Furnace Leaking Water? Why It Happens & How to Fix It

A furnace leaking water is usually a drainage problem. High-efficiency furnaces make water as they burn gas, and when the drain, trap or condensate pump clogs, it spills onto the floor. In summer, the water usually comes from the AC coil sitting on top of the furnace. Clearing a clogged drain or trap typically costs $75 to $250. A leaking heat exchanger is the expensive exception, at $1,000 to $3,500, and it's often covered by warranty.

Typical pro cost

$75 to $500

LowHigh

Urgency: Within a day or two
DIY difficulty: Easy to moderate
Time to fix: 30 minutes to 2 hours
Prices: US typical range

Last reviewed October 2026

First, find out which kind of furnace you have

This one check tells you more than anything else, and it takes ten seconds. Look at the pipe that carries exhaust out of the top or side of the furnace.

White or gray plastic (PVC) pipes, usually two of them: You have a high-efficiency, or "condensing," furnace, rated around 90% or higher. It's designed to make water. A gallon or more a day in cold weather is normal, as long as it goes down the drain and not onto the floor.
A metal flue pipe going into a chimney or a metal vent: You have a standard-efficiency furnace, around 80%. It shouldn't make any water inside the cabinet. Water here usually means a venting problem, or it's coming from something nearby.

High-efficiency furnaces squeeze extra heat out of the exhaust by cooling it until the water vapor in it turns to liquid. That water is mildly acidic. It runs into a plastic trap, then down a drain line to a floor drain or a condensate pump. Every one of those parts can clog, crack or come loose, and that accounts for the large majority of furnace leaks.

Turn it off first: If water is pooling around the furnace, switch it off at the thermostat, then at the service switch on or near the furnace. Mop up the water. Standing water near furnace wiring and a control board is a real hazard, and it can short out a $300 board.

Leaking in winter or in summer? It matters

Leaking when the heat is on

Look at the furnace's own drainage: the condensate trap, drain hoses, drain line and pump. On standard furnaces, check the flue pipe for condensation. A whole-house humidifier is another common source in winter.

Leaking when the AC is on

The water is almost always coming from the AC's evaporator coil, which sits on top of the furnace. Its drain pan or drain line has clogged, or the coil froze and is now melting. The furnace just happens to be underneath.

If it's summer, our guide to an AC leaking water inside is the better place to start.

The most common causes

Clogged condensate trap

The number one cause on high-efficiency furnaces. Sludge and rust settle in the trap's loop and block it. Water backs up and overflows inside the cabinet.

Clogged or disconnected drain line

Slime builds inside the drain tube, or a hose slips off its fitting during a service visit. Look for a tube hanging loose or dripping.

Failed condensate pump

If the furnace is in a basement with no floor drain nearby, a small pump moves the water. When it fails, the tank overflows. See condensate pump not working.

Frozen drain line outside

If the drain ends outdoors, it can freeze in a cold snap. Water backs up into the furnace even though nothing is clogged.

Humidifier leak

A whole-house humidifier on the duct has its own water line and drain. A clogged pad, stuck valve or cracked drain tube can drip down the side of the furnace.

Flue condensation (standard furnaces)

On an 80% furnace, exhaust that cools too much in an oversized or uninsulated flue can condense and run back down. Have it checked, because it can also mean poor venting.

Leaking heat exchanger

On high-efficiency models, the secondary heat exchanger can corrode and leak acidic water inside the cabinet. It's less common, expensive, and needs a technician to confirm.

Something else nearby

Water heaters, laundry lines and sweating cold-water pipes often sit right next to furnaces. Before blaming the furnace, follow the water to where it's coming from.

How to fix a leaking furnace, step by step

These steps are safe for most homeowners and fix the majority of high-efficiency furnace leaks. You'll need a flashlight, a bucket, towels, and ideally a wet/dry vac.

