AHU Condensate Drain Trap Calculation (P-Trap Depth)

Quick answer: The trap depth is set by fan static pressure, not by plumbing habit. Convert static to a water column (mm H₂O = Pa × 0.102). For a draw-through unit the trap seal must be deeper than the fan suction: seal ≈ static head + 25 mm, with total trap depth ≈ 2 × static head + 50 mm. For a blow-through unit, seal ≈ static head + 25 mm to resist blow-out.

Why the trap depth depends on the fan

On a draw-through AHU the cooling coil and drain pan sit on the suction side of the fan, so the pan is below atmospheric pressure. The fan is effectively trying to suck the trap dry and pull air up the drain. If the seal is shallower than that negative static, no water leaves the pan — it simply stands there and overflows. Blow-through units are the mirror image: the pan is pressurised and the air tries to push the seal out.

The formula

Static head:  mm H₂O = Pa × 0.102

Draw-through (negative pressure at pan):
  Seal depth  = static head + 25 mm
  Total depth = 2 × static head + 50 mm

Blow-through (positive pressure at pan):
  Seal depth  = static head + 25 mm
  Total depth = static head + 50 mm

Worked example

A draw-through AHU with 500 Pa static at the drain pan: static head = 500 × 0.102 = 51 mm H₂O. Seal depth = 51 + 25 = 76 mm. Total trap depth = 2 × 51 + 50 = 152 mm. A shallow 50 mm plumbing trap here would never drain.

Common trap mistakes

• Using a standard plumbing trap — 50 mm seals are far too shallow for most AHUs.
• Sharing one trap between units — the higher-pressure unit pushes condensate into the other pan.
• No cleanout — deep traps silt up with biofilm and cannot be rodded.
• Trap runs dry in winter — when cooling is off the seal evaporates and lets air through; a trap primer or seal device helps.

Frequently Asked Questions

How do you calculate AHU condensate trap depth?

Convert the fan static pressure at the drain pan to a water column using mm H2O = Pa x 0.102. For draw-through units the seal must exceed that head plus about 25 mm safety, and the total trap depth is roughly twice the head plus 50 mm.

Why is a draw-through condensate trap deeper?

Because the drain pan sits at negative pressure, the fan pulls water up the drain leg. The trap needs depth on both legs to overcome that suction and still hold a seal, so it needs roughly twice the static head.

What happens if the condensate trap is too shallow?

On a draw-through unit the fan pulls air up the drain instead of letting water out, so the pan fills and overflows into the ceiling. It is the most common cause of AHU water damage.

Can two AHUs share one condensate trap?

No. Units at different static pressures will push condensate into each other pans. Each drain pan needs its own trap sized for its own fan static.