Outdoor Wood-Burning Furnace: What You Need to Know

New technology takes heating with wood to the next level with an outdoor wood-burning furnace.

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Outdoor wood-burning furnaces might make one feel a little safer for the combustion occuring away from the home. Photo courtesy Central Boiler
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Even though this technology takes heating with wood to the next level, properly storing and seasoning firewood remains important to obtain the cleanest, most efficient burn. Firewood should be seasoned for at least a year before burning. Photo courtesy HeatMaster
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Put a lot of thought into where you locate your device. Wind direction (smoke) and the woodpile are two key considerations. Photo by Brandon Hodgins
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It’s always a good idea to take lots of photographs, document splices, and measure out where things are located for future reference. Photo by Brandon Hodgins
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Hot-water supply and return lines need to be properly insulated, and the author recommends using a 12-inch balloon of closed cell foam for this application. Photo by Brandon Hodgins
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A 4-foot trench, spanning 140 feet, that ran from the outdoor wood-burning furnace to the house was a lot of work, but you’ll be glad when it’s done exactly the way you want it. Photo by Brandon Hodgins

The wood burners of the past have been much-improved and refined with better efficiencies for producing heat. There are modern versions of the old potbellied stove that are both stylish and capable of heating a large area. Add-on wood furnaces can be adapted to existing fuel oil and propane furnaces providing an optional or dual form of heating. In more recent years, the popularity of outdoor wood-fired boilers (OWBs) has increased substantially for people who have access to wood and are looking to replace traditional heating methods.

How do OWBs work?

Wood-burning hydronic heaters – also called outdoor wood-fired boilers or furnaces – are residential or small commercial wood-fired water heaters that are located outdoors away from the space being heated. Typically, a water jacket surrounds the furnace’s firebox heat exchanger and heated water is circulated to the home or other structure through insulated underground pipes. Once the heated water is brought into the building, it can be designed to work with existing heating systems. Water-to-air or water-to-water heat exchangers or direct water circulation conveys the heat generated into forced air furnaces, radiant baseboard or radiant floor heat. OWBs have the ability to heat multiple buildings from one furnace. Garages, workshops, greenhouses, barns and even swimming pools or hot tubs can be heated with OWBs.

System benefits

There are many manufacturers of OWBs in the U.S., with most located in the upper Midwest or the Northeast. Most manufacturers seem to agree in their literature and on their websites about the benefits of using outdoor wood-fired boilers. These benefits include reduced utility costs, increased safety and cleanliness by locating wood burners outside, the use of a renewable abundant energy resource, a zero net carbon contribution when wood is burned efficiently, and overall total efficiency of today’s OWB. Also located in the manufacturers’ literature and on their websites are numerous testimonials from satisfied customers who are pleased with their purchase.

Health issues

With the increase in popularity of OWBs, questions have been raised about the health effects of smoke produced by the units. According to the Environmental Protection Agency (EPA), there are more than 200,000 OWBs installed in the U.S., with another 14,000 units expected to be sold this year. The EPA states that there have been many studies that report potentially serious adverse health effects from breathing smoke emitted by residential wood combustion. Smoke contains fine particle pollution and a number of air toxins. Fine particle pollution is linked to a variety of health problems including asthma, reduced lung function and bronchitis. Older OWBs manufactured before 2007 tended to be less efficient in burning wood which translates into more smoke and fine particles.

The manufacture of OWBs has never been regulated, but to address some of the health issues, the EPA launched a voluntary program for manufacturers of OWBs to demonstrate that their models are 70 percent cleaner than unqualified models. The goal of the program is to achieve emission reductions and protect public health without a federal rule.

New technology

The voluntary program initiated by the EPA has produced some desired results, as manufacturers of OWBs are rising to the challenge and creating more efficient units. New certified OWBs burn the wood gasses more efficiently, causing less smoke. Increased efficiencies mean owners will also save money and resources by having to burn less wood.

