I know someone who gets through the winter off their fireplace. Really old timber house with riverrock chimney. Their fireplace looks nothing like what you think of a fire place looking. You can’t see the fire, there is like this big iron door in front of it. They go through a huge pile of wood every winter, along with a couple electric heaters for rooms or office.

I assume most decorative fireplaces on the other hand are not built to heat the house.

I grew up like this. We had special glass plates on the doors so you could see the fire (though this meant cleaning the glass every week).