How to Calculate the Correct Condensate Trap for Negative-Pressure AHUs
Updated: 2 days ago
In a draw-through AHU, the cooling coil and drain pan are usually located upstream of the supply fan. Because the fan is pulling air through the coil section, the drain pan area operates under negative static pressure. This negative pressure tries to pull air upward through the condensate drain pipe, which can stop water from draining properly.
If the condensate trap is undersized, the AHU may suffer from water carryover, overflowing drain pans, microbial growth, ceiling leakage, and nuisance maintenance complaints. A correctly sized trap is therefore not a small plumbing detail; it is part of proper AHU design.
1. What Is a Negative-Pressure Condensate Trap?
A condensate trap is a water seal installed in the drain line from the AHU drain pan. Its purpose is to prevent air from being sucked into the AHU through the drain pipe and to allow condensate water to discharge freely.
In a negative-pressure AHU, the pressure inside the drain pan is lower than atmospheric pressure. Without a proper trap, the fan suction can hold the water inside the drain pan and prevent drainage. (How to Calculate the Correct Condensate Trap for Negative-Pressure AHUs)
Common examples include:
Draw-through AHUs
FCUs with fan after the coil
Packaged units with negative-pressure drain pans
Fresh air handling units with cooling coils before the fan
Download the full design guide PDF with detailed calculations, sizing, code compliance, and real project insights. (How to Calculate the Correct Condensate Trap for Negative-Pressure AHUs)
For detailed calculations, project-specific design, and authority-compliant solutions, contact our engineering team.
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