6" x 4" x 4" Duct Wye Branch HVAC Ductwork AC Duct Fittings High Quality Sheet Metal Air Duct Fitting. SMK has made AC Repair and ventilation modification easy. Wye Branches are the correct way to split your vent duct. Easily add a new branch onto your ductwork, but don't forget the damper! Works with both flexible and rigid air conditioning duct.