Do you know the proper way to size a P trap on an HVAC unit?

The pull of the water to the drain has to exceed the sucking of the supply fan (“negative pressure”).Rule of thumb… add 2.5” to the static pressure before the fan (return and internal to the unit) and that’s the long end (closer to the drain of the unit. The end further away leg can be 2.5”.

For example, if you have an AHU (draw through) with a ducted return, the fan may have .5 esp for the return grille and and associated ductwork, plus the internal static pressure of the filter, cooling coil and cabinet (which would be another 1.5″ esp). The result in this case would be .5 + 1.5 + 2.5= 4.5. In the cut below, dimension A would be 2.


Dimension A is all the static pressures before the fan; ie the return duct and all the internal static losses before the fan. Sketch Courtesy Of United Cool Air

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