Condensate Trap Depth on a Cold-Climate Air Handler
A 3-ton upflow air handler with a draw-through evaporator coil is installed in a conditioned mechanical closet of a Fairbanks home. The coil is upstream of the blower, so the drain pan sits in the negative-pressure section. The installer runs a 3/4 in. PVC drain with a single 4 in. vertical drop to a floor drain, no trap, and the unit cools fine in July. In January the homeowner reports water on the closet floor and ice forming at the drain termination where the pipe passes through an exterior wall to daylight. Static pressure at the coil is measured at 0.35 in. w.c. negative. The closet stays at 65°F.
1. Explain why the drain leaked and specify the trap geometry needed.
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A draw-through coil puts the pan under negative static pressure, so air is pulled into the drain line unless a trap with a water seal deeper than the pressure can hold it back. The seal must exceed the fan pressure converted to inches of water - here 0.35 in. w.c. - plus a margin, so a trap with at least a 2 in. (commonly 3 in. to be safe) water column on the negative side is required. The current 4 in. drop with no trap provides no seal at all, so the pan never drains and overflows. Correct fix: install a properly sized P-trap with the outlet leg extending below the trap to a height that clears the fan pressure, add a cleanout/vent tee at the top of the trap, and slope the line continuously to the discharge. The 4 in. drop is adequate depth for 0.35 in. w.c., but the trap must be built, not just a straight drop.
2. Address the ice at the exterior termination separately.
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The ice is a freeze problem, not a trap problem. Draining condensate through an exterior wall to daylight in Fairbanks guarantees a frozen plug in winter, which backs water up into the closet regardless of trap quality. Route the drain to an indirect waste receptor inside the conditioned envelope (floor drain, laundry sink, or condensate pump to a drain), never to an exterior termination in a heating climate. If a pump is used, confirm it discharges indoors and is rated for the lift.