Made-to-Order Duct Transitions & Reducers

Why transitions almost always need to be custom

A transition exists to connect two different duct sizes — say a 20×8 supply trunk stepping down to a 14×8 branch. The number of possible inlet/outlet/length combinations is effectively infinite, which is why supply houses can only stock a handful and why field crews spend so long fabricating them by hand.

Made-to-order fabrication solves it directly: you specify both openings and the taper length, and the flat pattern is developed and cut by CNC so the finished piece meets each duct exactly.

What to specify

• Inlet width × depth (the larger opening, usually)
• Outlet width × depth (the smaller opening)
• Length — the taper distance between them; longer = gentler angle = less turbulence
• Concentric or eccentric — centered, or flat on one side to sit on a joist or ceiling
• Gauge and material — 26–20 gauge galvanized, aluminum or stainless
• Connection type at each end — slip-and-drive, TDC flange, or raw edge

Taper length matters more than people think

A short, abrupt transition is cheap in metal but expensive in static pressure — it creates turbulence, noise and pressure drop that the blower pays for continuously. SMACNA guidance favors a gradual slope, commonly around 15° per side or gentler where space allows.

Because you control the length when you order, you can dial the taper to the space you have instead of accepting whatever a stock fitting forces. When headroom is tight, a shorter transition is available — you just make the tradeoff knowingly.

Reducers: the round cousin

A reducer does the same job in round duct — stepping a larger diameter down to a smaller one over a set length. The same custom inputs apply: large diameter, small diameter, length, gauge and material. Concentric reducers center the two; eccentric reducers keep one wall straight for drainage or clearance.