1
Turn the furnace off at the thermostat and the service switch. Dry the floor and the bottom of the cabinet so you can see where fresh water appears.
2
Replace the air filter if it's dirty. A clogged filter can freeze the AC coil in summer and makes everything run harder.
3
Remove the front panel. Find the condensate trap, a plastic loop or box with clear or black tubes going into it. Follow every tube with your flashlight and look for one that's loose, cracked or dripping.
4
Clean the trap. On most furnaces it unclips or unscrews. Take a photo first so you can put it back the same way. Empty it into a bucket, rinse out the sludge with warm water, refit it, and pour in a cup of water so it's primed. A dry trap can let exhaust escape.
5
Clear the drain line from the far end with a wet/dry vac, or flush it with warm water. If the line runs outside, check that the end isn't frozen or buried in snow.
6
If there's a condensate pump, unplug it, clean out the tank, and pour in water to test that it switches on and empties.
7
Replace the panel, turn the furnace on, and watch the trap and floor through one full heating cycle. Dry floor means you're done.

Safety: Always turn off power before removing panels. Don't take apart the inducer, the vent pipes, or anything in the burner area. If you see rust-colored water dripping from inside the heat exchanger area, or smell exhaust, turn the furnace off and call a technician. Make sure your carbon monoxide alarms work.

Tools and materials

Flashlight and phone for photos
Bucket, towels and a wet/dry vac
Screwdriver for the front panel
New air filter

Approximate DIY cost: $0 to $40

Call a pro when

Water is coming from inside the heat exchanger area
A standard (metal flue) furnace is dripping from the vent
The leak returns within days of cleaning
The control board or wiring got wet
You smell exhaust or your CO alarm goes off

Cost to fix a furnace leaking water

Typical US prices, including labor and parts where a pro does the work
Job	Typical cost
Diagnostic or service call	$100–$200
Clean condensate trap and clear drain line	$75–$250
Replace drain hoses or trap	$100–$300
Condensate pump replaced	$150–$500
Humidifier repair (pad, valve or drain)	$100–$400
AC coil drain pan replaced	$150–$600
Flue or vent pipe correction	$150–$500
Secondary heat exchanger replaced	$1,000–$3,500

Ranges are typical US prices compiled by FixHomeCost from published cost guides. Heat exchangers often carry a 20-year or lifetime parts warranty, so you may only pay labor. Check your paperwork before agreeing to the repair.

Repair or replace?

Almost every furnace leak is worth repairing. A trap, hose or pump costs a fraction of a new furnace. The decision only gets real with a heat exchanger leak on an older system. If the furnace is over about 15 years old and the heat exchanger isn't under warranty, a new furnace is often the better value, especially if you'd be paying $2,500 or more for the repair. Our repair or replace calculator and furnace replacement cost guide can help you compare.

How to keep it from happening again

Have the condensate trap cleaned at every fall tune-up. Ask for it specifically.
If the drain runs outside, insulate it or reroute it to an indoor drain so it can't freeze.
Replace the humidifier pad every year, before heating season.
Change the air filter every 1 to 3 months.
Leave a little space around the furnace so you can see a leak early.

Common questions

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

On a high-efficiency furnace, the condensate trap, drain line or condensate pump is usually clogged or disconnected. A humidifier leak is the next most common cause.

Is it normal for a furnace to produce water?

Yes, for high-efficiency furnaces with plastic vent pipes. They make water as they burn gas, and it should drain away through a hose. Standard furnaces with metal flues shouldn't produce water inside the cabinet.

Is a furnace leaking water dangerous?

Usually the bigger risk is water damage and a shorted control board. If the leak is from the heat exchanger area or you smell exhaust, turn the furnace off and have it checked, because venting problems can be a carbon monoxide risk.

How much does it cost to fix a furnace that's leaking water?

Most fixes cost $75 to $500, such as clearing a drain, replacing a trap or replacing a condensate pump. Replacing a leaking secondary heat exchanger costs about $1,000 to $3,500, and the part is often under warranty.

Why is my furnace leaking water in the summer?

In summer, the water almost always comes from the AC evaporator coil above the furnace. Its drain line has clogged, the pan is overflowing, or the coil froze and is melting.

Can I run my furnace if it's leaking water?

It's best not to until you find the source. Water can damage the control board and floor. Clearing a clogged trap or drain often takes under an hour.

This is general information, not a professional inspection. Gas furnace work can be dangerous; hire a licensed professional when in doubt. See how we research costs.