🌀 HVAC

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

Calculate AHU condensate P-trap seal and total depth from fan static pressure. Draw-through and blow-through configurations. Free tool for MEP engineers.

📐 Standard: ASHRAE / SMACNA
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

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.

📐 Condensate P-Trap Depth Formula

ASHRAE / SMACNA

Convert fan static to water column:
  mm H₂O = Pa × 0.102

Draw-through (negative pressure at pan):
  Trap seal depth  = static head + 25 mm safety
  Total trap depth = 2 × static head + 50 mm
  (fan suction tries to lift water up the drain leg)

Blow-through (positive pressure at pan):
  Trap seal depth  = static head + 25 mm safety
  Total trap depth = static head + 50 mm
  (fan pressure tries to push the seal out)

Rule: the seal must always exceed the fan static it fights.

❓ Frequently Asked Questions

How do you calculate AHU condensate trap depth? +
Convert the fan static pressure at the drain pan to a water column (mm H2O = Pa x 0.102). For draw-through, the trap 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 trap deeper than a blow-through trap? +
A draw-through pan is at negative pressure, so the fan pulls water up the drain leg. The trap needs enough depth on both legs to overcome that suction and still hold a seal. A blow-through pan is at positive pressure and only needs enough seal to resist being pushed out.
What happens if the condensate trap is too shallow? +
On a draw-through unit the fan pulls air through the drain instead of letting water out, so the pan fills and overflows into the ceiling. On a blow-through unit the air pressure blows the seal out and conditioned air escapes down the drain.
Should each AHU have its own trap? +
Yes. Never manifold drain pans from units at different static pressures into a shared trap - the unit with higher pressure will push its condensate into the other pan. Each pan gets its own trap sized for its own fan static.
Do condensate traps need a cleanout? +
Yes. Condensate lines block with biofilm and debris more than almost any other pipe. A removable cleanout or clear trap section makes maintenance possible without cutting the pipe.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

🌀 AHU Condensate Trap Calculator
Reference: ASHRAE / SMACNA
Enter the fan static pressure at the drain pan in Pa and select draw-through or blow-through. Results give the required trap seal depth and total trap height in mm.