How Do I Automatically Size Ducts?

Design Goals

A "good" duct design is one that reaches a suitable balance between a number of (sometimes competing) requirements, namely:

• Suitability for the application
• Reliability, which affects the system's maintenance cost
• Manufacturing or capital costs
• Pressure loss, which affects the system's operating cost
• Air balancing, which affects the commissioning cost
• Acoustics, which is an environmental cost

The parameters that you use in Plandroid will determine how these requirements are balanced in a design made by the program.

Sizing Ducts

You can automatically size the ducting in your design to the required flows using the Auto-size Ducts toolbar tool () in the Design page. This will calculate the duct flows as required to supply the specified zone requirements (regardless of the output of your selected A/C unit), and it will then automatically size the ducting to handle that flow. You can investigate your system flows after doing an automatic resize by selecting the parts in your design and noting the flows given in the part descriptions in the status bar. The flows shown are the required flows, and not the flows due to the output of your A/C unit. You can compare the required flows to the actual flows provided by the A/C unit you are using by using the Solve Air Flows tool ().

The program will search the loaded catalog for parts of the same sub-type that have the required size, and replace them as required. (Therefore, resizing will only work properly if the catalogs that were used to create the design are still loaded when you use the automatic duct sizing tool). Any parts which require a size which is not available in the loaded catalogs will be disconnected, and an error given. If you do have errors, you can use the Check Design toolbar button (, or press C) to highlight the disconnected parts so you can locate and adjust them manually.

The part sizes are determined by the duct flow velocity limits table for the current design mode in the Tools -> Options -> Design Modes options dialog. This table sets the maximum flow velocity limit for each duct branch position (main, branch, final for example).

If a downstream duct would be larger than the upstream duct because of its branch position, the upstream duct will be increased in size to match the larger duct. Duct may only get smaller as it gets further from a unit.

Parts must be properly connected to each other, and to a single A/C unit, for the sizing to work. Ducting must be properly terminated - ducts that don't terminate with an outlet or an inlet are considered blocked and carry no airflow. Outlets that service zones which are switched off carry no airflow, and will not be resized unless that is required to keep the design consistent. The resizing works only for flexible ducting, and not for rigid ducting.