TL;DR

Water overflow is almost always a clogged condensate drain line. Flush the drain with a wet-vac, clean the pan, and verify the P-trap is correctly primed.

What you might see

water dripping from the unit into the space below
standing water in the drain pan
algae or slime visible in the drain pan
musty smell from the supply air

Likely causes

Tap to mark as ruled out

Condensate drain line clogged with algae, scale, or debris causing the drain pan to overflow
P-trap absent, undersized, or evaporated during extended non-cooling periods, allowing the drain to air-lock
Drain pan cracked or corroded, leaking directly
Unit not level after installation or roof settling, tilting the pan away from the drain outlet

Required tools

Wet-vac with drain hose adapter
Spirit level
Diluted bleach solution
Condensate treatment tablets
LOTO kit

Safety first

Lock out the unit before opening access panels.
Diluted bleach used for pan cleaning is a skin and eye irritant. Wear gloves and safety glasses.

Procedure

7 steps · ~20 min

1
Lock out the unit.
2
Locate the condensate drain pan under the evaporator coil. Check for standing water and visible algae growth.[1]
3
Connect a wet-vac to the drain outlet at the unit and pull suction to clear the blockage. Flush the drain line with a cup of water to confirm flow.[1]
4
Clean the drain pan with a diluted bleach solution (1 oz bleach per quart of water) to kill algae and prevent re-clogging.
5
Install a condensate treatment tablet in the drain pan. Replace at each PM cycle.
6
Check the P-trap: fill it with water. If there is no P-trap, install one to prevent air locking.
7
Verify the unit is level front-to-back using a spirit level on the base rail. Shim the unit or adjust the roof curb if it has settled.

Sources

Trane Voyager Commercial Rooftop Service Literature, Trane (Trane Technologies)
Trane Voyager Commercial Rooftop IOM, condensate drain maintenance and pan cleaning procedures (general)
View source

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