Free AHU Condensate Drain Trap Calculator (P-Trap Depth)

The P-trap on an AHU drain is not a plumbing formality - its depth is set by the fan static pressure at the drain pan. Get it wrong and the unit either cannot drain (draw-through pulls air up the pipe and the pan overflows) or blows the seal out (blow-through pushes water out of the trap). This calculator sizes the trap seal and total depth for both configurations.

Draw-through units are the usual troublemakers. The pan sits below atmospheric pressure, so the fan is effectively trying to suck the trap dry - if the seal is shallower than the negative static, air is drawn up the drain and water simply stands in the pan. That is why the draw-through trap needs roughly twice the static head as total depth, not just the seal. Blow-through units are the mirror image: positive pressure pushes water out, so the seal must resist blow-out. Deep traps also need a cleanout and should never be shared between units at different pressures - one fan will push its condensate into the other pan.