Cassette Started Dripping After Routine Cleaning: Drain Trap Dried Out

• •Concern: Worry was that the indoor coil had cracked and replacement would disrupt office operations.
• •Key check: Performed a controlled drain flow test and confirmed the trap had no standing water seal

Because cooling was stable and the coil showed no ice formation, we ruled out refrigerant issues and hardware faults early. A unit that cools well but drips has a drainage problem, not a cooling problem. We focused on the drain path before considering anything else: the trap, tray, and pipe run. In cassette units, the drain trap is a key component that maintains a water seal to allow gravity or pump-assisted drainage. If that seal is broken, condensate backs up even though the pipe itself is clear.

• •Panel and filter clean with no ice formation on the coil.
• •Drain line physically open. No blockage in the pipe run.
• •Trap section had no standing water seal.
• •Condensate backed up in the tray during steady operation and spilled at the panel edge.

The drain trap lost its water seal during the cleaning sequence. The trap relies on a standing column of water to create a seal that prevents air from entering the drain line. When that seal was broken (likely by draining the trap during filter removal and not re-priming it afterward) air entered the line and disrupted the siphon effect that drives condensate through the pipe. Condensate still formed on the coil and collected in the tray as normal, but the impaired drain rate meant water could not discharge as fast as it was produced. Over several hours of steady operation, the tray gradually filled until it overflowed at the panel edge, producing the afternoon dripping pattern the client described.

We re-primed the drain trap by pouring water into the trap section to restore the standing water seal. After priming, we ran a controlled flow test to confirm that condensate discharged steadily through the drain line under normal operating conditions. The tray level was monitored over an extended run to verify it remained stable and did not rise toward the overflow point. No coil replacement, pump repair, or pipe work was needed. The entire fix involved restoring a water seal that had been broken during the cleaning sequence.

The afternoon dripping stopped. Tray level held stable through a full day of operation, and no ceiling tiles needed replacement.

Post-cleaning drips: why drainage deserves the first look.

• •Normal cooling with new dripping almost always points to the drain path. A cracked coil or pump failure would show cooling loss or no drainage at all.
• •The drain trap needs a standing water seal to work; if that seal breaks during cleaning, condensate backs up in the tray even though the pipe itself is open.
• •Afternoon dripping that starts hours after the unit turns on is a classic tray-overflow pattern. The tray fills gradually, then spills once condensate production exceeds the impaired drain rate.
• •After any maintenance that involves the drain path, a simple flow test takes under a minute and catches trap issues before the unit goes back into service: pour water into the tray and confirm steady discharge.