Hello Erkka! Thank you for the wonderful blog posts, pictures, and all of your creative endeavors. You are making me want to take a trip to Finland when this virus settles down! As I was looking at the DIY sauna stove drawing I had a thought...have you ever heard of Rocket Mass Heaters? They're wonderfully fuel efficient as well as very easy to build, they operate similar to the first drawing however the flue is longer and the firebox is smaller. They take advantage of a large thermal mass being heated by the super hot gases and smoke flowing through the flue. I'm thinking it would be very easy to create a variant of one specifically for sauna use, as in using sauna stones as the thermal mass the heater needs to operate effectively. Here's a link to a great homesteading and permaculture forum where I learned about these heaters: https://permies.com/f/260/rocket-mass-heaters

Thanks again for everything, I've only recently discovered UnReal World and I have to say it's one of the best games I've played in a long time!

PS If you are interested I can send you a PDF that explains how Rocket Mass Heaters work in greater detail and includes plans on how to build one yourself