A plenum is the sheet metal box attached directly to a furnace or air handler that collects and distributes air. The supply plenum connects the equipment outlet, often above the cooling coil, to the main supply trunk; the return plenum gathers air from the return ducts into the blower. Plenum size and shape strongly affect airflow and noise.
Why it matters to you
When a furnace or heat pump air handler is replaced in an older Fraser Valley home, the new unit rarely matches the old plenum's dimensions. A poorly sized or makeshift plenum transition can add static pressure, noise, and air leakage.
Practical guidance
On equipment replacement, ask whether the plenum and transitions will be built to fit the new unit and sealed at every joint. Rattling, booming, or whistling near the furnace can point to a plenum or transition problem.