Why does a shallow condensate trap make an AHU drain pan overflow?
A draw-through unit can hold water in the pan if the trap does not overcome negative pressure. Verify unit type/static and rebuild the drain trap per manufacturer detail before water damages the cabinet.
['On some AHUs, the fan can pull against the condensate drain. If the trap is too shallow or piped wrong, air gets pulled through the drain and water cannot leave the pan properly.', 'This is not just plumbing-looking PVC. Check the AHU manufacturer trap detail, negative/static pressure condition, drain pan elevation, venting, slope, and whether it is draw-through or blow-through. Repipe only to the approved trap profile.']
Stop if
- Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.
Check
- A bad condensate trap can flood the unit, damage controls/electrical areas, stain ceilings, and create startup headaches.
Steps
- Confirm whether the coil/fan arrangement is draw-through or blow-through.
- Check the manufacturer condensate trap detail and required dimensions for the actual unit/static.
- Look for shallow trap, missing cleanout, wrong vent location, no slope, blocked drain, or pan outlet issue.
- Check for water damage near electrical/control compartments.
- Stop/contain water before it damages equipment or ceiling.
More in Joints, TDC, Flanges & Connections