Water Leaking from the Indoor Unit
Drain pans, traps, and coils. A leak at the air handler is almost never random — it's condensate that lost its path to the drain. Here's how to trace it back to the real cause instead of just mopping it up.
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In This Guide
First: Is It Even Condensate?
Before you pull a panel, confirm the water is actually coming from the system. Condensate is clean, odorless, and shows up only when the equipment has been running in cooling (or when a high-efficiency furnace is producing flue condensate). If the water is dirty, appears after rain, or drips when the system has been off for hours, you may be chasing a roof leak, a sweating supply plenum, or a nearby plumbing line — not the coil.
A quick tell: dry the area completely, run the system in cooling for 15–20 minutes, and watch where the water reappears. Condensate leaks track the run cycle. On a horizontal air handler in an attic, water on the ceiling below almost always means the secondary pan is doing its job and the primary path has failed.
The physics in one line
A 3-ton system on a humid day can pull 1–3+ gallons of water per hour out of the air. That's a lot of liquid that has exactly one job: get to the drain. Anything that interrupts that path ends up on the floor, the ceiling, or inside the furnace.
Where the Water Comes From
The evaporator coil runs below the dew point of the return air, so moisture condenses on the fins and runs down into the primary drain pan molded into or set under the coil. From there it flows through the primary drain line — through a P-trap — and out to a proper termination. A correctly built system also has a second line of defense: either a secondary (auxiliary) drain pan under the whole cabinet or a secondary drain port on the coil, protected by a float switch that cuts the cooling call before water ever overflows.
When you get a leak call, one of those components has failed to move or contain the water. Your job is to figure out which link in the chain broke.
The Six Common Causes
In the field, indoor water leaks almost always trace back to one of these. Roughly in order of how often you'll see them:
- Clogged condensate drain line — algae, biofilm, and dust turn to sludge and block the line. Water backs up and overruns the pan.
- Frozen evaporator coil that melts — low charge or low airflow ices the coil; when it thaws, meltwater sheets off far faster than the pan can handle.
- Dry or missing P-trap — a draw-through coil under negative pressure pulls the trap dry (or air blows it out), so water sits in the pan or the pan sweats and the fan slings droplets.
- Cracked or rusted-through drain pan — older galvanized pans rust; plastic pans crack or the drain fitting fails.
- Improper slope or a blown termination — a drain line that runs uphill, sags, or terminates against a wall won't carry water away.
- Failed condensate pump — on units below the drain, a stuck float, seized impeller, or unplugged pump lets the reservoir overflow.
Don't stop at the clog
Clearing a clog fixes today's leak, but if the coil is icing because of low charge or a dirty filter, you'll be back next week. A leak is a symptom — always ask what let the water get loose in the first place.
Step-by-Step Diagnostic
Here's a worked sequence for a typical call: "water dripping from the ceiling under an attic air handler, system still cooling."
Step 1 — Kill power & check the float switches.
Pull the disconnect. Inspect the primary pan float and any secondary-pan float. If a float is up but the system was still cooling, the switch or its wiring failed — that's why the pan overflowed instead of shutting the unit down.
Step 2 — Open the coil compartment.
Look for standing water in the primary pan, biological slime at the drain outlet, rust streaks, and ice on the coil face. Ice tells you to go hunt airflow and charge before anything else.
Step 3 — Test the primary drain.
Pour a quart of water into the primary pan. If it doesn't drain freely, the line is clogged downstream. Check the P-trap: if it's bone dry, air is short-circuiting through the line and water is pooling.
Step 4 — Verify airflow & delta-T.
Measure total external static pressure and the temperature split across the coil. A healthy cooling delta-T is roughly 16–22°F. A split above ~22°F with a dirty filter points to low airflow — the classic coil-freeze setup.
Step 5 — Confirm the diagnosis.
In this example, the pan was full, the line was blocked with slime, the trap held water, and airflow checked out. Root cause: a clogged primary drain plus a float switch that never tripped.
Result: Clog + dead float
Clear the line, refill and treat the pan, and replace the failed float switch so the safety actually works next time. Two fixes, one callback avoided.
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Clearing a Clogged Drain
A blocked primary drain is the single most common leak cause, so it's worth doing right. The most reliable method is to pull the clog toward you from the termination with a wet/dry vacuum rather than pushing it deeper.
- Attach a wet/dry vac to the outdoor or utility-sink termination and run it 60–90 seconds to pull the sludge out of the line.
- Open the cleanout tee near the air handler and flush with water to confirm free flow all the way through.
- Some techs finish with a nitrogen blow-out from the coil side — keep pressure low and stand clear of the outlet.
- Never blow raw nitrogen or compressed air backward through the coil pan hard enough to crack fittings.
- Treat the pan with condensate tablets or a manufacturer-approved biocide. Skip bleach on aluminum coils and cased-coil pans — it corrodes them.
Water damage already done?
If the leak soaked drywall, insulation, or a furnace's control board, document it and flag it to the customer. A shorted board or a rusted-through heat exchanger from chronic condensate is a safety issue, not just a cosmetic one.
Traps, Slope & Pan Reference
Most chronic leaks come back to install details that violate the mechanical code. The IMC (Section 307) and IRC (M1411) set the baseline; here are the numbers you'll actually check against:
| Item | Requirement / Target | Why It Matters |
|---|---|---|
| Drain pipe size | 3/4" min. (nominal) | Undersized line clogs and floods faster |
| Slope | 1/8" per foot min. | Flat or back-pitched lines hold water and sludge |
| P-trap | Deeper than fan static | Shallow traps blow dry on draw-through coils |
| Secondary protection | Pan, port, or float | Required over finished space to stop overflow |
| Cooling delta-T | ~16–22°F | High split flags low airflow / coil freeze |
| Termination | Approved, air-gapped | No direct connection to sanitary drain |
The trap depth trap
On a draw-through (blower-after-coil) air handler, the coil pan sits under negative pressure. If the trap isn't deep enough to overcome the fan's static, air pulls the trap dry and water stops draining. Size the trap so the total depth exceeds the operating static — and always leave a cleanout so it can be primed and serviced.
Preventing the Callback
A leak fixed once but never explained is a callback waiting to happen. Close the loop before you leave:
- Confirm the float switch actually interrupts the cooling call — lift it and verify the system shuts down.
- Run the system 20 minutes and watch the pan drain fully with no standing water.
- Set the customer up with a fresh filter and a maintenance interval so the coil stays clean and airflow stays high.
- Leave condensate tablets in the pan and note the drain layout for the next tech.
- If you found a rusted pan, a cracked heat exchanger, or a marginal trap, write it up — don't let a band-aid become the permanent fix.
Ninety percent of indoor water leaks are a clog, a dry trap, or a frozen coil. Work the chain from pan to termination, and you'll find the break every time.
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