How it connects
The tabbed end inserts into a circular cutout in the plenum wall until the outer flange stops against the exterior. The projecting sheet-metal tabs are then manually bent radially flat against the interior plenum wall to clamp it tight.
Male and female halves
Male spigot with bendable locking tabs mating into a circular cutout/hole receptacle in a plenum or trunk duct wall.
Usually connects to
- circular hole in sheet metal plenum or trunk duct
- circular hole in fiberglass duct board plenum
Common sizes
- Typically 4 in to 20 in (100 mm to 500 mm) nominal diameter, most commonly 5 in to 14 in for residential branch takeoffs.
Where it is used
residential HVACcommercial HVAC air distributionplenum and trunk duct branching
Variations
- short tab (for sheet metal plenums)
- long tab (designed to penetrate and clinch thicker fiberglass duct board)
- takeoff collar with integrated balancing damper
- flat flange vs flared bead flange
How to recognise it
Recognized by a circular galvanized steel collar having an outer ring of slotted, finger-like sheet metal tabs around one end, positioned just past a stop flange or bead.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| Adhesive-flange or gasketed sheet metal takeoff | Uses a flat solid flange fastened with sheet metal screws or peel-and-stick mastic rather than bendable through-hole tabs. |
| Crimped male duct connector | Has continuous corrugations for slip-fitting into pipe rather than slotted bendable fingers/tabs. |
Also called
tabbed starting collartabbed takeoff collardovetail collartabbed spin-in collartabbed start collar
Good to know
Commonly referred to in HVAC trades as a tabbed starting collar or dovetail collar. In practice, after tabs are bent outward, installers frequently seal the joint exterior using mastic or foil tape to prevent air leakage.
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