TL;DR
Condensate overflow is almost always a blocked drain line or a plugged p-trap. Clear the drain line first, then inspect the trap depth to ensure it holds sufficient water seal against the negative pressure inside the unit.
What you might see
- water dripping from the AHU base or access panels
- high condensate level switch alarm in the BAS
- mold or biological growth visible in the drain pan
- water staining on the ceiling or floor below the unit
Likely causes
Blocked condensate drain line from biological slime or debris accumulation
Shallow or dry p-trap losing its water seal due to negative AHU pressure, allowing the drain line to act as an air intake
Drain pan not pitched toward the drain, allowing water to pool at the far end
High-performance coil at high load producing more condensate than the drain can handle at the current cleaning frequency
Required tools
- Wet-dry vacuum
- Flexible drain-cleaning brush
- Diluted bleach solution for pan cleaning
- Measuring tape for trap depth check
- Nitrile gloves
Safety first
- AHU drain pans can harbor Legionella bacteria in standing water. Wear nitrile gloves and avoid face contact during cleaning. Dispose of drain water per your site biological waste procedure if Legionella risk is elevated.
- Lock out the AHU before entering the drain pan section.
Procedure
- 1
Shut down the AHU and open the drain pan access door. Confirm visually that the pan is overflowing and measure the water level.[1]
- 2
Clear the drain line by pouring clean water into the pan and observing drain rate. If the water stands, the line is blocked.
Warning: Lock out the AHU before opening drain pan access doors. Standing water in an AHU drain pan may contain Legionella or other biological organisms. Wear nitrile gloves and avoid splashing. - 3
Use a wet-dry vacuum or a thin flexible drain-cleaning brush to clear the drain line from the pan fitting. Avoid sharp tools that could damage the drain connection.
- 4
Inspect the p-trap depth. The trap water seal depth must be at least 1 inch greater than the maximum negative static pressure (in inches WC) inside the AHU. Shallow traps break seal under pressure.
- 5
Clean the drain pan with a diluted bleach solution (1/4 cup bleach per gallon of water) and brush to remove biofilm. Rinse with clean water and confirm the drain line flows freely.
- 6
Confirm the condensate high-level float switch activates when the pan is tested with water and that the BAS receives the alarm signal.
- 7
Restart the AHU and verify the drain pan remains dry during normal cooling operation over the following 4 hours.
Sources
Trane Trane Performance Climate Changer Air Handling Unit (AHU) general technical documentation, Trane
AHU condensate drain pan maintenance, p-trap sizing, and drain line clearing, general HVAC references (general)
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