Running 3.72beta on Windows 10 via Steam. Installed mods: BAC 3.72b-R2
In earlier versions the fireplace could be used to make a sauna, and this is stated in the documentation both in game, on the wiki and in the forums. Using the new beta, the fireplace would not work as a sauna. Solution: Dismantle fireplace then build sauna stove.
This may be a problem caused by the BAC mod? If not then it needs a fix as a sauna is part of the advanced game course. If this is intended, then the documentation should be changed to reflect that.
That is nowadays intended, and only sauna stove will work for sauna purpose.
The change came with the version 3.63 with option to build sauna stoves by player character, and changes to mechanics of throwing water on the sauna stove.
I quickly checked out advanced game course information regarding sauna bath, and to me it speaks of finding a sauna in particular - although any fireplace worked earlier. But there are layers of old mechanics described so that is something to change.
(Wiki updates are required here and there, constantly, and especially after each new version, but that remains as a community thing.)