Plenum Problems That Show Up as Comfort Problems

The plenum is the transition between the equipment and the duct system. On the supply side it is the box that feeds every branch. On the return side it is the box that brings air back to the blower. When people say one room is fine and another never is, the plenum and the first few joints are as likely as the furnace itself.

What goes wrong

Tape and mastic dry out. A takeoff that was tabbed into the plenum works loose, and that branch loses most of its air into the attic or the crawlspace. You paid to heat or cool air that never reaches a grille. A return plenum leak does something sneakier: the blower pulls attic or crawlspace air instead of room air. That air is dusty, and in summer it is hot. The filter loads up and the house still feels off.

Internal liner can sag onto a coil or across the blower inlet. Airflow drops. A cooling coil freezes, or a furnace cycles off on the high limit, and the equipment gets blamed for a piece of insulation.

Clues you can see without tools

• Whistling or a flapping sound at the furnace when the blower starts.
• A room that died after other work in the attic, not after a cold night.
• Dust streaks on the outside of the supply plenum. Air leaking out leaves a mark.
• A supply grille with almost no air while the grille next to it is strong.

Do not tape a plenum with cloth duct tape and call it sealed, and do not block a return to force more air into a weak room. Closing returns makes the leak and the blower both worse.

If the equipment runs and the air is not arriving, say that on a diagnosis. We look at where the air actually goes, not only at the box that made it.

Common questions