Definition
The central air distribution box attached to your furnace or air handler where supply and return ducts connect.
Why this matters for your home
The plenum is the large sheet-metal box mounted directly on your furnace or air handler where the main ducts connect. The supply plenum distributes conditioned air into the duct runs, and the return plenum collects air coming back to be reconditioned.
Because all of your airflow funnels through it, a poorly sealed or undersized plenum quietly undermines the whole system. We check plenum sealing and sizing during installs and major service, since fixing it here improves airflow to every room downstream.