Why does an inspector note whether a furnace's return and supply plenums are sealed at the cabinet?

The plenums are where the duct system connects to the furnace cabinet, and they carry the entire system's airflow, at the highest pressure differential in the system. Leakage there is therefore disproportionately costly compared with leakage anywhere else in the ductwork. On the return side, unsealed joints pull unconditioned air out of the basement, crawl space or attic — air that may be dusty, humid or, in a space containing a combustion appliance, carrying combustion products. On the supply side, conditioned air that was paid for is lost into the same space before reaching any room. Both effects reduce delivered capacity and can produce pressure imbalances in the house. Sealing is done with mastic or foil-backed tape rated for the purpose, not with common cloth-backed duct tape, which fails as its adhesive dries.

The plenums carry the full system airflow at the highest pressure difference, so leakage there draws in unconditioned air on the return side and loses conditioned air on the supply side, wasting capacity.

Duct leakage at the plenums affects the entire system airflow at the greatest pressure differential. Return leakage draws unconditioned air from the surrounding space; supply leakage loses conditioned air. Sealing uses mastic or listed tape.

All the Air Goes Through Here, at the highest pressure in the system. A gap at the plenum costs more than a gap anywhere else.

Where the furnace sits in a space with an atmospherically vented appliance, return leakage is also a combustion safety concern, not just an efficiency one.

• •Accepting cloth duct tape as an adequate seal
• •Checking supply but not return connections
• •Missing the combustion safety angle where an atmospheric appliance shares the space