Proper Flex Duct Connection Procedure
Before attempting a connection of any kind, all mating surfaces must be clean and dry.
Mixing Box Taps:
The hole in the mixing box should be cut round and slightly smaller than the tap Outer Diameter (OD) top assure a snug fit. The edge of the hole should be coated with a two inch wide 1/8th inch thick layer of UL 181 mastic. The tap should then be slid into the hole and the fingers bent back against the inner wall of the mixing box. The tabs should be bent sufficiently to make a tight fit between the tap flange and the mixing box out wall. A bead of mastic should be squeezed out of the mating surfaces. A thin 1/8th inch layer of mastic should then be painted over the flange and mixing box wall. This will sandwich the flange with an inner and outer layer of mastic.
Flex Duct Connections:
The male mating surfaces, whether they be the mixing box tap collar or the boot throat, should be painted with a 1/8th inch layer of UL 181 approved mastic. The inner liner of the flex duct should then be slid over the mastic covered mating surface and mechanically fastened. A second layer of UL 181 mastic should then be applied to a thickness of 1/8th of an inch to the outer surface of the inner liner. This mastic should extend beyond the inner liner all the way to the tap or boot flange. The outer wrap should then be pulled over the connection, snugged to the mating surface, mechanically fastened, taped to the mating surface, and sealed with another layer of UL 181 approved mastic to the interface.
The mastic must be permitted a minimum of 24 hours to cure before the hvac system can be operated.