Free HVAC Condensate P-Trap Sizing Tool
Calculate minimum P-trap vertical drop ($A$), trap outlet rise ($B$), and total water seal depth to prevent drain pan backup in negative-pressure draw-through AHUs.
🪠 AHU Fan Configuration & Static Pressure
📊 Sized P-Trap Dimensions & Pipe Size
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The Physics of HVAC Condensate Trapping
A cooling coil condenses gallons of water every hour as it cools humid air. In a Draw-Through Air Handler, the fan is downstream of the coil, creating a partial vacuum ((-1.0") to (-4.0") w.g.) inside the drain pan casing. If the condensate pipe is untrapped or shallow, atmospheric air rushes backward through the drain into the AHU, violently aerating and holding the water inside the pan until it overflows.
Draw-Through P-Trap Sizing Equations (ASHRAE Standard)
- Inlet Vertical Drop ((A)): The distance from the bottom of the drain pan to the centerline of the trap discharge outlet must exceed static pressure: $$A = |P_{static}| + ext{Safety Margin (1.0")}$$
- Trap Discharge Rise ((B)): The distance from the discharge outlet down to the bottom of the trap U-bend: $$B = rac{|P_{static}|}{2} + 0.5 ext{ inch}$$
- Total Trap Height: (H = A + B). Ensure the unit curbing or housekeeping pad is elevated sufficiently to accommodate dimension (H).
Frequently Asked Questions
Where should the trap vent tee be placed?
The open atmospheric vent tee must ALWAYS be installed DOWNSTREAM of the trap weir outlet. Placing a vent between the drain pan and the trap breaks the suction seal and causes immediate pan overflow.
What happens if a condensate trap runs dry during winter?
When heating runs without cooling, water in the trap evaporates. In a draw-through system, sewer gases or mechanical room fumes will be sucked directly into the supply airstream and distributed into tenant spaces.