Why is my AC or furnace leaking water?

Wondering what's going on in there?

Where the water comes from, in plain English

Your AC and a high-efficiency furnace both make water as they run. That's normal. A leak means the water isn't getting out of the house the way it should.

Each number matches a step below. Dashed lines show a pump that some homes have.

The AC coil sweats. In summer, the cold coil on top of the furnace pulls water out of the air, the same way a cold drink sweats. Carrier says that water is normal.
A pan catches the drips. The drain pan under the coil collects the water.
A drain line carries it away. A skinny white plastic pipe runs from the pan to outside, a floor drain or a sink. A capped T near the unit is where you can pour vinegar to keep it clear.
Some homes use a pump. When the water can't run downhill, a small condensate pump pushes it out. It plugs into an outlet, and a float turns it on, like the one in a toilet tank.
High-efficiency furnaces make water too. In winter, a furnace with two white plastic pipes cools its exhaust so much that water forms (Trane). Small tubes and a trap send it to a drain or a pump. A standard furnace with one metal pipe shouldn't drain water.

So a leak usually comes from one of these:

A clog in the drain line, the pump or the furnace's trap.
A pump that stopped: no power, a stuck float, or worn out.
A frozen coil that leaks as it melts, from a dirty filter or low refrigerant.
A loose tube, or a cracked pan.
Something else nearby: the water heater, a pipe or the humidifier.

The questions at the top check these in that order. Water by the water heater instead? See a water heater that's leaking. Ice on the pipe and warm air? See an AC blowing warm air. Furnace turning on and off? See a furnace that keeps turning on and off. No heat at all? See a furnace that won't turn on.

AC won't start at all? See our guide to an AC that won't turn on.

Who to call

Rather not do it yourself? Call an HVAC tech.

That's a fine choice. Maybe you're busy. Maybe you're selling the house and just want it done right. An HVAC tech (a heating and cooling pro) can find where the water is coming from and stop it.

$75–$250Clear the drain line

$100–$450New condensate pump

Most companies charge $75–$250 to come out, and many put that toward the repair.

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What to say

Calling a pro? Here's what to say.

A few lines to read when they pick up, and the questions to ask before anyone comes out.

Before you call

Under 10 years old? Call your installer first.

Home warranty? Call them first.

4.5 stars or more, and a license number.

Say this

Hi. There's water leaking around my furnace. When's the soonest you can get here? And what's your fee for the visit? Does it go toward the repair?

Always ask

What's your fee to come out, and does it go toward the repair?
Will you tell me the full price before you start any work?
Are you licensed and insured?

Watch out if

They push a whole new system on the first visit for a small problem.
They won't give you a price before starting.
They want to "just add refrigerant" every year without finding the leak.

Feeling pushed to decide on the spot? You can say no and hang up. You owe nothing for a phone call.

Know what's wrong before they come

Answer up to 5 quick questions. We'll give you the words a pro uses.

Answer the questions →

More of a list person?

Common causes of a leak, tests and costs

Find the row that sounds like yours. The quick test is something you can check yourself in a few minutes.

What you notice	What it probably is	Quick test	Do it yourself	With a pro	Verdict
Water by the water heater, or a pipe next to the furnace	The water heater or a pipe is leaking, not the furnace	Dry the floor; see which spot gets wet first	Free	Not needed	FIXSee the answer: The water heater or a pipe is leaking, not the furnace
Water drips from a box on the furnace's duct	The furnace's humidifier is leaking	Look under the box with the thin water tube	Pro job	$130–$404	CALLSee the answer: The furnace's humidifier is leaking
Ice on the copper pipe; water as it melts	Not enough air froze the coil	Hold the filter up to a light; look for frost on the pipe	$5–$40	Not needed	FIXSee the answer: Not enough air froze the coil
Ice on the pipe even with a clean filter	Low refrigerant or a dirty coil	Ice with a clean filter; hissing at the indoor unit	Pro job	$100–$2,000	CALLSee the answer: Low refrigerant or a dirty coil
Pump box full and silent; unplugged, or a dead outlet	The condensate pump has no power	Check the plug, the outlet and the breaker	Free	Not needed	FIXSee the answer: The condensate pump has no power
Pump plugged in, tank full, but silent or only humming	The condensate pump is dirty, or its float is stuck	Pour in water and watch the float lift	Free	Not needed	FIXSee the answer: The condensate pump is dirty, or its float is stuck
Pump won't run after cleaning, or its tank is cracked	The condensate pump is worn out	Pour in water: the float lifts, but nothing runs	Pro job	$100–$450	CALLSee the answer: The condensate pump is worn out
Water in the pan, little or nothing at the pipe's end	A clogged drain line	Look for standing water in the pan under the indoor unit	$0–$30	$75–$250	FIXSee the answer: A clogged drain line
A tube off its fitting, or out of the floor drain	The drain pipe came loose	Follow the drain tube from end to end	Free	$75–$200	FIXSee the answer: The drain pipe came loose
Drain works, but the pan is rusty or cracked and drips	A cracked or rusted drain pan	Shine a flashlight under the pan	Pro job	$200–$600	CALLSee the answer: A cracked or rusted drain pan
Leaks while heating; high-efficiency furnace	The furnace's drain is clogged or frozen	Look for ice where the drain tube comes out outside	Pro job	$75–$250	CALLSee the answer: The furnace's drain is clogged or frozen
Rust or soot inside the furnace, or a CO alarm	A leak inside the furnace's heat exchanger	None. Turn the furnace off	Pro job	$100–$3,000	CALLSee the answer: A leak inside the furnace's heat exchanger
Standard furnace with a metal pipe drips while heating	Water is coming down the exhaust pipe	Look for water on the metal pipe or its joints	Pro job	$75–$200	CALLSee the answer: Water is coming down the exhaust pipe

