How to Install a Secondary Drain Pan and Float Switch

Why this matters

An air handler over a finished ceiling will eventually have a primary drain stop: algae in the trap, a line that sags, a pump that fails, a cleanout cap a homeowner left off. When it does, the only thing between the coil's water and the bedroom ceiling is the secondary pan and whatever it does with the water. A pan with a drain that discharges where nobody looks fills a soffit. A pan with a float switch wired to the wrong conductor, or not tested, fills to the rim while the compressor keeps making water.

A float switch that stops cooling is cheap, and a customer who calls because "the AC quit and there's water in the pan" is a customer whose ceiling is dry. The install takes under an hour; the part people skip is the last ten minutes, where you fill the pan and prove the system actually stops.

The code basis (when auxiliary protection is required, the four methods, the pan's minimum size) belongs to Condensate Drain Code and Trap Field Card. This is the installation.

Safety before the cabinet is opened

The air handler carries line voltage for the blower and, on a heat pump, often for heat strips. Shut it off at its disconnect or breaker, lock and tag it, and prove absence of voltage at the unit with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024) with work practices under 29 CFR 1910.333(b)(2). Killing power also protects the 24 V circuit: a stripped R wire brushed against C or the cabinet blows the board fuse or the transformer.

In the attic, walk on framing or boards, watch for roofing nails at head height, and plan around the heat; a full pan of water is also heavy, and it goes on framing, not on drywall between joists.

The install, on one attic heat pump

The job: a 3-ton heat pump with a horizontal air handler in an attic over a finished bedroom. The primary drain and trap are good. There is no secondary protection. The air handler's footprint is 22 inches by 48 inches (as measured on this unit).

1. Pick the method the space allows. The four options are in the code card. In an attic over a finished ceiling the usual choice is an auxiliary pan under the whole unit with a float switch, with or without the pan's own drain line. A drain from the pan helps only if it ends where an occupant will notice it, typically over a window or out through a soffit at eye level; a drain tied into the primary line is not secondary protection, because whatever blocked the primary blocks it too.

On this job: no route to a noticeable discharge point, so the method is a pan with a water-level switch that shuts the equipment off, and the pan's drain fitting is plugged as the pan maker supplies.

2. Size the pan to the unit. Per the code card, at least 1-1/2 inches deep and at least 3 inches larger than the unit in both width and length, in a corrosion-resistant material. Also allow room for the primary trap and any service panel that needs to swing over the pan.

On this job: 22 + 3 = 25 inches minimum width and 48 + 3 = 51 inches minimum length. A stock 30 by 54 inch pan, 2 inches deep, gives 30 over 25 and 54 over 51, and leaves room for the trap at one end.

3. Set the pan on structure, level, and set the unit in it on pads. The pan sits across joists or on a platform, never spanning drywall alone. Level it in both directions so water spreads evenly and reaches the float wherever it collects; if the framing forces a slight tilt, put the float at the low corner. Set the air handler on the pads or rails the maker allows inside the pan, so the cabinet is not sitting in water and the pan bottom is not dented by the unit's weight.

On this job: two 2x4 sleepers across four joists under the pan, shimmed until a level read flat both ways. Air handler on four rubber pads.

4. Mount the float where it sees water early. A pan-edge float switch clips onto the pan's rim at the low point, positioned so it trips well below the rim. Keep it clear of the primary trap and the unit's cabinet so nothing holds the float down. For a coil with no room for an auxiliary pan, the alternative is a float in the equipment's secondary drain port or in the primary pan, per the code card, which uses the same wiring below.

On this job: float clipped at the trap end of the pan, adjusted so its float sits about half an inch above the pan floor.

5. Choose which call the switch breaks. The switch is normally closed and opens on high water. Where it opens decides what stops:

• In Y, between the air handler and the condenser, it stops the compressor and leaves the blower, the heat strips and the thermostat working. This is the common choice on an air conditioner or heat pump: no compressor, no new water.
• In R, it stops every call, including the blower and heat. Use it where the shop or the maker wants everything off, or where the same pan catches condensate from a condensing furnace, whose water comes in heating mode and is not stopped by breaking Y.

