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Every time I get called out for water leaking from a system, I already know what the customer was told last time: the drip pan is rusted out. It is the fastest answer in the trade. It is also usually wrong.

Here are three leaks I worked, back to back. Same complaint. Three completely different causes.

1. The drain was plugged

Water was overflowing the pan and running down the cabinet. Nothing wrong with the pan at all. The condensate line was packed with slime.

I pulled the cleanout cap, vacuumed the line out from the termination, and flushed it. Water drained the way it should and the leak stopped.

Swap the pan on this one and the customer pays for a part and still has a leak the next humid week.

2. There was no P-trap

An install problem, not a maintenance problem. The condensate line came straight off the pan fitting with no trap at all.

On a draw-through coil the blower is pulling a vacuum on that cabinet. So instead of water running out the drain, the unit was pulling air in through the line and holding the water in the pan. It filled, then spilled over the edge.

I cut in a proper trap, deep enough to hold against the static in that cabinet, and vented it after the trap. The pan was fine. It just never had a chance to drain.

3. The coil was never sealed after replacement

A First Co. air handler that had a coil replaced by another company. Water showing up on the floor, but the pan was dry and the drain flowed clean. Took me a minute to catch it.

Whoever changed the coil never sealed it back to the cabinet. Air was bypassing around the coil instead of going through it, and that leaking air was carrying moisture right off the coil face, past the pan, and down the outside of the cabinet. The pan can only catch what falls into it.

Flip the switch below to see the difference.

Air handler with an unsealed evaporator coilCutaway of a vertical air handler. A gap has been left between the edge of the evaporator coil and the cabinet wall. Part of the airstream bypasses the coil through that gap, picks up condensate from the coil face and carries it past the lip of the drip pan. The water runs down the inside of the cabinet and onto the floor, while the drip pan continues to catch water and drain normally.Supply airBlowerEvaporator coilCondensate drain+ P-trapCoil sealed to the cabinetEvery bit of air has to passthrough the coilFloor stays dryUnsealed gapAir bypasses the coil edgeand carries water with itWater on the floorIt never reached the panDrip panCatching water anddraining normally
Same unit, same coil, same drain — one difference. With the coil sealed to the cabinet, all of the air passes through it and the condensate runs down the fins into the pan. Leave a gap at the coil edge and part of that air shoots around the coil instead, picks up water off the fins, and carries it past the lip of the pan and down the inside of the cabinet. The pan is still catching water. The drain is still clear. And there is still water on the floor.

Sealed the coil to the cabinet and closed off the bypass. Dry.

What I take from these three

The pan is where water ends up. It is the last place in the chain, which is why it is the first thing people blame. If you want the actual cause, work backwards from where the water landed to where it started. Check the drain, check the trap, check that the air is going where it is supposed to go.

A rusted pan is a real thing. But rust does not happen by itself. It happens because water sat in that pan for years, and something else put it there.

Most of what goes wrong on a leak call was decided the day the system went in. Here are the checkpoints we hit on every install so it never gets to this: how we install it right.

Water showing up around your air handler? We will find the cause before we sell you a part.

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