Condensate Trap on a Rooftop Unit
A 12-year-old rooftop packaged unit serves a small office. During a humid week the ceiling tiles below the return-air plenum show damp staining that spreads along a seam. The condensate drain leaves the unit through a PVC line that runs horizontally for 3 m before dropping 1.5 m to a floor drain. A technician finds the drain pan holding 15 mm of standing water and the pan outlet partly blocked by biofilm. The unit's fan is on the draw-through side, so the drain pan sits under negative pressure. The trap is a shallow field-fabricated U with a vertical depth of about 40 mm on both legs. The building operator wants a quick fix: clear the biofilm, refill, and return the unit to service the same afternoon.
1. Explain why clearing the biofilm alone is unlikely to stop the staining, and identify the two design faults in the condensate arrangement.
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Clearing biofilm restores flow but does not change the pressures acting on the pan. With a draw-through fan the pan is at negative pressure relative to the space, so air is drawn backward through the drain line unless the trap provides enough water seal. The trap here is about 40 mm deep on each leg, which is far too shallow to hold against the fan's static pressure, so the seal is blown or sucked dry and the unit pulls humid room air (and possibly unfiltered air) across the pan, causing carryover and the staining pattern along the ceiling seam.
Fault one is the trap depth: the seal must be sized from the fan's negative static pressure at the pan, with a margin, not guessed at 40 mm. Fault two is the horizontal run: 3 m of near-flat pipe gives biofilm and silt a place to settle and creates a second trap effect that the shallow U cannot overcome. Standing water in the pan confirms drainage is marginal even when the outlet is clear.
The correct response is to redesign the drain: a properly sized trap with adequate seal depth and a cleanout, continuous fall of at least 1 in 100 along the whole run, and a shorter or re-routed path to the drain. Then clear the pan and verify the seal holds with the fan running.
2. The operator proposes simply deepening the U-trap on site. Assess this proposal and state what must be checked before the unit is restarted.
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Deepening the U helps only if the new depth is calculated from the actual pressure difference across the pan, not chosen by eye. The seal depth must exceed the fan's negative static pressure at the pan, expressed as a water column, plus a margin so normal fluctuations and partial blockage do not break the seal. A trap that is too deep is also a problem: an over-deep trap on a shallow pan can be slow to prime and can hold water that stagnates.
Before restart, check four things. First, confirm the fan's static pressure at the pan from the unit data or a measurement, and compare it with the proposed seal. Second, confirm the trap has a cleanout and that both legs are accessible for priming. Third, verify the horizontal run now falls continuously and has no sag that would form a second trap. Fourth, check that the pan drains fully and that the overflow or secondary outlet, if fitted, is clear and discharging to a visible location.
Only after those checks should the unit run, and the ceiling should be inspected after a humid operating period to confirm the staining has stopped. A same-day refill and restart without these checks risks repeating the fault.