On a heat pump, Y also carries the heating compressor call, so a tripped switch in Y in winter leaves the house on strips (where the thermostat brings them on) until the pan is cleared. That is acceptable; a pan does not fill in heating, because the indoor coil is the warm side and makes no condensate.

On this job: Y, at the air handler, in the conductor that runs to the condenser.

6. Wire it in series. Thermostat wire to the condenser normally lands on the air handler's Y terminal alongside the thermostat's Y. Take the condenser's Y conductor off that terminal, splice one float lead to the terminal, the other float lead to the condenser conductor. If the switch has three leads, use the common and the normally closed lead, and cap the normally open one. Thermostat Wiring Reference covers the terminal designations.

 THERMOSTAT     AIR HANDLER BOARD          CONDENSER
   R ---------- R  24 V hot
   C ---------- C -------------------------- C
   G ---------- G  blower                    |
   Y ---------- Y --+                   contactor
                    |                     coil
              float switch, NC,              |
              opens on high water            |
                    |                        |
                    +----------------------- Y

With water low, the switch is closed and Y passes through to the contactor coil exactly as before. With water high, the switch opens and the contactor drops out. Nothing else in the circuit changes.

7. Make the splices solid and the wire secure. Wire nuts or crimps sized for 18 AWG, inside the air handler's low-voltage area, and the float cable strapped so it cannot be pulled off the pan edge by a service tech moving the trap. A float wire left hanging across the pan is a float that gets knocked off its clip the first time someone changes a filter.

Restore and prove it

8. Restore power and confirm normal cooling. Remove your lock, close the disconnect while standing to the side of it, and set a cooling call. Confirm the condenser starts and the blower runs. A three-lead float landed on its normally open lead, or a loose splice, shows up here as a system that will not start at all.

9. Fill the pan until it trips. With the system running on cooling, pour water slowly into the pan at the float end. Watch the condenser, or have a second person at it. When the float lifts, the outdoor unit should stop within a few seconds. Measure the water depth at the moment it trips: it has to be well below the rim.

Confirm it electrically if you want a number: with the meter on the low-voltage splice, condenser-side Y conductor to C, at the air handler and away from any line-voltage terminal, read about 24 V before the trip and near 0 V after.

On this job: the condenser stopped at 5/8 inch of water in a 2 inch pan, leaving 1-3/8 inch below the rim. Y to C at the splice read 24.6 V running and 0.1 V tripped.

10. Drain the pan and confirm the restart. Sponge or wet-vac the water out. The float drops and the switch closes; the condenser should restart on the next call, often after the anti-short-cycle delay many boards and thermostats impose, commonly several minutes. Wait it out before deciding the reset failed.

On this job: the condenser restarted about five minutes after the pan was dry, on the board's delay. Pan wiped dry and left empty.

When the test does not go the way it should

• The pan fills past the float and the condenser keeps running. The switch is not in series with the condenser's Y (often the condenser conductor was left on the Y terminal with a float lead spliced alongside it, which puts the switch in parallel with nothing to break), or the float is held down by the cabinet or a wire. Kill power and re-trace before the customer gets a false sense of protection.
• The system will not start with a dry pan. A three-lead switch landed on normally open, or a splice is loose. The open circuit looks exactly like a tripped switch, which is the point of a normally closed device: a broken wire fails safe.
• It trips with almost no water. The float is set too low or the pan is tilted toward it hard. Fine for protection, annoying for the customer; adjust so normal condensate film does not trip it.

Recording it

Write the method used, the pan size against the unit footprint, which conductor the switch breaks, and the water depth at trip. Leave a tag on the air handler: "Float switch stops cooling if this pan holds water. Check the primary drain before resetting." The next person who finds the AC off and water in the pan will know what happened.

References

• NFPA 70E, current edition, Article 120 (establishing an electrically safe work condition); 29 CFR 1910.333(b)(2)
• Air handler and float switch manufacturers' installation instructions
• See related: Condensate Drain Code and Trap Field Card (methods, pan size, code basis)
• See related: Thermostat Wiring Reference; How to Build a Condensate Trap for a Draw-Through Coil