Attic horizontal air handler: crew set it on a new 2-inch aux pan with a float switch in the pan, ran the primary 3/4 PVC out to the eave with a P-trap, put a vent tee between the coil and the trap, and tied the float switch into the R wire.

The primary error is the vent tee placement between the coil and the trap.

Here's why:

Code Violation:

• IRC M1402.4 requires drain piping to maintain a "self-draining" configuration. A vent tee installed between the coil and trap creates an air break that prevents proper priming and siphoning action.

What Happens:

1. The vent tee allows air into the line between the coil and trap, breaking the water seal
2. Without a proper water column, condensate can back up and overflow at the coil connection
3. The aux pan catches overflow, but if the float switch wiring is incorrect or the pan isn't properly sloped, water can still escape before the switch activates

Additional Issues:

• Float switch wiring: Tying into the "R" wire is correct for safety, but verify the switch is normally closed and opens on high water
• Pan slope: Ensure the aux pan has proper drainage to the secondary pan or overflow

Fix:

• Remove the vent tee between coil and trap
• Install the vent tee after the trap (on the outlet side) if venting is needed
• Verify aux pan has 2% slope toward drain
• Test float switch operation

The vent tee placement is the root cause of the ceiling damage.

Primary error: The float switch is wired to the R wire. This only kills the thermostat’s call for cooling. The air handler’s blower (G) and electric heat strips (W) remain energized. The coil keeps producing condensate (or the blower re-evaporates pan water) while the switch is tripped, overflowing the aux pan.

Secondary errors (code violations):

1. No secondary drain line. IMC 307.2.3.1 requires a separate auxiliary drain line from the coil’s secondary outlet, terminated conspicuously. An aux pan with a switch is an alternative (IMC 307.2.3.2), not an addition to a missing secondary line.
2. Vent tee location. The vent must be after the P-trap (downstream). Placing it between the coil and trap (upstream) allows positive plenum pressure to blow air up the vent, preventing the trap from sealing and stopping drainage.
3. Trap depth. A 3/4" PVC trap requires a minimum 2" water seal (IMC 307.2.4). Shallow traps break seal under static pressure.

Fix:

1. Rewire float switch in series with Y (cooling call) and G (blower) at the air handler control board (typically break R to the board, or use a 24V relay to kill Y/G).
2. Plumb a dedicated 3/4" secondary drain line from the coil’s secondary port to a visible eave/soffit location per IMC 307.2.3.1.
3. Move vent tee to downstream side of trap; ensure 2"+ trap seal.