For those of us living out beyond the last power line, wood is the primary heating fuel. In terms of cost, it ranges between free and cheap, and it doesn’t require electricity or winter deliveries over bad dirt roads. It does require tree felling, limbing, bucking, hauling, splitting and stacking, and that amounts to a lot of work that never gets any easier. So, we owe it to ourselves to burn that wood in the most efficient heating appliance we can afford: in our wood burning stove.
I decided to do a minor rebuild of our woodstove this summer for that very reason. Over the last 17 years, we’ve stuffed over 80 tons of wood into our Vermont Castings’ Dutchwest 2460 catalytic stove. We have converted 70 cords (around 9,000 cubic feet) of pine, oak, cottonwood and juniper into 1.4 billion BTU of heat energy. This is more than enough to bring a block of ice the size of an Olympic swimming pool to a full boil. If the thing suddenly disintegrated into a pile of rust, I would scatter its remains with reverence and go buy another just like it. Put in that perspective, the couple hundred bucks I spent on the rebuild is a bargain.
Burn It Again, Sam
Light a match to a gas stove and you’ll see a clean blue flame that emits nothing more harmful than CO2 and water vapor. That’s because natural gas and propane are simple molecules. Each made up of a few atoms of hydrogen and carbon that happily bond with atmospheric oxygen to make other simple molecules. Wood is more complex. It contains dozens of high-molecular-weight substances that all burn at different optimal temperatures. Some, like water and minerals, don’t burn at all.
The whole idea of burning something is to liberate heat. This is released by breaking high-energy molecular bonds to form simpler molecules with lower-energy bonds. But, when it comes to wood, this is not the end of the story.
Ignite a pile of kindling in a woodstove and the flames spread. The moisture trapped within its cellular matrix will be converted to steam. The higher the wood’s moisture content, the more energy it takes to complete the process. Once most of the moisture is gone and the wood reaches 540 degrees F., the first stage of combustion, called primary combustion, commences. At this point, the cellulose, hemi-cellulose, lignin, and volatile oils ignite. About half the energy available in the wood is liberated in the form of heat. A number of secondary gases, such as methane (CH4), methanol (CH3OH), hydrogen gas (H2), and carbon monoxide (CO) are formed in the process.
In an old, inefficient woodstove, these secondary gases would just go right up the flue and into the atmosphere. This is a real waste, considering that they contain half of the wood’s available energy. But harnessing this energy is tricky. Secondary combustion occurs at and above 1,100 degrees F. and requires a lot of oxygen. But, because the wood still engaged in primary ignition is eating up all the oxygen that comes along, there is generally not much available.
Two Designs, One Purpose
There are two basic ways to get around the problem of volatile, unburned gases. One is to burn these secondary gases in a secondary-combustion chamber with its own oxygen supply. The other is to channel the gases through a catalytic converter that burns them at a much lower temperature.
Because a catalytic stove can burn secondary gases at an easily sustained temperature (between 500 and 600 degrees F.), it will burn wood more efficiently and hold a fire much longer than other stoves. Our catalytic stove has no problem maintaining a fire throughout the night.
On the downside, the catalyst wears out every few years and must be replaced. They cost of around $200. Additionally, you can ruin the catalyst by burning coal, treated lumber, glossy newsprint, or any of a number of non-wood fuels. Though unseasoned wood won’t permanently hurt the catalytic unit, it will muck it up, requiring you to remove the top of the stove to clean it.
Non-catalytic (Secondary-Combustion) Stoves
Lacking a catalytic unit, a secondary-combustion stove is easier to operate. Because there is no bypass damper to close at just the right stage of combustion, you can pretty much light it and let it do its thing. It is also far more tolerant of unseasoned wood and other fuels you would never dare to put in a catalytic stove.
This ease of operation comes with a cost, however. Because secondary combustion is only achieved above 1,100 degrees F., a non-catalytic stove must burn hot to sustain maximum efficiency, for once it begins to cool, the efficiency drops dramatically (as the energy-laden secondary gases then go out the chimney). Unfortunately, a drop in efficiency means a corresponding drop in burn time.
The small secondary-combustion stove in our guest cabin is a study in simplicity. It’s great for guests unschooled in the wood-burning arts. It starts in a jiffy and burns hot and clean. I love it. But a long-burner it is not.
Wood Burning Stove Maintenance
All wood stoves require a certain degree of maintenance. Chimneys should be cleaned periodically to prevent buildup of fine ash and creosote. Though there should be virtually none of the latter if the stove is operating properly. Because internal airflow is a carefully engineered process, door gaskets should be replaced every few years to prevent air from being drawn into the stove through improperly sealed doors and disrupting the flow of gases. It is a simple chore to pull out the old gasket, remove the hardened gasket adhesive with a scraper and wire brush, and apply new adhesive prior to installing the new gasket (which costs only a few bucks).
Catalytic stoves require additional steps. The catalytic unit and the baffle that protects it should be cleaned a couple of times each year. They should be replaced when they are no longer serviceable. We generally get three or four years out of a catalytic unit, a couple more out of the baffle.
Which stove would I recommend? While I love a secondary-combustion stove’s ease of operation, we invest a lot of time, sweat, and blood putting up firewood every year and we want to squeeze out every BTU we possibly can. So when our Dutchwest (which is no longer in production) finally bites the dust, I’ll probably go with a flex-stove, one designed to burn in both catalytic and non-catalytic modes. It is nice to have a choice, after all.
This article is from a previous issue of The New Pioneer. Grab your copy at OutdoorGroupStore.com.
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