Stop Manual Drafting: The Ultimate AutoLISP SCRIPT for HVAC Ducts

Palgaurav

Stop Using External Duct Sizers: How I Automated HVAC Duct Design in AutoCAD

Every HVAC design engineer knows the frustration of the traditional drafting workflow: switching back and forth between AutoCAD and an external duct sizer or Ductulator. You calculate the airflow, size the duct, manually draw the lines, and then painstakingly add the text annotations for dimensions and airflow.

Over the course of a large commercial project, these repetitive steps burn hours of valuable time.

To solve this, I decided to bridge the gap between engineering calculation and drafting. I developed a custom AutoLISP script that brings the duct sizer directly inside AutoCAD, automatically sizing, drawing, and annotating ducts in a matter of seconds.

Here is a breakdown of how the script works and how it streamlines the entire HVAC layout process.

The Core Features

I wanted this script to be flexible enough to handle any standard design scenario, whether you are working on local projects or international layouts.

  • Dual Unit Support: The script fully supports both Imperial and Metric systems. You can input airflow in CFM or L/s, and work with physical dimensions in inches or millimeters.

  • Engineering-Driven Sizing: You aren't just drawing boxes; the script actually sizes the duct based on your preferred engineering parameters. You can choose to size by Target Velocity or Target Friction Loss.

  • Automated Annotations: Once the duct is drawn, the script automatically tags it with the calculated dimensions and the assigned airflow, eliminating manual text entry.

  • Integrated Fittings: Creating transitions is seamless. The script includes commands for bends (allowing you to set the radius at 0.5x width or 1.0x width) and allows you to easily reduce duct sizes while specifying the reducer length.

The Workflow: Step-by-Step

The goal was to make the user experience as fast and intuitive as possible. Here is what it looks like in action:

1. Activate the Script
Simply type the shortcut SM into the AutoCAD command line to initiate the program.



2. Select Airflow Units
The prompt asks you to select your airflow unit system: Metric (L/s) or Imperial (CFM).





3. Define Dimension Units
Next, select your preferred input and output drawing dimension units (mm or inches).





4. Choose the Sizing Method
Select how you want to govern the duct size: by Velocity or by Friction Loss.




5. Input the Parameters
The script will prompt you for the specific design data:

  • Enter the required airflow.

  • Enter your target design parameter (the specific velocity or friction rate).

  • Enter your preferred duct height.

6. Draw the Duct
Finally, just click your starting point and ending point in the model space. The script instantly calculates the required width, draws the duct segment, and places the dimension and airflow text directly on the duct.

Click Here to Download

Why This Changes the Game

By keeping the entire workflow inside AutoCAD, this script eliminates data entry errors and drastically reduces drafting time. You no longer have to break your focus to crunch numbers on a separate screen. You just punch in your CFM, pick your friction loss, and draw.

Developing custom tools like this using AutoLISP opens up incredible possibilities for productivity in MEP design, and I am excited to continue expanding its capabilities.

You can watch the video Below.




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