When we lost power for 10 days a few winters back we attempted to use the fire place for heat. It was a fail. Post and beam house (large wide open floor plan) with a large transfer from 1st to 2nd floor, and apprently my lack of skill for optimizing heat over beauty in the fireplace, left us without much of a thermal bump. To this day I swear we were pulling heat out of the chimney faster than we were heating the house; I cooled the house with fire.

An open fire is not a particularly warm thing to have unless you’re directly in front of it. Most of the heat goes straight up the flue, and it uses an enormous amount of air to keep burning - it will pull huge volumes from rvertwhere it can. This is why these old buildings didn’t suffer from damp issues - the open fires burning were ventilating them.

Yeah, if you actually want to heat the house with fire you’ll want an insert or a wood stove. Otherwise most fireplaces in most houses are decorative, and one pays for that decoration with heat loss.

That’s not uncommon, but having grown up in a house heated by wood fires I knew that when building our current house. The main fireplace is on a central wall and has enormous thermal mass. Beauty and utility can be combined.

To use it effectively you want one with water jacket and just use that hot water with your normal house heating system. You don't need much power to run circulation pump so UPS + some solar panels should be enough even in deep winter. There are also systems that get it out of the exhaust but that doesn't get you much heat storage, just instant heat and generally less efficient.

Old school version of that were masonry stoves that come with ton+ of mass for the bricks and smoke being routed all over (often including a place to sleep) to take as much heat as possible from it.

If I had money for that I'd put a big hot water tank for buffer, heat it normally with heat pump, and just had emergency water-sheathed fireplate, with big buffer you can just fire it up once and have tank slowly give the heat back to the building. Or fire it up at the coldest days to save some heat pump power in days where there is barely any solar.

the fresh air inlet should be piped along the chimney walls, this would also recover the condensation heat of the water produced during combustion, but its not trivial to design while keeping in mind things like maintenance, different chimney column temperature (and thus different convective forces), capturing and effluence of the condensed water, ... the heated fresh air should not directly go to the fire but piped into the room.

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.

It's pretty much 10 to 20% efficiency for an open chimney and 80% for a good wood stove so your result isn't surprising.

Open floor plans also destroy the efficiency as the heat goes up which made your already inefficient heating even worse.

Combine both together and you probably have 5% efficiency.

Yeah fireplaces don’t make sense to me. Hot air rises and it sucks the existing heated air in the house which all flows out. The only way to heat the space is you need something with a lot of thermal mass that heats up in the process and then radiates heat. So a lot of bricks around the fire, some sort of baffle to enable the heat transfer and a system that sucks in air from the outside.