A poorly designed condensate drain trap lets conditioned air blow straight through the drain line or lets condensate back up and overflow the drain pan; both failures trace back to trap depth not matching the section static pressure.
Why Traps Exist on an AHU
A comfort AHU cools and dehumidifies air, so water constantly forms on the coil and collects in the drain pan. The pan outlet must discharge that water while preventing the unit's static pressure from escaping through the pipe. A trap uses a water seal: a vertical leg of water that blocks airflow but lets liquid pass. If the seal depth is less than the section pressure, air blows through; if the trap is missing an air break or vent, the pan siphons or floods. In a large plant room a failed trap can quietly dump litres per hour onto a ceiling until staining appears months later.
The pressure on the drain side differs by section. A cooling coil sitting in a negative-pressure plenum (draw-through) sees suction pulling air inward, so the trap must hold against that vacuum. A coil in a positive-pressure section (blow-through) pushes air outward, and the trap must resist that positive head. Getting the sign right is the whole job, and the two cases need different seal depths even on the same unit.
Sizing starts with knowing how much water there is. A cooling coil removing latent heat condenses roughly 1 litre per hour for every 0.7 kW of latent capacity, so a 100 kW unit at a typical sensible heat ratio can shed 30 to 50 litres per hour in humid weather. That flow has to leave a pan often only 50 mm deep, which is why the pan itself needs a fall of at least 1% toward the outlet and a stainless or coated finish that will not corrode where water sits permanently. Flat pans with a central drain are the most common source of standing water and the microbial growth that follows it.
Trap Depth by Section Pressure
As a rule of thumb the water-seal depth should exceed the section static pressure expressed as a column of water, with a safety margin. Roughly, 1 Pa equals about 0.1 mm of water, so a 250 Pa negative section needs a seal deeper than 25 mm plus margin, typically 50โ75 mm in practice. Deep traps are common on large draw-through units, while small blow-through units may need only 25โ50 mm. The mistake is assuming one trap size fits every coil.
| Item | Specification | Note |
|---|---|---|
| Negative section | 50โ100 mm seal | Draw-through coil, resists suction |
| Positive section | 25โ50 mm seal | Blow-through coil, resists pressure |
| Safety margin | + 25 mm | Over the static pressure head |
| Vent requirement | Open air break | Prevents siphoning of the seal |
| Pipe slope | โฅ 1:50 | Toward the floor drain |
| Condensate rate | ≈ 30–50 L/h per 100 kW | Humid ambient, latent load |
| Drain pan fall | ≥ 1% to outlet | Prevents standing water |
A two-seal (P-type) trap with an air vent on the outlet leg is the robust choice for negative sections because it resists both blow-through and siphonage. The vent equalizes pressure so condensate leaves by gravity instead of being held by a vacuum, and the second leg keeps the seal from being pulled dry when the pan level drops during part-load cooling.
One trap per drain point is the rule broken most often. Where a unit has a cooling coil and a heat-recovery section that both produce water, joining the two tail pipes upstream of a single trap lets the section at higher pressure push air through the lower-pressure pan; the result is a drain that gurgles and a pan that overflows. Each pan needs its own seal, and pipes may only be combined downstream of the air break. In freezing climates the tail pipe and trap also need trace heating or an internal route, because a 32 mm pipe holding a 75 mm water seal will freeze solid long before the duct does.
Installation Checklist
- Measure the section pressure: Use the unit's fans-on static, not the catalog nameplate, to size the seal.
- Provide an air break: Never hard-connect the trap outlet directly to a sealed waste line without a vent.
- Slope the tail pipe: Keep at least a 1:50 fall so water clears the pan and air cannot pocket.
- Prime after shutdown: In dry seasons the seal can evaporate; specify a primer or trap seal holder.
- Size the pan outlet: Match the outlet to the condensate rate so the pan never backs up before the trap.
- Estimate the flow first: Work out litres per hour from the latent load before choosing a pipe size.
- One trap per pan: Combine drain lines only downstream of the air break, never upstream of the seal.
Design Significance
Correct trap design turns a recurring maintenance complaint into a non-event: no fogged ceilings, no overflow calls, no microbial growth in a stagnant pan. Keruite details trap configuration on its AHU submittals so the seal depth matches the actual section pressure rather than a generic guess, and the construction team gets a drawing it can install without reinterpretation.
Drain detailing is a small part of an air handling unit that generates a disproportionate share of warranty calls, so getting it onto the submittal drawing pays for itself. More than 30 years of unit fabrication in an 80,000 m² plant and an ISO 9001:2015 quality system mean pan, outlet, and trap geometry are treated as a controlled detail rather than a site improvisation, while CCC certification covers the electrical and safety side of the same unit. Exporting to more than 50 countries also means the standard drawing set is annotated for both floor-drain and open-gully practice, which differ significantly between markets.
Our engineers respond to selection enquiries within 24 hours, including drain and trap sizing for combination air handling unit projects.