The supply plenum sits on the discharge of a furnace or air handler and spreads air into the trunk ducts; the return plenum gathers air back. In commercial buildings, the space above a drop ceiling is often used as a return air plenum, and anything run there, such as cable, must be plenum-rated for fire and smoke. Static pressure is usually read in the plenums.
Drill a port in the supply plenum above the coil and one in the return, and let's see what the static looks like.
Related terms
Static pressure
The pressure air exerts outward on duct walls, the resistance a fan works against, measured in inches of water column.
Return air
Air drawn back from the conditioned space to the air handler to be filtered and conditioned again.
Duct leakage
Air lost from supply ducts or pulled into return ducts through gaps at joints, seams and connections.
AHU
Air handling unit: the cabinet of fans, coils, filters and dampers that conditions and moves air through a building's ducts.