Recently, you may have read the news about the fire at Driftwood Public Sauna in Milwaukee. Fire caused a total loss of the newly built facility, which looks to be a building on a trailer parked near the waterfront.
My sympathies to the owner, Derek Collins, and his team who clearly put their heart and soul into this project, which looked beautiful in the before images. As a small business owner, this is a worst nightmare scenario. Even with good insurance, recovery will be difficult (there is go-fund me page).
But let’s not all go running away from wood fired saunas with this image glued to our minds. Woodburning sauna stoves installed correctly are not dangerous.
Sure, the old Finns would build their saunas away from the house because, well, sauna fires do happen, and even the old sauna at Podunk clearly toed that line many times, evidenced by the seriously charred wood we found when we deconstructed it. But modern stoves and the associated chimneys are engineered and tested to heat a sauna. They are not cobbled together like the way the pragmatic old Finns did things, fabricating as much as they could from scrap metal. And neither should you be cobbling things together and taking safety shortcuts.
The fire is “under investigation”. Looking closely at the image of the fire, I know exactly what caused it. Fire rises; the lowest charred point is the point of origin.
I know that stove: the Harvia Legend Duo 300. It is meant to be fired from the outside— which is perfect for a public sauna where the fire tender has to do their job while staying invisible. According to the instructions, It is meant to be fired through a masonry firewall (see above instructions) with a minimum of 12“ of masonry one each side, yet in the photo of the fire I see plywood right next to the stove. I don’t want say it, but WTF?
On the inside there is (was) a tiled fire shield with what looks like the appropriate 1” air gap behind the stove extending to the ceiling. This looks nice, but a fire shield is not a firewall. A Firewall is entirely non-combustible, a shield simply protects a wooden surface behind it.
The plywood clearly got too hot (remember Ray Bradbury’s Farenheit 451: the temperature wood and paper combusts!) A hot stove will reach well over 1000°F on it’s surface, and so will anything touching it. I’ll actually let mine get to a dull cherry glow on top (that’s 1300°F) just to test my installations.
I go to job sites where there other contractors working, sometimes on my designs, and I commonly see manuals tossed in the trash and my written instructions totally disregarded.
I know reading is hard ( and harder when you turn fifty and need cheaters to read anything), but, sometimes the obvious is right in front of you: you just have to read it. I cringe when I see online images of woodstoves installed too close to combustibles. It’s not like speed limits—10 inches means 10 inches. And the sauna elf will enforce that!
Wood fired saunas can be dangerous—but so can anything installed improperly. Me, I’d rather run from a burning sauna naked then get zapped with 240 volts while barefoot. Let’s keep building wood-fired saunas, but safely.
The old sauna at Podunk had two rooms: a small dressing room and the larger hot room. The old Nippa stove sat between them, embedded in a masonry wall. Sitting on the benches we stared at the business end of the stove with its pile of rocks, and the stove was tended from the dressing room side.
This arrangement always made sense to me and is how I have been building my saunas for thirty years. I learned to weld in art school and set up my own studio soon after. Ozzie, the owner of the Podunk sauna, would send people my way for stove repairs. After seeing how other stoves failed, I designed and started making my own stoves using much heavier plate on the top. In the older stoves, the heat would soften the thinner steel and typically lead to collapse under the weight of the rocks. I also kept to the external feed (thru-wall) and designed my stoves to be fired exclusively that way. As kids, we loved to pretend we could speak Finnish by stretching vowels and jamming consonants together to make up Finnish sounding nicknames for each other. I called my stove the Lämpimämpi by combining Lemp and Memp. Finns will chuckle at this because it translates to warmer.
I called my stove the Lämpimämpi by combining Lemp and Memp.Finns will chuckle at this because it translates to warmer.
There are so many advantages of the external feed (thru-wall):
The fire-tending, and associated ash debris, is kept out of the hot room, and you don’t have to tramp in and out with your boots on to tend the fire.
Venting a small space can be complicated; a sauna stove requires significant combustion air which can create drafts, or worse, steal oxygen from the hot room. The external feed draws air from the dressing room or outside.
Any stove front requires thirty-six inches of clearance to combustibles in front of it. This can’t be mitigated by heat shields. This severely limits the layout of the hot room. However, it is easy to get three feet in front of the stove in the dressing room.
Any stove also requires a noncombustible hearth (stone) eighteen inches in front of the stove. Hot ash and coals falling from the stove are a major source of fires. In a crowded and dark sauna room, these hot coals can easily be overlooked, fall under duck boards, etc.
A flickering flame may be romantic site, but it is the soft heat off the rocks you want, not the searing radiant heat you get from sitting in front of a blazing fire.
Typically, the fire may be almost out by the time the sauna is ready. The rocks should be the focal point. Also, following the 36-inch rule above, you can’t have the stove front facing the bathers, unless the sauna is excessively big.
If you are providing a sauna experience for others, you can discreetly tend the fire without interrupting the bathers or invading their privacy.
The external feed or thru-wall stove heats the dressing room just enough to allow hanging out and watching the fire while the sauna heats up.
