I’ve seen combustion air ducts connected directly to return plenums on conventional forced air furnaces many times. Just curious of other’s views on this installation. I suggest to clients to drop this duct to the basement floor to also supply combustion air for the other gas fired appliances in the house. It seams this would also be not be an energy efficient installation considering this duct is pulling in cool outside air (I’m in MN) rather than recycling indoor air.
I see this installation enough to wonder why it’s so common and the logic behind it.
Gadget Home Inspections