2026 U.S. averages, rounded. Pro prices include parts and labor. Most companies charge $75–$250 to come out, and many put that toward the repair. Nights, weekends and big cities cost more. See all repair prices

Want proof the drain is clear?

The cup-of-water test

Carrier's techs test a drain line by pouring clean water through it. You can do the same after you clear it.

Set the thermostat to OFF.
Find the small cap on the T in the drain pipe near the indoor unit, and take it off.
Slowly pour in a cup or two of water.
Go to the pipe's end (outside, or at the floor drain), or listen at the pump.

Water comes out the end, or the pump starts and empties: the line is clear.
Water backs up at the cap, or fills the pan: it's still clogged. Clear it with a wet/dry vac, or call a pro.

Put the cap back on when you're done.

Not sure it's safe to keep running?

Turn it off right away if…

Water is near wiring, an outlet, or the furnace's power parts. Carrier says not to run a furnace if that could put water on electrical parts.
A ceiling is wet or sagging. Stay out from under it. Wet drywall can fall.
A carbon monoxide alarm goes off, or you see rust or soot inside the furnace. Get everyone outside and call 911.
You smell gas. Don't touch any switches. Leave the house and call the gas company from outside.
You see ice on the copper pipe or the coil. Set it to OFF and FAN to ON until it melts.
The pan or the pump is overflowing.

To turn it off, set the thermostat to OFF. If water is near wiring, also flip the furnace switch or its breaker off, but only if you can reach it without standing in water.

Got a big repair quote?

Should you repair it or replace it?

Most leaks cost little to fix. A pro charges about $75–$250 to clear a drain line, $100–$450 for a new condensate pump and $200–$600 for a drain pan. The bigger bills are a new coil or a heat exchanger. Ask yourself three things:

Is it the coil? HomeAdvisor and Fixr list $600–$2,000 for a new one, and HomeGuide up to $4,500.
Is it the heat exchanger? Trane lists $1,000–$3,000, and says an older furnace may be worth replacing instead.
The half rule. If the repair costs more than half the price of a new system, get a replacement price too.

Example: a $300 pump on a 6-year-old system is worth doing. A $2,500 heat exchanger on an old furnace is worth comparing with a new one.

Ran into a word you don't know?

AC and furnace words, explained

Read these before you start. The drawings show roughly what each part looks like.

Thermostat: The control on your wall where you set the temperature. OFF stops the system.
Indoor unit: The furnace or air handler, usually in a closet, attic, basement or garage. The AC coil sits on top of it or inside it.
Furnace: The big metal cabinet that heats air. In many homes the AC coil sits on top of it.
Coil: A grid of thin metal tubes that gets cold when the AC runs. It pulls water out of the air.
Drain pan: A shallow tray under the coil, or under the whole indoor unit, that catches the water.
Drain line: A skinny white plastic pipe or soft tube that carries the water outside, or to a floor drain or sink.
Drain access cap: A capped T on the drain pipe near the indoor unit. You pour vinegar in here.
Condensate pump: A small plastic box with a pump, beside the unit. It pushes the water out when it can't run downhill.
Float switch: A safety switch with a float. It shuts the AC off when water backs up into the pan.
Wet/dry vac: A shop vacuum that can suck up water. Held on the drain pipe's end, it pulls a clog out.
Air filter: A flat screen that catches dust before air reaches the coil and furnace. Change it every 1–3 months.
Vent (register): The grille in a floor, wall or ceiling where air comes into a room.
Refrigerant: The special fluid that carries heat outside. It runs in two copper pipes; the fat one wears a foam jacket.
Vent pipes: On high-efficiency furnaces, two white plastic pipes that go outside. One brings in air, the other pushes out exhaust.
Condensate trap: A curved piece inside a high-efficiency furnace that drains the water the furnace makes.
Exhaust pipe (flue): On a standard furnace, one metal pipe that carries exhaust out of the house.
Heat exchanger: The metal part inside the furnace that keeps the flames and exhaust apart from the air you breathe.
Humidifier: A box on the furnace's air duct that adds moisture to dry winter air. A thin tube feeds it water.
Furnace switch: A switch on or near the furnace that looks like a light switch, often on a red plate.
Breaker: A safety switch in your breaker box that shuts off power if something draws too much.
Carbon monoxide alarm: An alarm that warns you of carbon monoxide, a gas you can't see or smell.
Water heater: The tall tank that heats your home's water. It often sits right next to the furnace.

