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Forest Products Extension

Outdoor Wood Furnaces

High natural gas prices have increased interest in burning wood for heating homes. A relatively new version of this old technology is the outdoor wood furnace. These wood burning systems have some advantages but also some significant disadvantages.

An outdoor wood furnace (OWF) is a wood burning stove that is located outside the house that it heats. The firebox of these stoves is jacketed in water, and the hot water is circulated through pipes that go underground into the nearby house. Once the hot water is inside the house, it can be run through radiators or a heat exchanger that is part of a forced-air duct network. The cooled water is then circulated back outside to the furnace to be reheated.

An OWF has a number of advantages over traditional wood burning stoves, and their use is increasing, especially in the Northeastern United States . OWFs are usually quite large, and thus may only need to be loaded with wood once or twice per day. The large size also means that large, irregular pieces of wood can be burned. In addition, wetter wood can be burned because the large fuel load can offset the lower efficiency of burning unseasoned wood. In addition to heating an entire house, large OWF systems can also be used to simultaneously heat shops, garages, or barns. And, despite the fact that burning wood inside is safe if properly done, many people feel that OWFs are safer because the fire is far away from the home. Finally, because the OWF keeps the wood outside, there is no bark, dirt and ash mess to clean up inside the house.

Unfortunately, there are a number of drawbacks to OWFs. They are generally expensive to install. Although they can be fitted to work with existing oil- or gas-powered forced air systems, the stove and the water circulation system by itself will usually be much more expensive than traditional wood burning systems. OWF's also require electricity to run the water-circulation pumps and fans, thus they don't provide the same insurance against power outages that a normal wood stove does. Many people also will miss the feel and look of having a fire burning inside the house.

The biggest problem with OWFs, however, is increased smoke. OWFs generally operate colder than normal wood stoves, in part because the water surrounding the fire box can reduce burn temperatures. Low combustion temperature results in incomplete combustion and, thus, low efficiency. While modern wood burning stoves are around 70-80% efficient, OWFs are only about 50% efficient. This may not be a problem for people who get their wood for free but lower efficiency means more problems with thick smoke. Low temperature wood burning results in increased levels of particulate matter and volatile organic compounds in the smoke released from the stoves. These compounds make the smoke more “smoky”: dark in color and smelly.

Traditional inside-the-home wood stoves are regulated by the EPA but OWFs currently are exempt from those regulations. However, the increased levels of smoke from OWFs have led to so many complaints that some municipalities are starting to develop new rules and regulations.

Burning wood can be a clean, inexpensive and safe way to heat your home. There are a number of good options for wood burning systems but it is important to think through the pros and cons before deciding on an outdoor wood furnace.

Information on burning wood can be found at http://www.woodheat.org/

For more information, contact:

Adam M. Taylor
Tennessee Forest Products Center
2506 Jacob Drive
Knoxville, Tennessee 37996

Phone: 865-946-1125
Fax: 865-946-1109

Adam Taylor's email