Finnish Sauna - Heater and Stones

Choosing a Heater and Stones

Heating a room to 100 degrees Celsius is no small task. The heater must be properly designed to be able to produce and sustain such extreme temperatures. Even if you prefer your saunas a bit cooler, the right heater and good stones are necessary for proper löyly.

Kiuas: The Heater

The heater has two functions in the sauna: heating the room and creating löyly. It was originally just a pile of stones around a fire, which made it very slow to heat and quick to cool down. Smoke saunas did not even have a chimney and the smoke was cleared from the room only just before bathing. Since one could not keep a fire burning during bathing, a lot of stones were required to maintain the heat. The fireplaces were necessarily huge piles of stone, more than a meter across and almost as high.

The chimney made it possible to keep a fire during the sauna and hence reduce the amount of stones necessary. The second great innovation was electricity, changing sauna habits entirely. The heater has since evolved into a variety of alternatives fit for different needs. After a lot of experimentation, many sauna builders are now returning to the older methods, even smoke saunas are making a comeback.

Most heaters are now designed into a steel casing. Many have a layer of insulation protecting the external surfaces from the internal heat.

While you can use gas or oil to heat your sauna, the most common choices are wood and electricity. Some heaters are heated once for the whole duration of the sauna, while others are heated throughout the bathing. The latter is most common among electric heaters.

The Stones

Löyly is an essential part of the Finnish sauna, but creating steam demands a lot of energy. The stones allow the heater to retain its heat during the bathing and to release it quickly to make löyly.

You should place the stones so that the larger ones a on the bottom and the smaller ones on top. This will allow air to pass more easily through the stones. Also, when you throw löyly, the water will hit the denser layer of small stones first, creating a lot of steam, while the larger stones remain dry and hot.

The stones in a sauna heater are obviously under a lot of stress. Constant heating and cooling as well as water and steam are enough to crack normal stones. Some stone varieties support the heat better and will last longer than others.

Some good properties of a sauna stone are:

Heat resistance
Does not release acids or bad odours when heated or wet
No cracks
Good heat capacity
Not smooth surface: to release steam faster

Some good stone minerals are:

peridotite
olivine

If you do not want to buy stones from the heater manufacturer, you can use any stones you find if they pass a stress test. Heat the stones for an hour in a fire, the drop them into a bucket of cold water. Dry the stone and search for cracks. Hit it with a hammer and grind it against another stone. If you notice cracks or if an otherwise smooth stone makes a suspicious grinding sound, discard it. (This advice from Some Like it Hot and Mikkel Aaland.)

And while a certain stone might not be the optimal sauna stone, you may have other reasons for using it: it might be from you first summer camp or it might come from Finland. At the end of the day, if you are enjoying the sauna, who cares what kind of rocks you are using.

Whatever kind of stones you use, be aware that they will wear in use. You should change the stones as often as every two years if you bathe often. If you only bathe once a month, you can probably use them for five years or more.