied into either a hot water (hydronic) or hot air (furnace) system. This is done by piping the hot water from the OWB into the house and transferring the heat into the existing system using a heat exchanger. In a home with a hot water heating system, the heat exchanger is usually a stainless steel and copper device known as a flat plate heat exchanger. In the case of a hot air furnace, it is a copper coil located in the furnace bonnet through which the hot water circulates. The furnace fan then blows air across the coil, heating it and distributing it into the house.

Most OWBs have very large fireboxes, often ten times larger (12 to 40 cubic feet) than the firebox found in a traditional wood stove. This makes them best suited for large heating loads or multiple buildings. However, using such a large firebox to heat most homes is overkill, since the boiler quickly satisfies the heat load and then spends the rest of the time idling. Idling results in lower efficiencies and the production of more smoke and pollutants than when the boiler is running under a load.

Such large fireboxes, together with improper burning techniques, have resulted in numerous complaints about OWB smoke and smell. Some states and localities have either regulated or banned them, and many others are planning similar restrictions. It is important, therefore, for people considering an OWB to research existing and pending regulations in their area before buying one. Don’t count on a dealer to give you accurate information about regulations- either current or proposed or you could wind up owning a boiler that is illegal to use.

Beginning in 2007, some new OWBs were introduced with advanced combustion designs that pass a new voluntary EPA (Orange Tag) standard for outdoor units. If they perform as well in the field as they have in the testing labs, these new boilers will provide many benefits over the older technology, including more heat for less wood and less smoke and pollutants. In addition, the newer EPA-compliant units are likely to be exempt from the state and local bans currently being implemented.

Yukon Multi-Fuel and Add-on Furnaces - Burn wood/coal and oil/gas efficiently in one indoor furnace! Automatically switches between the fuels. For over 40 years, we’ve been the leader in UL approved multi-fuel hot air heaters. Multiple sizes as well as wood-only add-ons.


Greenwood Aspen EPA Approved OWB

Note that there are also some “hybrid” gasification boilers entering the market now (2008) which are basically indoor boilers housed in a shed and adapted for outdoor use. These are typically pressurized units which have smaller (5-10 cubic foot) fireboxes than traditional OWBs.

If you decide to buy an OWB, it is important to follow common sense wood burning wisdom such as:

1. Burn seasoned wood only: Green and wet wood will give you MUCH less heat, regardless of what the salesman tells you!

2. Do not burn trash, tires, plastics, etc. Doing so will not only pollute your neighborhood and the environment, but it may corrode the steel firebox of your boiler.

3. Raise the chimney for best operation: many OWBs allow for installation of a taller stack (chimney) than the short stub that comes from the factory. Extending the chimney will result in better operation and help dissipate any smoke.

4. Do not use year-round: Smoldering your wood in the spring and fall will cause excess smoke, low efficiency and may contribute to a shorter life of the steel in your boilers firebox.

5. Learn and follow both the manufacturers instructions and local codes pertaining to setback from property lines, distance to house, chimney height and other criteria for proper installation.

As with any wood burner, it is important to consider your wood supply before deciding on a particular appliance. An OWB with a large firebox will require copious amounts of firewood, often from 8 to 14 full cords per year, making them most suitable for folks with access to their own woodlots.

There are many manufacturers of OWBs, including both regional and national brands. Be certain to research the makers of any OWBs you may be considering to determine their reputation as well as the longevity and durability of their products. An OWB is a substantial investment and your unit should be built to last for at least 15-20 years.


EPA Hang Tag for OWB

With the new breed of efficient OWBs entering the marketplace, this tried and true approach to heating with wood is sure to remain popular among farmers, large landowners and other rural residents. With proper design, installation, fueling and operation, the new EPA OWBs can provide a clean way of heating with locally harvested firewood.


http://www.vtwoodsmoke.org/ - State of VT Outdoor Wood Boiler ban info

http://www.epa.gov/burnwise/index.html - EPA site detailing voluntary clean burn information for OWBs
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