They are registers for an old propane furnace in the cellar (which doesn't work anymore). The big one in the foyer is the intake, and all the little ones are the hot air ducts.

I am planning to only really live in the side where the floor is torn out right now, in that room and the kitchen. I'll cut a cat door out onto the front porch and screen it in so she can poop outside of the house where I can't smell it and it is easier to clean up, and then I will shut off that side of the house so Ms Kitty can't go into the bathroom or the upstairs or the extra room that has my bed in it at the moment (it will eventually be a cat-free utility room where I do reloading and stuff that Ms Kitty shouldn't be into). So only those two main rooms will have cats, and I will move my bed into the room that has no floor right now, so that I can get Kitty Snuggles at night. :3

I plan to put a pellet/biomass stove in the room that has no floor at the moment, and then fill in the holes in the ceiling that let the heat upstairs. Then I want to repurpose the furnace ducts that run to the bathroom and the utility room into warm air vents that connect to an intake in the room next to the pellet stove. Then I will use a thermostat to turn a fan in the intake duct on and off, to blow hot air from the main living area into the bathroom and the utility room to keep it warm enough on that side of the house that the pipes underneath don't freeze.

I also plan to put small/cheap wall mount propane heaters in the utility room and bathroom, but to keep them turned down very low. Only as backup in case the pellet stove goes out or can't keep up with the dead of winter.

I should make a video about all that! :D Maybe tomorrow lol. I still need to edit today's!