High Efficiency and Combustion Air from Inside...thoughts

[ICE]

Expanding on JAR's example, with the furnace being assumed as installed within a basement would the basement now be considered a plenum?IPC 502.2 Rooms used as a plenum.

Water heaters using solid, liquid or gas fuel shall not be installed in a room containing air-handling machinery when such room is used as a plenum.

No.

Plenum.

An air compartment or chamber including uninhabited crawl spaces areas above a ceiling or below a floor, including air spaces below raised floors of computer/data processing centers or attic spaces, to that one or more ducts are connected and that forms part of either the supply-air, return-air or exhaust-air system, other than the occupied space being conditioned.