Fire, like sauna, is a keystone of my life. There is a mystery to fire that even science cannot unravel. It is more like the fourth state of matter—plasma—than the familiar trio of liquid, gas, and solid. I have played with and studied fire since I was a kid, sometimes barely avoiding serious trouble, other times under the guidance of elders like in Boy Scouts when my troopmates and I boiled a quart of water in five minutes during a fire-building competition. As I grew to an adult, burning wood became a way to heat our family home. Wood cutting and splitting became not only a chore but a workout and a way to get my angst out with each strike of the axe.
After I learned metalworking in art school, I started to apply my skills to making wood stoves and thinking about what happens inside a stove—the mysterious process where tons of fuel wood are reduced to a small amount of ash, carbon dioxide, particulate, and other emissions, carried away by the wind. The heat is generated when the atomic bonds of carbon molecules break, turning matter back into the energy of the sun that formed those molecules. There is something seductively simple in that balance of carbon in/carbon out, but as we now know, there is also something deviously complicated about the carbon cycle. I have warmed myself with wood heat over the years in my home, shop, and sauna. Each time I light a fire there is still a allure to the flames that draw me to them, calms my mind, and perhaps blinds me to greater issues.
In the past fifty years, wood stove technology has gone through several changes. Initially, stoves were simply boxes with a loose-fitting door and a chimney—like the Ben Franklin stove. These stoves burned uncontrollably and inefficiently and needed constant feeding. Later, airtightness became a thing: dampers were dialed in, rope caulk was added to the doors, and the fires were slowed down so they could burn all night.
But while the fire burned longer and cooler, not all the organic matter was combusted—more went up the chimney. Flammable wood gases called creosote condensed on the cool sides of flu pipes and chimneys, building up thick tar-like layers. Eventually, that compound would catch fire, sometimes taking the whole house with it.
In the ‘70s, the nation endured an energy crisis, and wood stoves became a very popular way to deal with the spiraling cost of oil. The ‘70s also saw the birth of the environmental movement and the Environmental Protection Agency (EPA). The EPA stepped in to regulate the developments in wood heating as did Underwriter’s Laboratory (UL). Stoves had to be made safer and cleaner burning. Expensive catalytic converters—like on your car—were added to the stove outlet to capture some of the nasty stuff. But neophytes, in their craze to burn wood, skipped the all-important step of letting firewood dry at least two years. The converters clogged up. The cats were dropped, and the focus turned to better engineering. Stoves are now designed with all kinds of baffles to get the wood to burn cleanly. They are complicated affairs, and many don’t work that great. They certainly don’t burn all night, or as I used to do with mine, burn non-stop all winter.
But sauna stoves are different beasts. Since they are “occasional use only” (only one model is UL listed) they are, thankfully, EPA exempt. Technically. But still, I don’t want to be that guy who smokes out the neighborhood every time I light my sauna—especially since this past summer when we all got a taste of the Smoke Armageddon coming down from Canada’s forest fires. So, despite my years of wood-burning experience, I continue to tweak the process and learn the idiosyncrasies of my stove. Every stove fires differently, and even a familiar stove can rebel when the wind changes or when a sauna is moved. When I light mine, I know it will smoke some. My goal is always to get it burning hot as fast as possible so combustion is near complete and the smoke will be minimal.
The three sides of the fire triangle are heat, fuel, and oxygen. A perfect balance gives a cleaner burn.
You can easily adjust the fuel and air, but the heat, not so much. The heat in a wood stove comes from the fire itself, so you need to get the stove very hot, as quickly as possible, to achieve a good balance. Above a certain temperature, wood emits gas when heated in a reduced atmosphere; this gas will burn cleaner than the wood itself. If you get your stove so hot that the wood gas burns before the wood, it will burn cleanly. (There are cars designed to run on wood gas. A heated tank of wood chips creates the gas to power the engine.)
Most stoves have a baffle or two and an upper chamber where the hot gases will hopefully combust when mixed with additional air. The real heat is at the top of stove, before it exits the flu. In my Lämpimämpi stove, the top plate that the rocks sit on is 3/8” steel. I will get this steel glowing a dull cherry red (about 1300°F). Any gas passing through this chamber will be burned. But until the upper chamber is hot, gases and particulates will escape up the chimney, and the sauna will smoke. Having a brick-lined fire chamber will help the fire get hotter faster. Wet cool days will make it worse as will a down-draft caused by the sauna being in the lee of nearby trees or structures. Wet wood doesn’t help either.
Up until this week my process has been to get a small fire going briskly, with the ash drawer open and stove door open until it starts to roar (I have an external feed, so no worries about embers falling out). Then I add larger sticks in one or two loadings until I fill the fire chamber (nothing bigger than my arm–scrap 2×4s are perfect), topping it with one or two small hardwood logs. The problem is, when I add the fresh wood on top of the fire, there is a period of incomplete combustion as the wood heats up, and the stove smokes a lot. If the wind is wrong, my neighbors will get smoked out. I tinker with the ash drawer or open the stove door to blast in more air until the smoke clears (another advantage of an external feed: I can watch the chimney.) I can add more air to balance the fuel, but I can’t add more heat. Think of it like the carburetor on an old car. Too open, it won’t run well, too closed, it sputters and smokes and clogs the engine. Not enough heat, and it won’t burn well either. I try to find the sweet spot. Unlike a wood stove in a house, I’m not worried about things getting too hot (better than too cool). It’s not uncommon for my stove pipe to glow red for a while, but that’s ok, because I know my installation is safe.
Recently, after reading an online post, I tried a new way to fire the stove (yes, old dogs can learn new tricks): from the top down! I load up the stove with thicker sticks at the bottom, then smaller, with short sticks crisscrossing between them. Then on top of this stack I put wads of newspaper with a handful of kindling and light that. The fire immediately starts heating the baffle and upper chamber as the fire slowly works its way down. This way the flames aren’t cooking the larger sticks before they are ready to burn. This solves the too much wood/not enough heat problem. Amazingly, it only takes five to ten minutes for the fire to reach the bottom, and a hot bed of coals forms quickly. I leave the ash drawer open slightly for twenty minutes and then add more wood. After that, I can’t see any smoke. When we take our sauna to one of the local parks, we can be clandestine; with no tell-tale smoke, passersby have no idea that our sauna is cranking hot inside.
I know there is a whole argument for decreasing our carbon footprint as much as possible and not burning any wood, but there is an opposing argument that says we need to maintain our ties to nature to want to save it. Controlling fire is not only as old as humankind but one of humanity’s defining traits.
Without getting into the debate, which I don’t lose sleep over, I admit again that I don’t want to be that guy. I want to remain sensitive to others and burn my stove as cleanly as I can. Learning how to master the art of fire building is one small step to take if I am going to cling to tradition and enjoy a really smokin’ hot sauna.
