My son is looking at purchasing a home where the duct work is formed into the slab. I looked under the furnace there doesn't seem to be any type of lining material. It looks like it was just formed before they poured the slab. I can see problems with this. Mold probably my main concern. Hard to add additional runs. Mildew odors. Any other issues to be concerned about?
Another concern location of furnace. Middle of the house in a closet water heater next to it. Combustion air supply?
Can a down flow furnace be converted to a up flow?
My fix for this would be to run everything in the attic. Probably not a cheap fix but well worth it.
Yes you can remove the counterflow and fill the hole with concrete, install a up flow heater and new ductwork in the attic or you can install a unit in the attic by hanging it from the framing, or you can put a baseboard system hot water heat in. All need to be sized for the house and a good design..