As anyone who routinely heats their home with forestry products can tell you, wood is a fairly energy-dense fuel. Pound for pound, well-seasoned hardwood contains nearly one-half the heat energy of pure natural gas and, curiously, over three times the heat energy of TNT. Add to this the facts that wood is completely renewable, carbon neutral and—depending on where you live—somewhere between affordable and free and it quickly becomes a very sensible way to heat your home.
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Today, there are many appliances designed to fulfill that purpose. Some, such as ordinary fireplaces, do a poor job of it. Others, like high-tech, computer-modulated, firebrick-lined wood furnaces, reach heat-conversion efficiencies nearing 100 percent. In between (where most of us live), there are fireplace inserts, pellet stoves, indoor and outdoor wood-fired boilers and woodstoves spanning a broad spectrum of efficiencies.
And then, set off in a class by themselves, are masonry heaters (also known as Russian stoves, Finnish stoves and Scandinavian fireplaces, among other names), massive structures of block, brick and stone that just may be the answer for anyone heating with wood in a cold-weather climate.
Whenever you set a match to a pile of wood inside a wood-burning appliance, you do so with the hopes that the greater portion of the wood’s available chemical energy will be converted into heat. This is not as easy as it may sound. While seasoned wood will readily burn to ash in a reasonably short time, that’s no guarantee that most of the wood’s energy hasn’t gone up the stovepipe unutilized.
The problem lies in the fact that wood is a lot trickier than it seems, since to really get the most out of it, it needs to be burned twice. The first burn, aptly called primary combustion, takes place at relatively low temperatures and breaks down the lignin, cellulose and hemicellulose comprising the wood fibers. About half the wood’s energy is released as heat during primary combustion.
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The other half, however, is contained in the gaseous products of primary combustion, such volatile compounds as methane, methanol, hydrogen and carbon monoxide. To salvage the heat energy in these gases, they must be burned at or above 1,100 degrees Fahrenheit in the presence of sufficient oxygen. Fortunately, virtually all modern wood-burning appliances are designed to enable secondary combustion. But that doesn’t mean they are all able to prevent a large portion of that reclaimed heat from rushing out the stovepipe.
The niggling truth is that the secondary burn produces heat faster than most woodstoves are able to store or radiate it, and consequently much of it is lost. And therein lies the beauty of masonry heaters. Because they are so massive—most weigh in at well over a ton—the stove itself becomes a heat battery, able to store large amounts of heat that it eventually radiates out into the room. The trick is to keep the hot exhaust gases inside the matrix of the stove for as long as possible—exposing it to as many surfaces as can reasonably be placed in its path—and urging it to give up its precious heat all along the way.
This is accomplished by forcing the exhaust gases through a series of baffles laid out in a serpentine arrangement, so that the gases spend more time flowing sideways and down than they do flowing up. The reason this works is because the gases are so hot when they enter the baffle chamber—as high as 2,000 degrees Fahrenheit—that they are naturally compelled to rise against all obstacles. As they do, they give up their heat to the interior mass of the heater.
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There are no slow, smoldering burns inside a masonry heater. By design, every fire is hot and furious until all the wood is consumed. And then the heater does its magic, slowly radiating that heat outward over the course of hours and even days.
For a properly sized masonry heater, one quick, hot burn every 24 hours is sufficient to warm a house all day and night, although an evening burn may sometimes be needed during stretches of especially cold weather. It’s not a lot of work, but is a chore that should not be neglected, since once the stove cools it can take days to warm it up again (a fact that has prompted some homeowners with hydronic backup heating systems to route hot-water piping though the stove’s interior to pre-warm the stone).
Heavy Heater Placement
Owing to their considerable mass, masonry heaters are most practically installed in new construction. Unlike a woodstove, which can be safely set upon a hearth above an unreinforced floor, a solid concrete (or concrete block) foundation built atop suitable footings is required to support such a heavy masonry structure.
The good news is that your masonry heater will be assembled onsite from manageably sized pieces, even if you choose to go with all or part of a kit, so it can come in through the door rather than through the roof. Masonry heaters are infinitely versatile in their design. Though most effective when placed in the center of a room, you can also build one into a corner space or into a wall like a fireplace. Horizontally designed heaters—often used as room dividers—are equally as effective as vertical ones.
Fifteen minutes on the internet will convince you that there is no end to the design possibilities. The only limitations are your imagination and, of course, your budget. (One site I found especially helpful both visually and imaginatively is inspirationgreen.com/masonry-heaters.html)
Unless you plan to do all the work yourself using locally sourced stone, expect to pay upward of $10,000 for a good masonry heater. Complete kits are available, most notably from Tulikivi, a Finnish company that has become the world leader in prefabricated masonry heaters. Tulikivi uses soapstone as the heat-transfer medium, which is a good choice since it can store up to three times more heat than conventional stone.
Or you can go half and half, buying an affordable prefab core—in pieces, of course—and then facing the heater core yourself with the stone of your choice. This allows you to bypass the tricky business of designing and executing an intrinsically confounding system of baffles while still offering you the creative freedom to determine the heater’s ultimate look and feel.
If you do decide to build your heater from the ground up, get ready to ascend a steep learning curve. Plans are available on the internet, but before you dive in, you should be well versed in both theory and practicalities. Whether you build from scratch or start with a prefab core, there are publications that can assist you. Masonry Heaters: Designing, Building, and Living with a Piece of the Sun by Ken Matesz is a good place to start.
There are several good reasons for choosing a masonry heater over other wood-burning technologies, but the one I find most satisfying is the fact that masonry stoves, heaters and fireplaces were originally built by true 16th century craftsmen who faced a driving need to heat their sub-arctic homes with scanty resources.
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This article was originally published in The NEW PIONEER™ Winter 2016 issue. Subscription is available in print and digital editions here.