Still have questions?

Questions people ask most

Is an AC or furnace leaking water an emergency?

A leak is an emergency when water is soaking a ceiling, reaching wiring, or comes with a gas smell or a carbon monoxide alarm. Otherwise turn the system off, mop up, and it can usually wait for a free check or a weekday call. Carrier says it's an emergency when it causes water damage to the structure.

Who do I call for a furnace or AC leaking water?

Call a heating and cooling company (an HVAC tech). Try the free checks first: the filter, the drain line's outside end, the condensate pump's plug. If the water heater is leaking, call a plumber; if you rent, call your landlord.

Can I still use my AC if it's leaking water?

Carrier doesn't recommend running an AC that's leaking, because it can make the water damage or the problem worse. Turn it off until you find the leak. If a safety switch already shut it off, it stays off until the water drains.

Why is my furnace leaking water when the AC is on?

The AC's indoor coil usually sits on top of the furnace, and its drain line, pan or pump overflows. A clogged drain line is the most common cause (Carrier, Trane).

Why is my furnace leaking water when the heat is on?

Only high-efficiency furnaces make water when they heat, and a clogged drain line, trap or pump makes it leak (Trane). A standard furnace with a metal exhaust pipe shouldn't drain water, so call a pro to check the exhaust pipe.

Is it normal for an AC to leak water?

Water dripping from a small pipe outside your house is normal in summer, but water inside the house is not. The AC pulls water out of the air, and its drain line carries it outdoors (Carrier). Worry if that outside pipe stops dripping on hot days while water collects inside.

How do I unclog my AC drain line?

Turn the AC off, then hold a wet/dry vac tightly on the drain pipe's outside end for a minute or two, Carrier says. Pour 1 cup of vinegar into the access point near the indoor unit to help keep it clear.

What happens if my AC drain line is clogged?

Water backs up into the pan under the indoor unit and can spill onto the floor or through a ceiling. Other signs are a musty smell, weaker cooling, or an AC that shuts off (Carrier). Many systems have a float switch that turns the AC off when the pan fills.

Is an AC or furnace leaking water dangerous?

The water itself isn't harmful, but if it sits it can damage ceilings and floors and lead to mold (Carrier). A leak inside the furnace is different: Carrier says a cracked heat exchanger can let carbon monoxide into the air. Keep CO alarms working and turn the furnace off if one sounds.

How do I stop my AC from leaking water?

Turn the AC off first so no more water builds up, Carrier says. Then check the common causes: a dirty filter, a clogged drain line and a full condensate pump. A dirty filter can freeze the coil, and the ice leaks as it melts, even after the AC is off (Carrier, Lennox), so change the filter every 1 to 3 months. Still leaking? Call an HVAC tech.

Why does my AC drain line keep clogging?

Dust from the air mixes with the water the AC removes and makes a sludge of algae, dust and mold that builds up in the drain line (Carrier). Carrier says to have the drain line checked and cleaned at least once a year; a cup of vinegar in the access point between cleanings helps keep it clear.

How much does it cost to fix an AC or furnace leaking water?

Fixing a leak is often free, like clearing a drain line or plugging in a pump. Pros charge about $75–$250 to clear a drain line, $100–$450 for a new condensate pump, $200–$600 for a drain pan, and $200–$1,600 for a refrigerant leak.

How we built this

The questions follow the order an HVAC tech checks a leak: where the water really comes from, when it leaks, ice on the coil, then the drain line, the pump and the pan, or the furnace's own drain. Each answer makes some causes more likely and others less likely.

Prices are 2026 national averages from published repair-cost guides, rounded. Your local prices will differ.

How we make money: some links go to parts stores that pay us a small commission, and some pro listings pay us when you call them. Verdicts come from the diagnosis, not from who pays more.