Central air conditioning was installed around Y2K and replaced in 2023. The air ducts and plenums were reversed, at that time, so heat now blows in from the outside walls of the house and the cold air returns are all in the center of the house, as it should be but almost never is. This system originally was installed with two zones and we later changed it over to one zone because it works better and more efficiently that way as long as the air duct loads are well balanced. (You don't need two zones in a house that is this well insulated.)
A gas fired on-demand hot water heater was installed in 2020 and rebuilt with a new heat exchanger in 2025. The gas furnace and the hot water heater both vent through the wall. Only the living room gas log fireplace uses the west chimney, which has two flues, one no longer used.
The top floor was a home office for a while in the 1990s. It has a high-output, high efficiency air conditioner, installed through the wall, which is almost capable of cooling the entire house. That unit was replaced in 2020 and has barely been used since then. The 240-volt power for that air conditioner runs up alongside the chimney and then through an exposed conduit that leads to the SW corner. Until recently, that conduit was hidden behind a built-in bookcase under the long butcher block table that fills the space under the window.