Using outdoor wood-fired heaters to heat a greenhouse or home is an idea that deserves praise, as using locally gathered and renewable materials (like wood!) to meet our heating needs is certainly a direction in which we should all be headed. However, the notion that such wood-burning furnaces are efficient and ecological is only partially true, especially in light of what better design and engineering can offer.

I would like to offer some other perspectives to readers who might be contemplating an outdoor wood-fired heater. (Full disclosure: I have no financial ties to any company.) I speak both from experience as an owner of such a furnace, which we used on our farm to heat our house, some outbuildings, and a couple of greenhouses, and from some research undertaken when I realized that the furnace was not as efficient as we had expected.
There are three major design flaws with virtually all of the outdoor wood-burning models currently on the market.

Design Flaw #1
Most outdoor units restrict the intake of oxygen into the combustion chamber once the temperature of the water (that will be used to carry the heat to its destination) reaches a certain preordained temperature. Some units will have a damper that will automatically close off virtually all of the air that enters the firebox, while others have an intake fan that will shut down by a thermostatically-controlled switch. No doubt that such safety features are essential in those systems, since without them, (and with a sufficient supply of wood in the firebox), the boiler could overheat, eventually boiling off the water in the system and damaging various components. However, restricting the combustion air supply produces a smoldering fire, yielding incomplete combustion resulting in significant smoke. Smoke is composed of creosote, carbon monoxide, aromatic hydrocarbons and respirable particulates.

In addition to the release of such toxic substances, the smoldering fire also indicates a loss of the potential BTUs that the wood could have provided. So not only are we spewing into the atmosphere particulates that are toxic to most oxygen-consuming mammals, we also are throwing away a significant portion of the BTUs available in the wood fuel.

A little lesson in combustion would help us here to explain what is going on inside the firebox. In a normal fire where wood is the only fuel, the temperature of the fire can, under the best of conditions (dry wood of proper size, with sufficient air) get to as high as 1500 – 1600 degrees Fahrenheit. At such a temperature, a high percentage of the original wood is oxidized, and the result of that oxidation — heat — can then be transferred into a liquid (usually water), which can then be relatively inexpensively pumped to where it is needed. However, at lower temperatures, which inevitably result when the volume of combustion air is restricted, (or when wood is wet, or when it is comprised of pieces of large diameter), a higher percentage of the wood is not completely burned off. Plumes of smoke coming out of the smokestack is an unfortunate indication of such incomplete combustion. A large quantity of “ash” left in the firebox is another indication of incomplete oxidation as wood is only 1% to 2% ash; thus what actually remains is unburned wood fuel.

Design Flaw #2
A second design flaw of most outdoor wood-fired equipment is that the fireboxes are of a very basic (almost crude) design, so that combustion temperatures rarely peak higher than 900 to 1000 degrees F. Relatively complete combustion can only occur above 1200 to 1400 degree F. Proof of complete combustion would be a minimal amount of ashes left over after the burning is complete, a minimal amount of creosote formed, and the absence of smoke coming out of the smokestack.

Design Flaw #3
Third, a portion of the heat generated in these simply-designed boilers ends up going right out the chimney. There is usually a short passageway leading from the combustion chamber into the chimney, so some of the heat is already out of the chimney before it gets a chance to be absorbed by the surrounding water tank. Proof of this is obvious to anyone who, either unintentionally or intentionally, touches (and burns their hands on!) the side of an exhaust stack of a working stove!

In our case, with our house sometimes downwind from the smokestack of the wood-boiler we installed six years ago, our bedroom was intermittently infiltrated with the smell of smoke. Although some may find this odor quaint, to us this eventually became repugnant, and we figured that there was something wrong with this situation. Installing a higher exhaust pipe can certainly help alleviate the odor in the bedroom, but does nothing to limit, if not eliminate, the smoke (and its pollutants) in the first place!

A search for answers led us to the work of Martin Lunde, a Minnesota Professional Engineer who designed a wood-fired hydronic heater that is vastly more efficient and less toxic. His firebox is comprised of two combustion chambers, in which higher temperatures (up to 2100 degrees F.) can be reached, insuring complete combustion of the wood. He accomplishes this through both the careful and detailed design of the combustion chambers coupled with an induced draft fan which insures sufficient oxygen for complete combustion. Furthermore, exhaust gases are routed through several heat exchangers, removing most of the available heat, leaving the final exhaust to be a ‘cool’ 100 degrees above water temperature. Upon complete combustion of the wood loaded into the firebox, the fire is totally out, so no smoldering at all occurs at any time. In order to capture the heat generated by such a hot fire, a huge water jacket surrounds the firebox available to store the heat. (Most other boilers have a limited thermal storage capacity.) With such a huge reservoir of hot water and BTUs, the firebox can then remain empty for many hours, or even up to a day or more, until another fire is needed. Because of the greater efficiency in burning, and in absorbing the heat generated by the fire, the owner can use at least 40% less wood to deliver the same amount of BTUs, and with fewer emissions to boot!

While there are downsides to this type of unit — one needs to build a new fire with each firing, installation of the boilers requires a little more on-site work, including placing the boiler within an existing building, and the initial cost is slightly higher — the benefits far outweigh the disadvantages. And some of the disadvantages may not be as significant as originally perceived; for example, building a new fire is actually quite easy in such a furnace, requiring little if any kindling, especially with the powerful fan to ‘blow’ on the fire, and having the furnace inside a building makes loading much easier on cold and stormy and rainy days and, at least for us, will finally give us a heated shop in which to spend our long Wisconsin winters — whether that be playing ping-pong, or fabricating our next weed-killing implement!

Martin Lunde’s website (www.dectra.net/garn/) gives some introductory information about his products and services, as well as further contact info. As he rarely, if ever, advertises, it’s usually by word of mouth that folks learn about his units.

Another very good unit for smaller applications is the Tarm wood-fired boiler. Information for this unit can be found at www.woodboilers.com. Note that this unit also requires the use of thermal (water) storage in order to attain its high efficiency rating. However, the thermal storage for this unit is separate from the boiler proper.