Building cavity space alone should not be used as a supply or return air pathway.
Return air ducts in wall cavity.
The duct layout should be considered in the initial framing design stage.
For the cavity to serve as a supply or return air pathway it must contain a sealed insulated duct made of approved duct materials.
The furnace or air handler is probably on the other side of the wall behind that door.
These include open floor truss cavities dropped ceilings and air handler platforms.
Aside from wall and floor cavities other framing cavities are also used as plenums or return air ducts.
Running a metal cold air return duct between studs requires knowledge of both hvac systems and the components involved in framing.
First building codes in many areas require that the cold air return line gets external insulation and therefore you will need to leave a gap for the insulation around the duct return line.
They should be on interior walls across the room from the supply ducts that should be washing exterior walls with conditioned air the red arrow points to a large wall mounted return air vent.
This article addresses the issue of unsealed wall stud cavities or panned floor joists used as a return air plenum resulting in risks such as indoor air quality issues due to negatively pressurized cavities drawing air through cracks along with any contaminants and also condensation by drawing in warm humid air.
Grossly undersized return air ducts on 2 three ton ac units being replaced with larger ducts to allow proper airflow.
Corrective measures are also suggested.
The original service call was to find.
When cavity spaces are used as return air pathways or supply air ducts a few issues will arise.