What you'll see

Water on the floor, a stained ceiling below an attic unit, or a full secondary drain pan. In cooling, the evaporator pulls moisture out of the air and it has to go somewhere — when that "somewhere" backs up, it shows up as a leak. Most of these are plumbing-side condensate problems, not refrigerant problems, but a frozen coil that periodically thaws will also dump water and masquerade as a drain issue. Sort that out early.

First confirm it's actually condensate. In heating season on a high-efficiency furnace, the water is combustion condensate from the secondary heat exchanger, and the failure points are different (see below).

Walk it in order

  1. Find where the water is coming from. Primary drain pan overflowing? Secondary (emergency) pan holding water? Dripping off the bottom of the coil cabinet? A wet secondary pan specifically means the primary drain failed and the backup caught it — your real problem is the primary.
  1. Check the primary condensate drain for a clog. This is the #1 cause. The line slimes up with algae and biofilm. Confirm flow by pouring water in, or by pulling the cleanout cap. No flow = clogged line. Clear it (wet-vac from the termination, or blow it out) and verify it runs free.
  1. Check the trap. A condensate trap that's dry, missing, or installed wrong (especially on a draw-through coil under negative pressure) lets the blower hold water back in the pan instead of letting it drain. A trap full of sludge does the same. Confirm there's a proper trap and it's clear.
  1. Check pan slope and the pan itself. The drain pan has to slope to the outlet. A cabinet that settled, a pan packed with debris, or a rusted-through pan all pool or leak water. Look for a cracked secondary pan too.
  1. Rule out a frozen coil. If the coil ices and then thaws, it overflows the pan all at once when it melts — looks exactly like a drain problem on the next visit. Look for ice or recent ice on the coil, water staining that suggests a big periodic dump, and check airflow and charge. A coil that froze from low airflow or low charge is the real cause; the water is a symptom (see the frozen-coil article).
  1. Confirm the float/safety switch works. If there's a float switch and the line clogged, it should have shut the system down before the floor got wet. If water reached the floor with a float installed, the float is stuck, wired wrong, or in the wrong spot.
  1. High-efficiency furnace in heating: if the leak is in heat mode, suspect the combustion-condensate path — clogged condensate drain/trap, a cracked condensate line, a plugged inducer drain port, or a failed pressure-switch hose collecting water. The acidic condensate also rots out traps and pans over time.

What "normal" looks like

  • Condensate flow: water you pour into the pan/cleanout runs out the termination freely, no backup.
  • Trap: present, water-sealed, sized and oriented for the unit's draw-through or blow-through configuration.
  • Pan: clean, sloped to the outlet, not rusted or cracked; secondary pan dry.
  • Float switch: opens the 24V when the pan fills, closes when it drains.
  • Coil: no ice, draining steadily as water — not freezing and dumping.

Common faults & what they mean

  • Clogged primary drain line: the classic. Algae/biofilm plug, water backs into the pan and over. Clear it and treat it.
  • Bad or missing trap: blower pressure holds water in the pan or sucks the trap dry; pan overflows. Install/correct the trap.
  • Pan problems (slope, debris, rust-through): water pools or escapes even with a clear line. Clean or replace the pan.
  • Frozen-then-thawed coil: big periodic water dump, ice evidence on the coil. Fix the airflow/charge cause, not just the water.
  • Failed float switch: water reached the floor despite a float — it didn't shut the system down. Replace/relocate it.
  • High-eff furnace condensate fault: leak in heat mode from a clogged inducer drain, cracked tubing, or a water-logged pressure-switch hose.

Tech tips & gotchas

  • A wet secondary pan means the primary already failed. Don't just empty the backup pan — fix the primary drain or you'll be back.
  • Clear and treat the line. Tablets or a flush keep the biofilm from re-plugging by next summer. A vacuum alone buys a few months.
  • Check for a frozen coil before you blame the drain. Vacuuming a clear line on a coil that's actually freezing wastes the trip and the customer's money.
  • Verify the float actually shuts the system off. Lift it by hand with a cooling call active — the system should stop. If it doesn't, the safety isn't protecting anything.
  • Negative-pressure draw-through coils need a deep enough trap. Too shallow and the blower lifts the water column right out of the trap and floods the pan.

Safety / code notes

  • Condensate disposal and overflow protection must meet local mechanical/plumbing code — the float or secondary pan exists to prevent water damage and isn't optional on equipment located above finished space.
  • High-efficiency furnace condensate is acidic; route and neutralize per code and protect the floor drain/materials it contacts.
  • De-energize before reaching into the blower or coil cabinet, and watch for sharp coil fins and standing water near electrical components.