1. Purpose

Resizes a rectangular duct while keeping its cross-section area (keeping the airflow): type a new width and the height computes itself, or vice versa. No hand arithmetic and no area mistakes when squeezing a duct under a beam.

2. Quick video (≤ 2 minutes)

[Video in production — follow the steps below for now.]

3. Steps

Select exactly 1 rectangular duct to resize (round or oval ducts won't work). [image 01: selecting one rectangular duct]
Click Area sizing in the VP_MEP tab, HVAC panel.
The dialog shows the current size (W × H) and the section area. Type a new width — the height auto-computes as area ÷ new width; or type a new height — the width computes likewise. [image 02: width/height inputs with the area held constant]
Click OK — the tool assigns the new W × H pair to the duct in one transaction, area unchanged.

4. Real-world example

An 800×400 duct clashes with a beam and must drop to 300 high while keeping airflow. Select it, run the command, type height 300 — the tool computes width = 800×400/300 ≈ 1067 and sets the duct to 1067×300, clearing the beam with identical area.

5. Common errors

Symptom	Fix
"Hãy chọn đúng 1 ống gió chữ nhật."	Select exactly 1 duct (not 0, not many), then rerun
"Element đã chọn không phải ống gió."	The pick must be a Duct — recheck
"Chỉ hỗ trợ ống gió chữ nhật có W/H chỉnh được."	Round/oval duct, or locked W/H parameters — pick another duct
Zero/negative input is refused	Type positive numbers only (mm)

6. See also

Tap a branch into the main duct — resize the branch before or after tapping.
Extend a fitting along its connection — stub out a short length from a free connector.