How do you take a sauna bath with the new fire mechanics?

[PALU]

I've tried several times with various amounts of firewood (20 on the last attempt) but never managed to satisfy the requirement to have hot sauna stones to use the vasta. I just get burnt out fires in the sauna (built by the character, but it is the new sauna stove, not a regular one). In the latest attempt I lit the fire, got the message that it will burn for a couple of hours, moved one tile to the side of the homestead and cut down two trees (which probably took 1-2 hours), and then returned to take a well deserved bath, but finding only burnt out ash in the stove. I even closed the door to the sauna (which I don't think matters to the game). It can also be mentioned that I have been able to smoke meat in the sauna, so it's definitely not completely broken.
If trying to take a bath shortly after the fire is lit the stones aren't hot either (which makes sense).

[Matti-patti]

4 branches and 30 firewood has been about the minimum for me. Mild steam after about hour and half of burning and proper steam for a while after three hours and the fire dies around the same time. Tested in end of Soil with temperature around zero, with a pre-existing burnt-out fire in the fireplace.

[PALU]

Thanks. That's a fair bit more than I've expected.

[JP_Finn]

Makes sense though. Modern stoves might heat up plenty good* with 12-15 firewoods. But good old smoke sauna stove, needs to be a heated for at least 6 hours, then water thrown on the stove for ‘häkälöyly’ aka ‘carbon monoxide steam’ with door and shutter(s) wide open. Wait for few minutes for safety, close the door (shutters can stay open for some time)

Or that’s how we did it. Other families, areas, regions might have different ways. And for the 6h burn, in game wood amount isn’t that much IMO.

*for the initial heat, longer bathing sessions, folks need to add few more firewood every 15-30min

[PALU]

I have no experience with fire heated saunas, only with normal fire places, in which you couldn't even fit 10 firewood in one go (or maybe you could, if you arranged them for storage rather than burning). However, heating over a length of time (abstractly feeding the firewood a little at a time) makes a lot of sense, rather than just an initial bonfire.

Thanks for the insight.

Edit:
I finally managed to take a sauna bath, but it's very fiddly.
- I lit 35 firewood in the sauna in the early morning.
- Started some building work, which was cancelled at 100% fatigue by noon.
- Went to the sauna and was informed that I wasn't warm enough (and the fire was out by that time)
- Lit a new fire from 10 firewood
- Not warm enough
- Waited to warm up
- Got warm and managed about 5 vasta sessions before I was informed that the sauna stone wasn't hot enough, and that was while the second, 10 firewood fire was about to expire, which indicates to me that the system doesn't allow for a gradual build up of the sauna temperature or for extension, but everything has to be done in one big lump.

Thus, the window during which you can actually take a bath seems to be rather narrow