Outdoor Furnaces
A “Back-to-the-Land” Alternative - Part I

An outdoor furnace is aptly named. It is a furnace that operates outdoors - but fills the needs for indoor heating. Outwardly, it resembles a small utility shed complete with a peaked roof and door.However, inside the protective shell is a wood-fired stove that heats a water jacket. The heated liquid in this boiler apparatus is water mixed with a mild rust inhibitor which is then pumped through underground, insulated piping to the home or other buildings designated for heating. These satellite uses can include barns, hot water tanks and swimming pools as well. In a standard outdoor furnace unit wood products from chips to four-foot logs are stacked in the firebox. The heated water is pumped right into a standard hydronic - or hot water-heating - system or it flows through a heat exchanger where a fan blows the heated air produced through exiting ductwork.

Picture courtesy of Taylor Water Stoves

This exchanger can be mounted on an existing furnace which is then used as a backup unit in case there is a problem with the system. There are several types of fuel than can be used for outdoor furnaces but we are going to first concentrate on outside wood burning units, or OWB’s. So what are the advantages of the outside furnace or OWB? It eliminates the fire-hazard of indoor wood-burning products. Placed away from brush and wooded areas, and mounted on a cement pad, the OWB is an insurer’s dream. There is no mess in the house or airborne pollution associated with the indoor units. Gone is the trail of wood chips, bark and dirt associated with carting wood inside, as well as the mess of ash and the dust that finds its way through the home. The OWB burns a reasonably cheap, abundant and renewable fuel: wood. Environmental; it saves on the effluent entering the air from fossil fuels.

However, the main attraction which towers above the rest is basic economics. The average OWB in Wisconsin, a state with a respectably cold winter, uses 10-13 cords of wood a year depending on type of wood chosen. (Hardwood would be more expensive even though it burns more efficiently and seasoned, fireplace-ready pieces may negate any savings over gas or oil.) Another advantage is that most of the OWB units can accept up to four foot lengths of whole, non-seasoned logs. This brings down the price even more over the indoor-version cut, dried and split variety. So even at $150 a cord for green wood (which is on the high side) the heat and hot water bill for a 3000+ sq. ft. home would be less than $2000 per year. Owners of barns and small manufacturing buildings give OWB's rave reviews. One small furniture plant in Nova Scotia fuels its whole operation on waste wood products which is a free byproduct. However, these commercial operations also have the advantage of forklifts and the ability and manpower to cut and process raw logs.

Diagram courtesy of The Wood Doctor

The cost for setting up an OWB ranges from $3000 to $10,000 depending on the size, labor and amount of extras needed. If you are handy, fit and have the time you can hand-dig the trench for the insulated piping but most homeowners get an excavator or rent a trenching tool so you are sure to get below the frost line. Then there is the insulated pipe, exchangers, fittings and accessories needed depending on whether your home is hot water heated or forced air. For those with an oil or gas furnace already installed this can be used as a back-up in case there is a malfunction in the OWB system. Most units are touted as good for 25 years and there are many designs out there. The drawbacks? Well, oil and gas require no lifting and you don’t have to go outside in a snowstorm to fill the firebox. (Oh, that’s another item I almost forgot to mention: the fireboxes have to be loaded twice a day in most models.) Even with a loading table beside the firebox a person should build an outer shed to protect the loading area from the elements. These two items, of course, add to the cost. I have also read some blogs which claim that the most efficient OWB’s have a firebrick lining and added insulation for better heating. This, of course, adds to the cost of the unit and shipping.

As well, there is an air quality debate on OWB’s. The design of the units is supposed to foster a slow burn to enhance heat transfer and burn less fuel. The cooler fires, however, have been reported to be inefficient and the source of clouds of smoke and creosote build-up. And many OWB owners burn trash, cardboard and even tires which leads to more elevated smoke levels and unpleasant odors.

This drawback has prompted the Environmental Protection Agency to take a long, hard look at OWB’s. In 1988, the agency established limits on emissions for indoor residential wood heaters due to the air pollution build-up in populated areas, especially in places where the airflow is light and the smoke can linger in dense clouds. As OWBs are the new kid on the block there are presently no regulations in place.

Diagram courtesy of Timber Ridge Inc.

However, government agencies in New York, Connecticut, Maryland, Massachusetts, Michigan, New Jersey and Vermont have petitioned the EPA to enact concrete federal laws on outdoor furnaces. As I write this article a review of OWB’s is being considered. So are they bad or good? For a rural home with plenty of available fuel OWB’s are a wonderful idea, especially if there is more than one building to heat – bar, garage, workshed, swimming pool. And the manufacturers have not been idle. Newer designs, where the smoke is re-combusted like a turbo-charger, have cut down on emissions and improved heating efficiency. For those of you in farming areas furnaces are now in use which burn hay bales and silage. Dried flax and other cereal chaff have been proven to have a higher BTU level than most woods. These units employ an automatic stoking mechanism for better burning because of the compact nature of straw. My suggestion: Go to your local distributor and find people in your area who own an OWB. Phone two or more of these owners and ask if you can go out and watch a demonstration. If your expectations match the benefits these owners are getting from their units, taking into account the labor and fuel accessibility, then you have got yourself a self-sustaining heat source completely independent of the fuel markets.

Oh, did I mention you might want to think about a battery back-up or small solar-electric power plant in case there is a power outage? Unfortunately, pumps and fans require electricity. So, in Part II of this article, I am going to take my own advice and see an actual OWB operation. Stay tuned!

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