I feel like there should be a running trap before the clean out tee. The trap would prevent air from coming out of the tee. I have worked with guys who have told me that the drain would not work with out the tee open to let air in. However I dont agree, use a running trap then a vent tee with a taller stand pipe.
Please do not install running traps, I end up changing them out on every service/maintenance. Ever notice how rusted and damp the blower side of the air handler is, P-trap is the way to go. Here in the south, a good 80% of the air handlers are in the attic, damn sure don't want water comming thru the ceiling. Float switch is a must also
I work on high end computing systems, so excuse my ignorance when it comes to optimal A/C installations. Could someone explain the P-Trap and where it should be installed? You guys have been a great help.