Ever-Ready Sauna in an Apartment

Depending on the source, there are probably between 2-3 million saunas in Finland. The population is about 5.6 million and there are about 2.6 million household-dwelling units. So there are many saunas there.

Saunas are quite typical even in condos and apartments. Many of those saunas are rather small and most if not all have electric stoves. Sometimes these saunas are frown upon by the locals because the air is considered often too dry and the steam is scorching. It is certainly very easy to ruin a small apartment sauna with an over-sized stove. However, if the steam room is big enough, there are ways to build very good saunas in apartments.

While most electric stoves are small and heated up about 45 minutes before going to sauna, there are also electric ever-ready alternatives available. While regular stoves typically have about 50 pounds of rocks on top, these keep about 200 pounds of rocks hot and ready to go in an insulated silo. The stove is always on and works a bit like a thermos bottle and the temperature in theat isolated silo is kept at about 250°C (480°F). The steam room temperature stays a little above normal room temperature. When you are ready to go to sauna, you just open the latch and let the room heat up for a bit and enjoy. In ways, the main idea in this sauna actually gets pretty close to traditional smoke saunas. The stove will not be able to keep the rocks hot enough indefinitely but you cannot pour so much water either during one evening that you would run out of steam.

My brother happens to have one of these saunas so we decided not only to enjoy the gentle steam but also make a science blog of it.

We started of by replacing the traditional sauna thermometer with a Ruuvi tag. When we started the session, the Ruuvi sensors showed only 30c temperature whereas that traditional showed about 20 degrees more. Most traditional sauna thermometers tend to show too high temperatures. The heat stratification is this kind of sauna is much less dramatic than in small electric saunas because these saunas are colder in general.

When we started the session, the sauna felt a bit cold. We could have left the latch open for a bit longer time before starting the session but I liked the idea of heating up the room with steam. A bit like how snow and tent saunas work in general.

The first pour of steam quickly more than doubles the humidity from 18g/m3 to 42 g/m3 and the temperature increased by 12°C immediately as the stove kicks some of that 200°C air to the steam room. The steam is very gentle and feels humid on the skin but not burning at all. Remember, this sauna is colder than what you are used to and that absolute humidity converts to about 73% relative humidity. It is hard to get anywhere near that in traditional saunas.

We did two sessions with a short break in between. The temperature eventually rose above 50°C and would have obviously risen higher if we did the third and fourth session. Absolute humidity stayed in the mid 40’s in g/m3. 200 pounds of rocks can generate a lot of steam so we had a pretty solid constant löyly in the room. At the end of the last round, the temperature was 52°C and absolute humidity was around 44 g/m3. This translates to about 48% relatively humidity. Still very humid and pleasant but it is just good to keep in mind that relatively humidity is generally a very poor metric in saunas.

One interesting finding was also that the dew point during our session remained at 36-38°C. It is quite easy to get the dew point to increase well above 40°C. At that point the steam from the sauna stove condenses on your skin so it feels like that you are sweating more when in reality you actually sweat less because that moisture doesn’t evaporate from your skin as easily which makes you sweat less. In any event, we did sweat a fair amount.

Conclusions

Overall, the ever-ready sauna was a different and a pleasant experience. I don’t see much value in having the stove always ready to go in minutes because most people can always wait for 45 minutes to get their sauna ready. However, the concept of 200 pounds of rocks is great and the steam is very gentle. Also, Finnish apartments are generally poorly air-conditioned so this kind of sauna is a great alternative there in summer to not to get the whole apartment too warm because of sauna. This kind of stove probably works the best for people that are not notoriously looking at the thermometer but rather enjoy smoke sauna like gentle steam.

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