Plenum
The sheet-metal box where your ductwork meets the equipment: the supply plenum distributes conditioned air out to the duct runs; the return plenum collects air coming back. The AC or heat pump's indoor coil typically lives in the supply plenum above a furnace.
The longer answer
Plenum size and geometry set the tone for the whole duct system's airflow: a cramped or badly transitioned plenum raises static pressure before the air even reaches a duct. It's also the natural mounting point for the big filtration upgrade — a 4–5-inch media cabinet sits in the return plenum path.
At coil-add time (adding AC or a heat pump to a furnace), plenum space decides how cleanly the coil fits — one of the specific things an in-person estimate checks that a phone quote can't.
The Seattle angle
Custom plenums and transitions are exactly what Glendale's in-house sheet metal shop fabricates — the odd geometries of older Seattle basements rarely fit catalog parts.
Where this decision happens
Nearby in the glossary
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