Condensate drain design for AHU and FCU removes the water condensed from cooling coils so it cannot spill into the pan, the ceiling, or the occupied space. A correct design sets trap depth to seal against the unit's static pressure, sizes pipe by slope and flow, and provides an air break to the building drain. In fan-powered units the coil face static pressure typically runs 25–250 Pa, and the trap seal must exceed that or the drain will not flow.
Where the Water Comes From
A cooling coil removes both sensible and latent heat. At a typical 7 °C supply temperature and 50% return humidity, everyๅ็ฆ of cooling can condense 0.4–0.7 L/h of water; a 10 kW FCU therefore produces roughly 4–7 L/h, and a 50 kW AHU section produces 20–35 L/h during peak cooling. That water must leave the pan continuously, not pool until the next service call. Undersized or untrapped drains are the single most common cause of HVAC water complaints in commercial buildings.
Trap Depth Must Beat Static Pressure
The trap seal depth in millimetres of water is approximately the unit static pressure in Pa divided by 9.81. A 50 Pa negative plenum needs at least a 5 mm seal, but codes and good practice call for 25–50 mm minimum to cover surges and fouling. Place the trap as close to the pan outlet as possible and keep its inlet below the pan water line so air cannot short-circuit the seal.
| Condition | No trap | Shallow trap (<20 mm) | Correct trap (25–50 mm) |
|---|---|---|---|
| Negative plenum (−150 Pa) | Air sucked in, drain blocked | Seal breaks on surge, gurgling | Stable seal, steady flow |
| Positive plenum (+100 Pa) | Air pushed out, odor leak | Marginal, leaks on peak | Sealed, no leak |
| Maintenance | Frequent overflow | Seasonal attention | Annual check only |
| Risk to ceiling | High | Medium | Low |
The table shows why a correct trap is not optional: it is the only element that keeps air and water moving in the right direction under live static pressure.
Pipe Sizing and Slope
Size the drain by gravity flow, not by coil tonnage alone. Use a minimum slope of 1:100 (1%) and prefer 1:50 for long runs; a 25 mm (1 inch) pipe reliably carries about 30 L/h, a 32 mm pipe about 60 L/h, and a 40 mm pipe about 110 L/h at 1% slope. Always provide a cleanout every 3–5 m and a primary and secondary pan with its own independent drain on ceilings below occupied spaces.
Material and Air-Break Rules
- Use PVC, PP, or stainless according to local code and condensate corrosiveness; insulated drain lines prevent re-sweat drips.
- Terminate at a visible air break to the building drain so sewer gas cannot enter the AHU.
- Add a P-trap on the building side of the air break to keep the seal complete.
- Support pipe every 1–1.5 m so slope is maintained over the life of the system.
Decision Checklist: When to Act
- Split the drain when one coil section exceeds 15 kW and would overwhelm a shared 32 mm line.
- Add a secondary pan with alarm when the unit sits above a finished ceiling or occupied room.
- Increase trap depth when measured static pressure exceeds 200 Pa or the unit varies speed widely.
- Insulate the trap and first metre of pipe in humid spaces to stop condensation on the drain itself.
- Keep the trap when swapping an old belt-drive fan for a variable-speed unit, but re-check the seal depth.
From Design to Reliable Operation
Good design is verified, not assumed. After installation, pour 10–20 L of water through the primary pan and confirm it clears in under 60 seconds with no gurgle and no ceiling stain. A unit that balances airflow to G2.5 and is performance-tested before shipment will still overflow if its trap is wrong, so the drain deserves the same engineering attention as the coil.
A Worked Sizing Example
Take a 25 kW AHU cooling section in a 26 °C, 60% RH space with 7 °C supply. With a latent ratio near 30%, condensate is about 0.5 L/h per kW, so the section makes roughly 12.5 L/h. At 1% slope a 25 mm drain rated near 30 L/h is comfortable, while a 32 mm line gives margin for a future 40 kW load. For trap depth, the plenum static is −120 Pa, so the seal is 120 / 9.81 ≈ 12 mm; apply the 25 mm code minimum plus a 10 mm surge allowance for a 35 mm trap.
Freeze and Re-Sweat Protection
In cold climates the trap and first metre of drain can sweat or freeze. Insulate both, and where the pan sits in spaces below 5 °C, add trace heat on the trap only rather than the whole line. A bottle trap is easier to service than an inline trap but needs firm support; an inline trap saves ceiling height. Either way keep the trap full of water by capping its vent, or the seal fails after a long idle and the next startup spills into the pan.
Drain Materials Compared
PVC is the default for neutral condensate up to 40 °C; above that, or with acidic drain from desiccant wheels, use PP or stainless. Insulate all exposed runs in spaces above 20 °C and high humidity to stop dripping. Where the drain crosses a firewall, fit a fire-rated collar so the penetration keeps its rating; omitting it is a common failed inspection that has nothing to do with hydraulics yet blocks handover.
Inspection and Lifecycle
Inspect the primary pan and trap every 6–12 months, more often in humid or dusty sites. Clear biofilm with a pan tablet or biocide, confirm slope with a sight glass, and test the overflow alarm. A clogged trap that backs up into a 50 kW coil can trip the unit within a day, so the small drain earns outsized attention in the maintenance plan and in the commissioning checklist.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
How deep should the condensate trap be?
Trap seal depth in millimetres of water is roughly the unit static pressure in Pa divided by 9.81, but apply a 25–50 mm minimum. A 150 Pa negative plenum needs at least 15 mm, yet codes require the larger safety margin to survive surges, fouling, and variable-speed operation without breaking the seal.
What pipe size is needed for a 10 kW FCU?
A 10 kW FCU condenses about 4–7 L/h, so a 25 mm drain at 1% slope (rated near 30 L/h) is adequate. Use 32 mm when the run is long or the unit is oversized. Slope the pipe 1:100 minimum and add a cleanout every 3–5 m for service access.
Why does my AHU still overflow with a trap?
Overflow with a trap usually means the seal is too shallow for live static pressure, the trap lost its water after a long idle, or the pipe is sloped wrongly and holds air. Re-measure plenum pressure, refill the trap, and verify 1% slope and a clear air break to the drain.
Do I need a secondary drain pan?
Yes whenever the unit is above a finished ceiling or occupied space. The secondary pan sits below the primary and must have its own independent drain plus a float switch or alarm. Codes in most regions require this redundant path so a primary failure cannot damage the ceiling below.