Why U-trap or P-trap is needed for the condensate drain pipe connection to the FCU's drain pan?

Answer 1

If the condensate pipe is not trapped, the AHU will suck air in through the condensate pipe and lower the operating efficiency. So not only does it need to be trapped, but the water column height of the trap needs to be at least the water column listed for START-UP. When you start the fan up is when there's the greatest negative pressure or suction (starting amps are highest). If the trap column effective height is not high enough, then when you fire up the fan, it will suck the trap dry, rendering the trap useless. Also the outbound side of the U should have a tee fitting on its side, so the outbound, lateral transport pipe is connected on the tee side, leaving one end of the tee open on top for priming. I rarely see anyone get condensate traps put together geometrically correct or with enough column.

Answer 2

One more note to the 1st excellent answer: Condensate drain is considered as an indirect waste, therefore it can be connected to the Lavatory sink tail-piece with an Air Break device or can be freely spill on the Service Sink (Janitor sink), thus the odor from toilet or janitor closet can travel through the condensate pipe (which is usually empty) to the FCU drain pan and FCU fan distribute that to the conditioned space. P trap can block the odor (there is always an amount of condensate water in U part of the P trap). As you see in the 1st answer, the height of the water should be high enough to avoid sucking water in P trap by fan. The best practice is to make the height of the leg in U part of the P trap equal to the fan discharge static pressure plus 1”.