Water on the floor: the condensate drain, from clog to float switch

Updated August 30, 2026 · Claude AC · 3 sources cited

Contents (8)

Your air conditioner is also a dehumidifier
Where that water is supposed to go
Reading the leak by where it appears
What you can safely do yourself
The float switch: reset it, never bypass it
When it is not the drain at all
What it costs, and what prevents it
Frequently asked questions

An air conditioner that leaks water is not usually broken. It is doing exactly what it was designed to do — pulling moisture out of the air — and the drainage that was supposed to carry that water away has stopped working. Understanding how much water is involved changes how urgent the problem looks, because most homeowners badly underestimate it.

Your air conditioner is also a dehumidifier

Cooling air below its dew point condenses moisture on the evaporator coil. That is not a side effect; on a humid afternoon it is a large fraction of the work the machine is doing. The arithmetic is straightforward. A ton of cooling is 12,000 Btu/h. Condensing a pound of water releases roughly 1,060 Btu. A US gallon of water weighs 8.34 pounds. A system spending a fifth to a third of its capacity on latent heat — a normal range in humid weather — therefore produces:

System size	At 20 % latent capacity	At 30 % latent capacity	Over a 10-hour run day
2 tons	0.5 gal/h	0.8 gal/h	5–8 gallons
3 tons	0.8 gal/h	1.2 gal/h	8–12 gallons
4 tons	1.1 gal/h	1.6 gal/h	11–16 gallons
5 tons	1.4 gal/h	2.0 gal/h	14–20 gallons

These are indicative figures for humid conditions — a dry-climate system in Arizona may produce almost nothing, which is its own diagnostic clue. But in the Southeast and along the Gulf, a stopped drain is not a slow drip. It is ten or twenty gallons a day looking for somewhere else to go, which is why ceiling damage from an attic air handler can appear within a single afternoon.

Where that water is supposed to go

The path is short and every element of it fails in a predictable way:

The primary drain pan, directly under the evaporator coil, integral to the coil box. In older systems it is metal, and metal pans rust through from below where you cannot see them.
The trap — a U-bend just outside the coil box. Its job is not smell, it is pressure. The coil usually sits on the suction side of the blower, so the cabinet runs at negative pressure; without a water seal in the trap, the blower pulls air backwards up the drain line and holds the condensate in the pan instead of letting it flow. A dry or missing trap produces a pan that overflows while the drain line is perfectly clear.
The primary drain line, conventionally 3/4-inch PVC, pitched downhill at not less than about 1/8 inch per foot, with a cleanout tee near the unit. Anything flatter holds water; anything holding water grows biofilm.
The termination — outdoors, or into a plumbing drain through an air gap. Water dripping steadily from that pipe on a humid day is the system working correctly.

Because a clogged drain in an attic or above a finished ceiling can wreck a house, the model mechanical codes most states adopt require a second line of defence for equipment in those locations: an emergency pan under the whole unit with its own drain, or a secondary drain routed to a conspicuous spot where dripping will be noticed, or a water-level switch that shuts the equipment off before the pan overflows. In practice you often have two of the three.

Reading the leak by where it appears

What you see	Most likely cause	Urgency
Puddle at the base of the indoor unit or furnace	Clogged primary line, dry or broken trap, rusted primary pan	Turn the system off now
Stain or drip on the ceiling below an attic unit	Emergency pan overflowing; no working float switch	Turn it off, call the same day
Steady drip from a pipe stub under the eave or over a window	The secondary drain doing its job — the primary is blocked	Not an emergency, but the warning it was built to give
System dead, thermostat calling, nothing runs	Float switch tripped on a full pan	Find the water before resetting anything
Water only in the first hot week of the season	Trap dried out over winter, or debris settled in a flat line	Routine
Sweating pipe with damp insulation around it	Uninsulated drain line condensing in a hot attic — not a leak	Fix at leisure
Ice on the copper lines, then a flood after shutdown	Frozen evaporator coil melting; a refrigerant or airflow fault	See the diagnostic order below

What you can safely do yourself

Switch the system off at the thermostat, and at the breaker if there is standing water near electrical components. Running it makes more water.
Mop up and check the filter. A smothered filter drops evaporator temperature and ices the coil, which then melts faster than the pan can drain. Filter selection is covered in our guide to MERV ratings and airflow.
Clear the line from the outdoor end. A wet/dry vacuum sealed over the termination stub with a rag, run for two or three minutes, pulls out the sludge plug that causes most clogs. This is the single most effective homeowner fix on this page.
Flush from the cleanout. Remove the cap on the tee near the air handler and pour through a cup or two of warm water, then distilled white vinegar. Vinegar is the sensible routine choice: household bleach is corrosive to metal pans, fittings and the equipment cabinet, and should not be a monthly habit.
Confirm flow. Water should reach the termination within seconds. If it does not, the blockage is beyond a homeowner flush.

