How to Detect Perforations in the Air-Conditioner Duct

Visually inspect the air ducts if they're not concealed behind drywall or paneling. Look for a dent or area obviously damaged, then inspect it closely for a perforation. Use a flashlight to illuminate areas of the air ducts that are in a shadow. Use a step ladder to check the top portion of the air ducts not readily visible.

Rent or purchase an air-duct pressure-testing unit from a hardware or ductwork specialty store. Place the pressure-testing unit's flexible duct over a return-air vent. Close the other air vents in the house by rotating or flipping the "Close" latch on the vent's cover. Cover each air vent with a piece of plastic tarp taped in place, so that no residual air can escape from the vent.

Turn on the pressure-testing unit's fan, then monitor the unit's control panel for information on the current pressure in the duct. After the pressure testing unit has run for the amount of time specified in its manual, the control panel or gauge will indicate whether it has detected a perforation in the duct, and the severity of the perforation, if one is present.

Place a fog machine in front of a return-air vent and turn it on. Go into the basement or room where the duct work is located, and look for an area with fog -- which indicates a perforation. This only works if the ductwork is not concealed behind a wall or ceiling.