AC Drain Trap Issue — Negative Pressure Loss / Odor Backflow

Trap missing or insufficient depth

Check: Indoor static = -0.05 to -0.10 in.WC (negative) → without trap, negative pressure pulls condensate back, flooding pan. Trap depth = 2× absolute static (in.WC). E.g., -0.07 in.WC = 14 mm + 50 mm safety = ≥64 mm total. Odor backflow = trap dry or missing.

What to do: Measure indoor static (-0.05 to -0.10 in.WC). Trap depth = 2× |static| + 50 mm safety. If absent or shallow = install P-trap (or inverted). After install, prime by pouring water. Inspect each season start.