Plenum (AC air distribution box)

In a ducted air-conditioning system the plenum is the air-distribution box attached to the air handler where air gathers before (or after) travelling through the ducts: the supply plenum is where cooled air collects and fans out into the supply ducts, and the return plenum is where air draws back to the unit. It's the junction that turns one air handler into a whole-home ducted system. Why it matters to a homeowner: an undersized, leaky or badly-transitioned plenum adds resistance (static pressure) and leaks expensively-cooled air into ceiling voids — so a poor plenum quietly cuts airflow and pushes up the running cost, cooling the roof space instead of your rooms. Sealing and correctly sizing the plenum and its takeoffs is part of getting a ducted system to deliver its rated airflow efficiently — which, over a long UAE summer, shows up directly in the cost of running your AC.