An essential sauna rule not to break is the so called “Ovi kii!” rule.
As you might guess, we burn decent amount of fuel of some kind to heat up the sauna and the sauna stones. You generate steam, aka löyly, by slowly pouring water on hot stones. This very inefficient, yet traditional way of generating steam practically means that the stones cool down relatively fast on their own and especially if you pour water on them.
The diagrams below show whether the (electric) stove is on or off and what happens to the stone temperature as you pour water (notice the peaks in humidity as I added water). Generally speaking, you get the best steam when the rocks are above 200c hot. If you see the steam, the rocks are too cold.
So löyly is precious and you can only get so much of it in a limited period of time. If you pour too much water on the stove, you’ll have to wain until the rocks heat up again. The challenge is then that if you keep the stove on for too long to re-heat the rocks, the sauna will get too hot.
This long intro should perfectly clarify why it is a crime to keep the door open in a sauna. However, many sauna novices don’t obviously know this so they often like to keep the door open to look inside and then slowly decide whether they want to come in to the sauna or not.
This ignorance will almost certainly result in more experienced sauna patrons yell “Ovi kii!” to you.
Ovi kii! this translates to “Shut the door for god sakes, you no-brain sauna noob! Now you ruined the steam and we have to do all this again”.
Passive-aggressive solutions to the problem
My first iteration to solve this ovi kii problem was to add a sign above the door. Not outside, to remind the person coming in but inside to remind, how to shout that phrase. The efficiency of this mitigation effort was neglible.
Luckily, Amazon is full of cheap electronic components, so the next iteration was to purchase a door sensor that would integrate with Home Assistant. This allowed us to build a logic where if the door remained open for three seconds, a smart speaker would yell Ovi kii in the sauna! This dramatically reduced trauma in my lungs.
But as opening the door while there is no löyly in the air is dramatically less serious demeanor than opening the door right after someone just added stream, we had to improve the logic with humidity sensor that uses even stronger language during those very misfortunate events.
All in all, we have now achieved the ultimate solution for the Finnish people: getting everyone in line without having to say a word. Hooray technology!
Hakka-Sauna 