Why is a flat-on-one-side 45-degree transition choking airflow?

Field answer

A 45° offset should move the duct without stealing throat depth. If only one side is sloped and the opposite side stays flat, the internal depth can shrink through the fitting and speed the air up through a choke point.

The recovery is usually a true parallel-plane offset, longer transition, or approved revised fitting that maintains the required internal area. Do not hammer the fitting into place and hope the air balance crew fixes it later.

What to check first

• Measure the duct depth through the narrowest point of the fitting.
• Compare the restricted area to the planned duct size.
• Check whether both top and bottom planes track the offset cleanly.
• Look for noise, high velocity, or pressure drop risk.
• Confirm revised fitting geometry before fabrication or install.

Why it matters

A choked transition can create pressure drop, noise, poor delivery, and TAB problems downstream.

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Final direction belongs to the foreman, approved drawings/specs, manufacturer instructions, pressure/material schedule, employer policy, and AHJ/code requirements.