A plenum is a large pressurized box-shaped duct section that connects the furnace or air handler to the supply and return ductwork. The supply plenum sits on top of the furnace; the return plenum is on the bottom or side, depending on configuration.
Why plenums matter
- They’re where the evaporator coil typically lives (on top of a gas furnace)
- They distribute or collect air from multiple duct branches
- They’re sized to keep static pressure manageable
- Air filters often sit at the return plenum interface
Common plenum issues
- Undersized plenums creating high static pressure
- Leaks at the seam where it meets the equipment
- Fiberglass duct board breaking down (older installations)
- Insulation failure causing condensation issues in summer
Related: Ductwork, Supply and Return, Evaporator Coil.
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Right Way Mechanical is a family-owned HVAC contractor serving Wheat Ridge, Arvada, west Denver, and the Colorado foothills.