Actually in VAV systems the air flow in the network is constantly changing, and this requires a mathematical model definitely complex.
Also the cost of the network of channels will change based on many factors.
The Excel spreadsheet not replace commercial software; it is a fast help for the...
I wrote a EXCEL macro for the calculation of the systems duct with equal friction and T-method.
You can compare the results of the two methods.
T-method duct optimization with Excel
Thank's
Dario