Condensate Traps for AHU: Positive vs. Negative Pressure Units

During AHU operation, condensate drains by gravity. The drain outlet may be in a negative or positive pressure zone, requiring different trap configurations.

1. Why a Trap Is Needed

• Negative pressure unit (drain in suction zone): Negative pressure sucks air in, preventing drainage and causing pan overflow.
• Positive pressure unit (drain in discharge zone): Positive pressure blows air out, causing air leakage, airflow loss, and splashing.

Trap function: Creates a water seal that blocks air exchange while allowing condensate to drain.

2. Negative Pressure Unit Trap

• Configuration: U‑type deep seal, high inlet – low outlet (inlet above outlet)
• Principle: Water seal height counteracts negative pressure, prevents air suction
• Requirement: Seal height > max negative pressure (mmH₂O), typically ≥50mm

3. Positive Pressure Unit Trap

• Configuration: U‑type deep seal, low inlet – high outlet (inlet below outlet)
• Principle: Water seal height counteracts positive pressure, prevents air blow‑through
• Requirement: Seal height > max positive pressure (mmH₂O), typically ≥50mm

4. Comparison & Common Mistakes

Common mistakes:

• Negative unit with positive‑type trap → seal sucked dry, no drainage
• Positive unit with negative‑type trap → seal blown out, air leakage
• Insufficient seal height → seal failure

 

 Conclusion: Select the correct trap based on unit pressure characteristics and ensure adequate seal height for proper drainage and no air leakage.