by Jerry Peck - Codeman on Wed Jul 28, 2021 7:17 pm
David, are you sure it is a plenum?
If so, then the water heater access could be an issue due to the plenum needing to be sealed up from the garage while also maintaining access to the water heater.
Sometimes things are just 'environmental air' closets and not a plenums. An example is a plenum closet in a hall way with a door (louvered or solid) with a water heater installed below the air handler unit. That would typically be considered environmental air and not a plenum ... unless the closet was sealed off from the hall way (solid door, with weather-stripping and a threshold which seals tight to the door, in which case it might be a plenum.
I don't have my codes with me, but I recall conversations about cable types suitable for use plenums back to around that time, however, development on those cable outer sheaths stopped (I don't recall why) and discussions started back up in the 2000s about plenum cables being rated. Thus the code may not have addressed that until the latter time frame, or even later.
Jerry Peck - CodeMan
AskCodeMan.com
Construction Litigation Consultant - Retired
Construction and Code Consultant - Semi Retired