Installing the external feed may seem daunting, but it is not that difficult. A firewall with the requisite size opening will be required. This can be achieved in different ways: solid masonry, which will add thermal mass (the sauna will take longer to heat); or a hollow insulated firewall with steel studs and cement board, tile or stone facing or stucco over metal lathe (which I typically use). A metal sleeve will be provided with the stove to dress up this opening and provide further heat shielding. My Lämpimämpi stove has an integrated heat shield/rock basket that works with the wall opening so that fresh air coming in is heated directly by the stove and directed over the rocks. This is an advantage over simply having the rocks sit on top inside a steel box. As with any installation, all listed clearances need to be adhered to, but with this method, the stove will take up less space in the hot room and make for a cleaner presentation. For your next sauna, consider this traditional, thru-wall approach to situating the stove.
Learn more about the Lämpimämpi Sauna Stove >
The custom-built wood-fired Lämpimämpi sauna stove that is packed with rocks on the inside and mounded up on top. This minimalists’ dream sauna stove has the elegance of a well-designed sauna stove and the function of workhorse getting to the high-temps you desire for many years. Hand-built with character. This sauna stove has been featured in our mobile saunas and freestanding saunas for over 30 years. About >
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.
You’ve probably heard that I’ve spent a lot of time in and around the saunas. But another hot spot I’ve spent a lot of time around is kilns. Specifically, foundry kilns and ceramic kilns. Unsurprisingly, there is a strong relationship between the two, as they both involve getting things hot. In the lost wax casting process, investment or ceramic shell molds are heated to roughly 1500° F. The extreme heat burns off the wax original, and thus, the lost wax of lost wax casting. This can take hours or even days depending on the mold type and size. A ceramic kiln can get much hotter, up to 3000° F. That is hot enough to melt steel and many other metals.
I learned how to do bronze casting in art school. It is an ancient process, and my classmates and I did it pretty much the same way that it was done thousands of years ago. We learned to determine how hot things were by using our senses. All objects emit radiation when heated but at about 1100-1300° radiation becomes visible. Peering into a hot kiln (safety glasses strongly suggested) is like looking at another world, perhaps on some gaseous alien planet. The blast of heat through the spy-hole is like a ray gun. Solid objects become transparent. Heat and light become one; the heated molds don’t reflect light but emit light. We rarely used pyrometers (hi-temp thermometers), and when we did, it was only to affirm what our senses were telling us. We recorded the smells of things burning off. When the smells were gone, the molds were clean and ready to accept the molten bronze.
When a kiln is loaded, there is always discussion about the hot spots—certain delicate molds need to avoid the highest heat while larger molds might need it more. There is always conjecture about how the heat circulates; a whole aspect of kiln building is dedicated to controlling the flow of heat within the kiln. Some of this conjecture is borne out in the results of a firing—whether things fire correctly or not. Ceramicists use cones: small tapering forms that bend at specific temperatures. After a firing, these devices will give a true telling of how the firing went. But despite the science, there is still a lot of mystery and art to the process, so much so that a firing of a large kiln can take on a ritualistic feeling. Staying up late to tend the kiln (as is done with wood fired and other non automated kilns), drinking beer, and heating up pizza on its surface add to the aura.
Thinking of all of this casting lore makes me think of sauna. Both processes have been done pretty much the same way for millennia, involving community and an aura of ritual. Both focus on fire and heat, and even as well studied and commonly practiced as they both are, there is still a bit of mystery involved in each process.
A kiln is like a sauna on steroids. The heat is so amplified that its flow and effects are unmistakable. Observing a kiln is a lesson in thermodynamics. In the sauna building culture, there is a lot of banter about how to best heat, insulate, and vent a sauna. Yet, all of it is conjecture, based on theory, until one sits in a sauna and feels the heat radiating off the rocks and the wave of löyly hitting the sensitive tips of your ears.
When I design a sauna, I draw from my years of kiln experience. I think of the heat as a visceral substance, almost visible, as in a kiln. I relish using my senses to discern quality rather than depending on technology. Even if the sauna is electric with a digital control panel, I rely on feeling, not the number on the display. I imagine the flow of heat like the way it flows in a kiln. My foundry experience has informed my understanding of sauna in ways that are hard to describe, but suffice it to say that I have always been drawn to fire and to the mysteries that it holds.
The kiuas, or heater, is the heart of the sauna. In a wood-burner, it is commonly referred to as the sauna stove, but a wood stove it is not! There is a lot of misconception around the kiuas and how it is different from a wood stove that you might use in your house.
