hm... strangely it does not warm up my character!?
i built mine next to my kota, and when i got out of the water after swimming training, i thought i could warm up but it didnt work. But perhaps the effect was just so small i didnt notice. Anyway, it is a great way to save on firewood and slow climate change!
Btw, i finally learned the difference between a heated fireplace and a lit fire when trying out cooking recipes and i wondered why i cant steam my turnips on the open fire. I should have known because im a chef in real life... they would burn of course
It was just another situation showing me again that realistic, plain logic is just a big part of this game and can answer many of our questions.
I think a stone fireplace is giving some warmth, but not as much as an open bonfire outside. Could be enough for sweating in the coldest environment, if you wear good fur clothing... Unless you were tanning naked...
In that case, it really would break the immersion.
But doing things naked(but swimming) to keep penalties low breaks immersion for me anyway