Drain System and Static Pressure

Somewhere in the drain there will (usually) be a trap to prevent unwanted air from moving in or out through the drain pipe. The net depth of the trap must be greater than the maximum expected static pressure of the AHU.

"causes/disadvantages for improper design"

Causes: no relevant knowledge, experience or education of the designer.

Father/uncle owns company allowing someone with, no relevant knowledge, experience or education to be the designer.

Disadvantages: The thing won't work right. Someone will lose money, hopefully a father or uncle.

You might consider some mechanical engineering courses, if you want to design stuff.

The following link may help you:

http://0323c7c.netsolhost.com/docs/Trane_-_Trapping_Design_Flaws%5B1%5D.pdf

or google for condensate drain or U trap drain AHU .