First, some history. The modern house stove is really a heating device designed to add comfort to your home while conforming to certain safety and smoke emission rules. Typically they are not used as primary heating appliances, unless you live in a cabin off-grid somewhere. Back in the 70s, during the energy crisis, woodstoves became popular as a way to save money. They were pretty much unregulated and varied in design from a kit that consisted of a door and a flue collar you could slap onto a used fifty-gallon drum, to a more complex Vermont Castings wood stove. Earlier stoves had little control over combustion; these evolved into airtight units that could keep a fire smoldering all night, if not for an entire season. I had one of these highly efficient stoves and didn’t let the fire go out all winter except to clean it. Cleaning the chimneys on these units is an imperative: when wood—especially if it has not been cured for two years—is burned slowly by reducing the combustion air to near nil, creosote forms. This is the result of the wood’s resinous gasses condensing on the inside of the cool chimney walls. As a result of the slow burning, these stoves emit a lot of smoke. After many houses were lost to chimney fires, safety regulations were put into place, and stoves are now required to use catalytic converters to reduce emissions, much like on your car. These regulated stoves require a religious adherence to the use of dry wood, lest your catalytic converter clog up, which they tend to do. Such stoves evolved into today’s models that use a carefully designed system of baffles and airflow to make fires burn efficiently. Now, all wood-burning home heating devices installed in the US must comply with UL (Underwriters Laboratory) safety standards and increasingly stringent EPA standards for particulate emissions. The stoves work well and are very cozy but, by design, they heat up slowly and are not meant to burn all night long not to mention all season long. Because they are intricate designs with interconnected parts, they are all cast iron. The exception is some stoves made in the pre-catalytic converter era, which were welded steel.
So, that is a wood stove. You may find a used one and think you can build a sauna around it, but the truth is, with the rare exception of one of those 70s all welded steel stoves (not the barrel ones!), you can’t. You can build a small hot room with a wood stove, but it will never be a real sauna. Here is why: A sauna stove, or kiuas, is designed to do one thing—heat sauna rocks. It is the hot rocks that heat the sauna and produce bursts of löyly steam, the essence of sauna. Early saunas did not have metal stoves. They did not even have the technology to make a metal stove, all they had was wood, earth, and rocks. The kiuas was essentially a hollowed out pile of rocks that lacked a chimney. A fire was lit within, the room filled with smoke, and after the rocks got hot, the fire was extinguished and the room cleared of smoke via vents and thereafter the rocks heated the room. The closer you can get to that smokey ideal—Savusauna experience—the better.
A sauna stove is not a wood stove; it fires hot and fast, it burns sticks not logs. Its job is to heat rocks. If fired correctly, you will never have to clean the chimney. The appropriate hot fire will combust all of the sticky wood gas and reduce creosote buildup. It is welded steel—can stand up to having water poured over it while red-hot. Cast iron cracks or explodes when subjected to this. It can take the weight of a hundred or more pounds of rocks sitting on top of it when cherry-red. Ferrous metal takes on specific colors when heated. At 1400° F, it is cherry red. At that temperature, an 1/8 inch plate of steel is as malleable as taffy on a hot summer day at the beach. I’ve repaired many sauna stoves with tops that looked like an egg carton from the stones pressing down on the hot metal. So I started making stoves (my Lämpimämpi sauna stove) with 1/2 inch thick plate at the top. I fire my stove so hot that I see dark, cherry-red glow underneath the stones. I swear that sometimes I can read a book by the glow coming off my sauna stove. If you fired your home-heating wood stove like that, you would be crazy. I like to test the limits of my stoves to know they are safe.
When you light a sauna stove, you want to fire it, that is, bring it up to temperature quickly. Use paper and dry kindling and then stuff it full of sticks, not logs (wood scraps from building saunas work great). Because sauna stoves are for intermittent use, they are exempt from the EPA particulate rules. But, the truth is once it gets going after about ten minutes, it should burn so hot that there is no smoke at all. Other than the shimmering light from the escaping heat, I can’t tell if my sauna is heating up by looking at the chimney. House wood stoves are tame devices, meant to be safe. Sauna heaters are another beast. That is why I will never install a wood-burning kiuas in a sauna in the home or attached to a house. Wood-burning saunas do burn down now and then.
If you are building your own wood-burning sauna, you may have a building inspector involved or have to get a wood-burning appliance inspection for your home insurance, and that may require a UL listing. The only heater with a UL label is the Lamppa Kuuma stove. Most others are made for the European or Canadian market, which use different standards. So, before you click on “buy” you should have a conversation with any inspectors involved. They may love the idea of a sauna, or they may think you are crazy to sit in a small hot room and throw water on a red-hot wood stove. In that case, you’ll have to convince the inspector that it’s something that’s been done millions of times without incident. In any case, you will need to make a safe installation of your kiuas. There are clearances and heat shields and floor hearths, none of which can be cheated on, unless you don’t mind owning one of the saunas that burn down. There is also combustion air to consider, which is why I like to fire mine from the outside. The sauna stove sucks up fuel and oxygen, so it’s better to not be sucking the air out of the tiny room you and your friends will be in. This is not such a problem with house wood stoves; although, it is an issue with newer air-tight construction and tiny homes.
So, before you purchase that old wood stove you find on Craigslist, do your research. Think hard about investing in a real sauna stove. The kiuas is not a wood stove. The kiuas is the heart of the sauna.
My stove pipe will get cherry red! After ten minutes, a hot fire should burn clean and smokeless.All stoves will smoke initially.Nothing bigger than your arm!This armload of ash will fire the sauna.Fill the stove and let it burn hot before closing the ash drawer.
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