The float switch: reset it, never bypass it

A tripped water-level switch is a system that shut itself off deliberately to protect your ceiling. Some reset themselves once the pan drains; some need a manual reset. What matters is the order: find and clear the water first, reset second. Jumping the switch out with a wire — occasionally suggested online, and occasionally done by a technician in a hurry — removes the only thing standing between a future clog and a collapsed ceiling. If a contractor bypasses one to keep you cool for the weekend, get it restored in writing.

When it is not the drain at all

Two failures masquerade as drainage problems. A frozen evaporator coil — from a blocked filter, a collapsed duct, a failed blower or a low refrigerant charge — produces a sheet of ice that overwhelms the pan when it thaws. The tell is weak airflow and poor cooling in the days before the water appeared; work through our AC not cooling walkthrough rather than flushing a drain that was never blocked. And a condensate pump, used where gravity drainage is impossible in basements and closets, is an electrical device with its own float and check valve; when it fails the pan fills exactly as if the line were clogged.

There is also a comfort-side lesson hiding in the water volume. A system that produces very little condensate in muggy weather is not removing moisture, which is the mechanism behind a house that reads 74 °F and still feels clammy — the subject of our guide to cold but humid houses.

What it costs, and what prevents it

Clearing a blocked condensate line is one of the cheapest service calls in cooling — indicatively in the low hundreds of dollars for a straightforward visit, more if the line has to be cut and re-plumbed or a rusted pan replaced, and more again for after-hours work in August. Prevention is genuinely effective here, unlike with most HVAC failures:

Flush the line with vinegar at the start of the cooling season and again mid-summer.
Change the filter on schedule — most drain sludge starts as dust that got past a loaded filter and settled on a wet coil.
Have the pan, trap, slope and float switch checked during a spring visit; they are on the list in our guide to what a real tune-up covers.
Insulate any drain line running through an unconditioned attic, so you never confuse condensation on the pipe with a leak from it.
Treat wet drywall and wet insulation as a mould question, not just a water question: the EPA's guidance is to dry affected materials within 24 to 48 hours or remove them.

Frequently asked questions

Is it safe to run the air conditioner while it is leaking water?

No. Every hour of runtime adds gallons to wherever the water is going, and if the leak is near the blower, control board or electrical connections you are adding a shock and short-circuit risk to a water-damage one. Switch it off at the thermostat, deal with the drain, then restart.

Can I pour bleach down my AC drain line?

It is a common recommendation and a poor routine one. Chlorine bleach attacks metal drain pans, fasteners and the cabinet, and where a drain ties into a plumbing line it may meet other cleaners. Distilled white vinegar clears biofilm well enough for a seasonal flush, and a wet/dry vacuum removes an actual plug far better than any liquid.

Why is water dripping outside from a pipe near my roofline?

That is almost certainly the secondary drain, deliberately terminated somewhere visible so that you notice it. It means the primary drain has failed and the emergency pan is now carrying the water. The system is not damaged, but it is running on its backup — clear the primary line before the backup blocks too.

How much water should my AC produce?

Anywhere from a trickle in dry desert air to a couple of gallons an hour on a five-ton system in Gulf Coast humidity. What matters is not the absolute figure but the change: a system that suddenly stops dripping outdoors in muggy weather has either stopped removing moisture or stopped draining it, and both are worth investigating.

Does a leaking AC mean I need a new system?

Rarely. A clog, a dry trap or a failed pump are repairs, not verdicts. A rusted-through primary pan on an older system is the one case where the arithmetic gets awkward, because the pan sits under the coil and the labour to reach it is significant — that is the point to weigh against the framework in our guide to repairing versus replacing.