A 20×20×48 supply plenum is a rectangular box used in forced‑air HVAC systems. It attaches to the outlet of an air handler, furnace, or coil and provides transitional volume for airflow before distribution into the branch ducts. Key purposes include reducing turbulence, allowing airflow to equalize, and giving a solid base for duct take-offs (branch connections). Depending on construction, the plenum might include insulation, lining, or flange(s) for easier connection.

Typical features:

Constructed from galvanized sheet metal (often 26 or 28 gauge).

Possibly lined or insulated in some models (to reduce noise, condensation, and heat gain/loss).

May include flanges or collars for ducts or evap/coil interfaces.

Used both in residential and light commercial HVAC systems.