The Condensate System: Pan, Trap, Pump, and Float Switch

What it is

Whenever the AC (or heat-pump cooling) runs, the evaporator coil pulls moisture out of the air and that water has to go somewhere. The condensate system is the plumbing that collects it and gets it out of the building: a pan under the coil, a drain line (usually with a trap), and — when gravity drainage isn't possible — a pump. A float/safety switch shuts the system down if the water can't get out, so a clog turns into "no cooling" instead of "ceiling caved in."

It's the simplest subsystem in the whole machine and it generates a wildly disproportionate share of service calls, because a slow drip from a plugged drain does expensive water damage and homeowners notice it fast. A tech who understands this system head-on prevents a lot of callbacks.

How it's built

• Drain pan. Sits under (or is built into) the evaporator coil/cased coil, sloped toward a drain connection — typically a 3/4" threaded fitting. It catches the water dripping off the coil's fins.
• Secondary / auxiliary pan. On equipment installed above a finished space (attic over a ceiling, for example), a second pan sits under the whole unit as backup, with its own drain (or a float switch). If the primary fails, this catches the overflow.
• Drain line. PVC (or sometimes other approved pipe) carrying the water by gravity to an approved termination — a floor drain, a condensate pump, or outside.
• P-trap. A U-shaped dip in the drain line near the unit. Required on most setups (more below).
• Vent / cleanout. A tee after the trap that vents the line and gives you a port to flush/blow it out.
• Condensate pump. Used when there's no downhill path to a drain — a small reservoir with a float-operated pump that lifts the water up and over to a drain. Has its own internal float switch and usually a safety/overflow switch output.
• Float / safety switch. A switch that opens (breaking the cooling call or the whole control circuit) when water rises too high — in the primary pan, in the secondary pan, or in the pump reservoir. It's the safety that prevents an overflow.

Why the trap matters

The trap isn't optional plumbing nobody thought about — it's there because the coil section is usually under negative pressure (the blower is pulling air through it). Without a trap, that suction either:

• Pulls air UP the drain line instead of letting water flow down — the water backs up in the pan and overflows, even though the line is "open." Or
• On a positive-pressure section (blower pushing through the coil, like some draw-through/blow-through configs), pushes air out the drain and can blow water out of the pan.

The trap's water seal blocks that airflow so the water can actually drain. A missing or dry trap is a top cause of "the drain is clear but it still leaks." The trap depth has to be matched to the unit's static pressure — too shallow and the negative pressure sucks the trap dry.

In the field

Condensate calls are almost always one of: plugged drain, dry/missing trap, failed pump, or a tripped float switch (which is doing its job because of one of the first three). The water itself is the clue — where it's pooling tells you where the system failed.

Normal values & targets

• Water production: a running AC produces real water — often on the order of several pints to a couple gallons per hour depending on tonnage and humidity. A bone-dry pan in humid weather with the system running means either it's not draining (clog backing up elsewhere) or the coil isn't getting cold.
• Drain slope: the gravity line should pitch downhill the whole way (commonly about 1/8"–1/4" per foot minimum) so water doesn't pool.
• Trap depth: sized to the unit's static; deep enough that the blower's negative pressure can't suck it dry.
• Float switch: opens before the pan overflows; verify it actually interrupts the call when lifted.
• Pump: cycles on as the reservoir fills and clears it quickly; lifts to the height the install needs (rated lift on the pump).

Common faults & what they mean

• Water dripping from the indoor unit / ceiling stain. Primary drain plugged (algae/biofilm slime is the usual culprit), dry or missing trap, or a low spot holding water. The pan overflows.
• System shut off on a hot day, pan full of water. Float/safety switch tripped — it's protecting you. Find why the water isn't draining (clog, trap, pump) instead of just bypassing the switch.
• Drain is clear but it still leaks. Classic missing/dry-trap problem — the blower's suction is holding water in the pan or pulling the trap dry. Install/correct the trap to the right depth.
• Condensate pump not running / reservoir full. Failed pump motor, stuck float in the pump, clogged pump check valve or discharge line, or no power to the pump. Its safety switch (if wired) should have killed cooling.
• Pump runs but doesn't empty. Clogged discharge tubing, failed check valve letting water run back, or lift beyond the pump's rating.
• Green slime / algae everywhere. Biological growth in the pan and line — the root cause of most clogs. Clean it and treat with pan tablets.
• Secondary pan has water in it. A warning — the primary system already failed and the backup caught it. Don't just drain the secondary; fix the primary.

Tech tips & gotchas

• Most "AC leaking water" calls are a plugged drain or a bad trap — not the equipment. Clear the line (wet/dry vac on the outside termination pulls clogs fast), confirm the trap holds water and is the right depth, and you've fixed the majority of them.
• The float switch tripping is a symptom, not the problem. It shut the system down because water backed up. Never just jumper it out to "get cooling back" — you've removed the only thing preventing a ceiling collapse. Find the clog.
• A dry trap mimics a clog. If the line is clear but it still overflows, suspect the trap is being sucked dry or is missing. Match trap depth to the unit's static pressure.
• Treat the slime, not just the symptom. Clearing a clog without addressing the algae means you're back in a month. Pan tablets/treatment and a flushed line buy real time.
• Pour water in the pan to test the whole path — pan, trap, line, pump, termination — and confirm the float switch interrupts the call when lifted. Prove it drains before you leave.

Safety / code notes

• Condensate disposal follows the mechanical/plumbing code condensate provisions: drain to an approved point, proper pipe size (commonly 3/4" minimum), and slope; it can't just dump where it causes damage or a nuisance.
• A secondary drain pan and/or a float/water-level safety switch is required where overflow from the primary would damage the building (e.g., units above finished space) — wire the switch to interrupt the equipment, not bypass it.
• Acidic condensate from high-efficiency (90%+) furnaces may require neutralizing before discharge to certain drains per local code — that's a combustion-side condensate, separate from the cooling pan but the same drain discipline.
• Never defeat a float safety to restore cooling — that's how a clog becomes a flood.