Many of these new OWBs use gasification technology to burn cleaner and operate at a higher efficiency then traditional wood boilers. The reason older OWBs have come under such scrutiny as of late is because of all the smoke they emit with fine particulate matter.

Gasification is a process where the emissions in the smoke are burned in a secondary chamber at a very high temperature. This process greatly reduces the particulate matter and increases the efficiencies of the units to 70 percent or higher.

Purchasing tips

If you are interested in purchasing an OWB, the Michigan Department of Environmental Quality has some tips. Look for a well-designed unit that utilizes secondary combustion, because they burn more efficiently and can be a good home heating option in rural areas. A few newer models have added technology to more fully incinerate particulate and gas emissions. Due to higher heating efficiencies, they use one-half to one-third the amount of wood to generate the same amount of heat and demonstrate much cleaner operation than most current models. Expect to pay more for a good quality unit – with OWBs you definitely get what you pay for. They also listed the following tips:

• Look for a unit with a lot of firebrick, which allows the unit to burn hotter with better combustion.
• Be a good neighbor – make sure the stack is at least as high as the chimney on your home.
• Ask for proof if the unit’s performance claims seem exaggerated.
• Only dry, seasoned wood should ever be used in wood-burning units.

Another source to find an efficient OWB is the EPA’s website Burn Wise. Look for the Hydronic Heaters Program, which lists the manufactures of OWBs that voluntarily submit their units for testing and approval.

DIY Installation: Do It Once, Do It Right

By Brandon Hodgins

With any outdoor wood-burning furnace purchase, there’s a major decision that needs made right away: Where do I want to locate it? Make sure the wood-burning furnace is placed in a convenient location. Can you load the stove easily, and back your truck or trailer right up to the woodpile to easily unload the day’s haul of firewood? If the answer is no, choose another location. Think about the prevailing wind to make sure the smoke doesn’t become a nuisance.

Another important decision is whether to purchase the prefabricated supply and return lines, or to make your own. The prefab lines work well, but they can come with a bit of sticker shock. They sold for up to $12 per foot when I was buying, a number that grows quickly when a great distance lies between the stove and the home to be heated.

If you’re brave enough to do it yourself, have a look at how the prefab lines are built and try to mimic that design. When I installed my supply and return lines in the spring of 2014, I dug a 4-foot trench that stretched 140 feet long. I blasted the bottom of the trench with around 4 inches of spray-foam insulation. I used wire ties and zip ties to hold the supply and return lines together, with 1-inch strips of foam board spaced between them. After placing the supply and return lines securely into the trench, I sprayed a 12-inch balloon of foam all the way around the lines, leaving no gaps for air or water intrusion. It’s been working wonderfully, as the 185-degree water coming from the outdoor stove barely loses any heat during its 140-foot trek to the house.

It’s important to use closed cell foam for this application. Soggy, wet foam loses its insulating properties in a hurry.

Before backfilling the trench, think about killing two birds with one stone. Have you been wanting to run a water line from the house out to the barn? Do you need to tie into your home’s electricity to run the wood furnace pumps? If so, save time and money by doing it now. If you’re burying a domestic water line in the same trench, bury it underneath the supply and return lines to be sure the frost will never make it down to freeze up your freshwater supply.

Avoid adding any splices into the lines underground, especially your hot-water supply and return lines. If a splice is an absolute must, measure the distance of the splice between the home and the wood furnace. Photograph it and document it as accurately as possible. This will make it much easier to find and repair if the splice fails in the future.

Cutting wood is a healthy lifestyle choice. Outdoor wood burners reduce dependency on liquid petroleum, fuel oil and natural gas, and they move the combustion from inside the home to a safe distance outdoors. Do it once, do it right, and reap the benefits of our most renewable resource for decades to come.

Tim Nephew, a freelance writer living in Minnesota, loves the outdoors and manages his 80 acres for wildlife. Tim is a regular contributor to the pages of Grit.