The Kiuas (is not a Woodstove)

Mar 4, 2019

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 70’s, during the energy crisis, woodstoves become 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 smoldering fire all night, if not for an entire season. I had one of these and didn’t let the fire go out all winter except to clean it. Cleaning the chimneys on these was imperative: when wood—especially if it is not cured for two years— is burned slowly by reducing the combustion air to near nil, creosote forms as the result of the resinous gasses condensing on inside of the cool chimney walls. As a result of the slow burning, these stoves emitted a lot of smoke. After many houses were lost to chimney fires, safety regulations were put into place and stoves were required to use catalytic converters, much like on your car, that reduced emissions. These required a religious adherence to the use of dry wood, lest your catalytic converter clogged up, which most of them did. Those evolved into today’s stoves that use a carefully designed system of baffles and airflow to make stoves burn efficiently. Now, all wood burning home heating device 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 don’t get very hot fast, nor are they meant to burn all night long, not to mention all season long. Because they are intricate 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 70’s all welded steel (but not a barrel!) stoves, 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, that produce the burst of löyly steam and that are true heart of the sauna. Early saunas did not have a metal stove- 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 a pile of rocks. The 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 and then the rocks heated the room. The closer you can get to that smoke or Savusauna experience, the better.

A sauna stove is not a wood stove; it fires hot and fast, it burns sticks, not logs. It’s job is to heat rocks. If fired correctly, you will never have to clean the chimney- a hot fire will combust all of the sticky wood gas. It is welded steel—which can have 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 color 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 repaired many sauna stoves where the top looked like an egg carton from the stones pressing down on the hot metal, so I started making stoves with half-inch thick plate at the top. I fire my stove so hot that I see dark cherry red glowing underneath the stones. If you fired your home heating wood stove like that, you would be crazy. I swear that sometimes I could read a book by the glow coming off of my sauna stove. I like to push the limits so that I know it is safe.

When you light a sauna stove, you want to fire it, that is, to 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 they 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 heat, I can’t tell my sauna is heating up by looking at the chimney. House woodstoves 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 or attached to a house. They 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 UL label on it is the Lamppa Kuuma stove; most of the others are made for the European or Canadian market, which uses 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 which case you’ll have to convince them 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 these can be cheated on, unless you don’t mind owning one of the ones that burns 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, better to not be sucking the air out of the tiny room you and your friends will be in. Not such a problem with a house wood stove (but with newer air tight construction and tiny homes, it is).

So, before you get that old woodstove you find on Craigslist, do your research and think about investing in a real sauna stove because the kiuas is the heart of the sauna and the kiuas is not a wood stove!