- This topic has 1 reply, 1 voice, and was last updated 2 days, 1 hour ago by
.
-
Posts
-
Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
On several projects the drain pans of air handling units overflow during operation even though a trap is fitted, and on others the trap is sucked dry and air whistles through the drain. The units are draw-through types with the fan downstream of the cooling coil, and a few are blow-through.
How should the trap dimensions be calculated from the fan static pressure? What plinth height does this require, and what are the other rules for condensate drains from AHUs and fan coils?
The trap is a water column that must balance the static pressure in the coil section. For a draw-through (negative pressure) section, the drain pan outlet must sit at least the negative static pressure plus about 25 mm above the trap outlet, and the water seal must be at least half the static pressure. For a blow-through (positive) section, the seal must exceed the positive pressure plus a margin.
1. Why these dimensions
Under negative pressure P at the pan, condensate can only leave when the water in the inlet leg stands P higher than the outlet weir. If the pan outlet is not higher than that, water backs up into the pan and overflows. At fan start, before condensate forms, the existing water redistributes: the inlet leg rises by P/2 and the outlet leg falls by P/2. If the seal is shallower than P/2, it breaks and air is drawn in through the drain.
For blow-through sections, positive pressure pushes the inlet leg down; the seal must be deeper than P so air cannot blow through.
2. Worked example (draw-through)
- Maximum negative static pressure in the coil section, with filters at their final (dirty) pressure drop: 600 Pa.
- In water column: 600 / 9.81 ≈ 61 mm.
- Pan outlet above trap outlet: H ≥ 61 + 25 = 86 mm → use 90 mm.
- Water seal: J ≥ 61 / 2 ≈ 31 mm → use 35 mm.
Add the pipe diameter and the trap body, and the drain pan outlet needs roughly 150 to 200 mm free height below it. This is why AHUs need a plinth or base frame of adequate height; check it on the plant room drawings, not on site.
Units of measure: 600 Pa ≈ 2.4 in wg, so in US terms the inlet leg is about 3.4 in and the seal about 1.2 in. Many manufacturers state the same rule in their installation manuals; follow theirs where it is stricter.
3. Other drain rules
- One trap per drain connection; do not join drains from sections at different pressures before their traps.
- Fall at least 1 % (1/8 in per ft, as in IMC 307.2) to an open tundish or air break; never connect directly to the soil system.
- Fit a cleaning tee or plug and prime traps at commissioning.
- Where traps can dry out in winter, use self-sealing or ball-type traps approved for the pressure.
- Outdoor and roof units: insulate and trace-heat drains against freezing.
- Size the drain for the condensate flow; for example 2 m³/s with a 5 g/kg moisture removal produces about 1.2 × 2 × 0.005 × 3600 ≈ 43 kg/h.
Common mistakes
- Using a standard sink trap with a fixed seal, regardless of fan pressure.
- Calculating with clean-filter static pressure.
- No plinth, so there is no room for the trap.
- Trap on the main drain only, several units sharing it.
Related: Condensate Drain Trap Design for AHUs and FCUs, AHU Sizing: Quick Rules.
Check the edition of the code that applies to your project.
- You must be logged in to reply to this topic.