Plenum
Updated
The plenum is the sheet-metal box that connects your furnace or air handler to the ductwork. There are two: the supply plenum sits on the outlet side and feeds conditioned air into the supply ducts, and the return plenum collects air coming back from the house.
The supply plenum is also where the evaporator coil for central AC usually lives, and the return side commonly holds the filter rack. Plenum size is not a detail. A box too small for the equipment chokes airflow at the very first step, before the air ever reaches a duct.
Common Questions
What is a plenum in an HVAC system?
The plenum is the transition box between the equipment and the ductwork: one on the supply side sending conditioned air out, one on the return side collecting air coming back. Every duct in the house branches off one of these two boxes.
What sits inside the supply plenum?
The evaporator coil for central air conditioning usually mounts in the supply plenum right above the furnace, so the same blower serves heating and cooling. That placement is why an AC coil replacement involves opening up sheet metal rather than just swapping a part.
Why does plenum size matter?
A plenum too small for the blower it serves restricts airflow before the air reaches a single duct, raising static pressure across the whole system. Matching the plenum to the equipment is part of a proper install, and mismatches are common when equipment gets upsized over the years.
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