Why is my furnace leaking water?
A 90%-plus furnace makes water on purpose; an 80% furnace shouldn't make any. Here's how to read the leak by furnace type, and which fixes cost $0 versus $3,000.
The short answer
On a high-efficiency (90%+ AFUE) condensing furnace, the usual cause is a clogged condensate drain, trap, or failed condensate pump. A leaking whole-house humidifier is next. On a standard 80% furnace, water usually means exhaust is condensing in the vent, which is a venting and carbon monoxide question for a licensed technician. Drain and pump fixes often run a few hundred dollars. A failed heat exchanger can run $1,000 to $3,000.
Estimate, not a final quote
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A 90%-plus furnace produces water every hour it runs. An 80% furnace isn't designed to produce any inside the cabinet. That one number, printed on the yellow EnergyGuide sticker or the rating plate inside the door, cuts the list of causes roughly in half before you touch anything.
Find out which furnace you have
Trane's definition is simple: a condensing furnace, rated 90% AFUE or higher, has two heat exchangers. A standard furnace, around 80% AFUE, has only the primary one. The second exchanger pulls so much heat out of the exhaust that the water vapor in it condenses into liquid. Lennox puts the exhaust leaving a condensing furnace at roughly 100°F to 150°F, cool enough to run through plastic pipe.
You can tell them apart without opening anything:
| What you see | Furnace type | What water at the furnace usually means |
|---|---|---|
| Two white or gray PVC pipes leaving the cabinet, often out a side wall; a small drain tube to a floor drain or pump | Condensing, 90%+ AFUE | Condensate isn't reaching the drain: clogged line or trap, failed pump, frozen line |
| One metal flue pipe going up to a chimney or roof vent; no drain tube | Standard, ~80% AFUE | Exhaust condensing in the vent, a blocked flue, or a leak from something else nearby |
| Water only when the AC runs, in summer | Either | It's the AC coil and drain pan above the furnace, not the furnace |
| Water near a box mounted on the supply duct, with a thin copper or plastic water line | Either | The whole-house humidifier |
If the water shows up only in cooling season, read our AC leaking water guide instead. Trane notes those leaks usually come from the evaporator coil and drain pan sitting on top of the furnace.
First, shut it down and dry the floor
Turn the system off at the thermostat, mop up, and check the filter, which is Trane's advice for every leak cause. Speed matters more for the floor than the furnace. EPA's guidance is that wet materials dried within 24 to 48 hours usually won't grow mold, so get a towel or wet/dry vac on carpet and drywall today, not after the service call.
If you smell gas or your carbon monoxide alarm is sounding, don't diagnose anything. Carrier's instruction is to leave the house and call emergency services first.
Condensing furnace: follow the drain
Carrier and Lennox both name a clogged condensate drain as the most common cause of a leaking high-efficiency furnace. Lennox lists dirt and debris in the drain hose, the drain line, or the condensate trap. The trap is the molded plastic fitting, usually on the side of the cabinet, that the drain tubes run through before the water leaves.
That trap is not a plumbing nicety. Nu-Calgon's condensate-neutralizer instructions state that every condensate drain needs a trap to stop flue gas from escaping, and that flue gas leakage can cause injury or death from carbon monoxide. Lennox's maintenance instructions call for the trap to be inspected for cracks, cleaned, and kept filled with water. Flat rule: if a tech or handyman offers to replace a cracked trap with a straight piece of tubing, say no.
Here's what a homeowner can safely check, without opening the burner compartment:
- The pump, if you have one. A condensate pump is a small plastic box with a tank and a discharge tube. Carrier lists a faulty or loose pump as a common source of water. Check that it's plugged in and the discharge tube isn't kinked. Lennox notes these pumps typically last three to five years, so a pump that hums but doesn't empty its tank in a furnace that's a few years old is a likely culprit.
- The outdoor end of the line. Carrier flags a frozen condensate line as a winter cause. When the line exits through a wall or crawlspace, an ice plug at the cold end backs water up to the furnace.
- The drain's slope. IRC Section M1411.3 requires condensate piping to slope at least 1/8 inch per foot toward its disposal point. A line that sags or runs uphill holds water and clogs faster.
Pulling the trap, clearing internal drain hoses, and checking pressure-switch tubing are a technician's job. Carrier says not to take internal components apart yourself.
Lennox's ML193UH manual warns that a restricted drain can flood the heat exchanger. That's why the 2018 IRC, at Section G2404.10, requires an auxiliary drain pan under a Category IV condensing furnace wherever a stoppage would damage the building, unless the furnace shuts itself down when the drain backs up. A furnace that leaked and stopped heating may have done exactly what it's supposed to.
Cost estimator
HVAC cost estimator
A budgeting range, not a quote. Move the inputs to see what changes the number.
Estimated national cost
Typical project lands near $8,250.
If a written quote comes in above the high end, that's not automatically wrong — ask what scope is driving it.
What this assumes
- Standard central HVAC replacement scope.
- 1,500 sq ft home size selected.
- No major duct, electrical, or structural repairs included.
What moves this price
- 01Equipment tier, SEER2 rating, and brand.
- 02Ductwork condition and attic or crawlspace access.
- 03Permit requirements, refrigerant changes, and local labor rates.
Walk into your quotes ready
This range is your anchor. Bring these to every contractor conversation so you can tell a fair quote from a padded one.
- What SEER2 rating and tonnage are you quoting, and why that size for my home?
- Is a full load calculation (Manual J) included, or are you sizing by rule of thumb?
- Does the price include new line sets, a permit, and haul-away of the old unit?
- What's the labor warranty, and is it separate from the manufacturer's parts warranty?
- A quote given over the phone without seeing your ductwork or home.
- Pressure to decide today or 'lose' a discount.
- An oversized system pitched as 'more powerful'. Bigger isn't better for comfort.
What each repair costs
| Cause | Who fixes it | Published cost |
|---|---|---|
| Pump unplugged, tube kinked, or ice at the outdoor end | You | $0 |
| Clogged drain line or trap, cleared and re-primed | HVAC technician | Service call of $50–$200, plus labor |
| Condensate pump replacement | HVAC technician | $250–$440 |
| Humidifier water panel | You, per the manufacturer | Price of the panel kit for your model |
| Inducer motor (if water has reached it) | HVAC technician | $400–$1,100 |
| Cracked or leaking heat exchanger | HVAC technician | $1,000–$3,000 |
Service-call, pump, inducer, and heat exchanger figures are Trane's published national estimates. Trane publishes no separate figure for clearing a furnace drain; that total depends on your labor rate, whether the trap has to come off, and whether the furnace sits in a basement or an attic.
The humidifier: the other water source on the furnace
If your furnace has a whole-house humidifier, it gets its own water supply and its own drain. Both Trane and Carrier list it as a common leak source. AprilAire's FAQ names the most likely cause: a water panel so clogged with minerals that water can't flow through it and spills over. Other causes are a clogged drain line, a unit mounted out of level, and a worn or missing orifice that meters water from the solenoid valve.
AprilAire says its 10, 12, and 35 water panels should be changed once per heating season, the 45 panel twice, and that you can swap one yourself. A cracked fitting or failed solenoid valve is a service call.
Water at an 80% furnace is a venting problem
A standard furnace sends its exhaust up a metal flue hot enough to keep water vapor from condensing. When you find water dripping from the flue connector, rusty streaks down the vent pipe, or water pooling at the base of an 80% furnace in heating season, the exhaust is cooling too much before it leaves the house. A partly blocked vent can do it. So can a vent or chimney that's too large or too cold for the equipment connected to it.
Carrier calls a cracked heat exchanger the least common cause of a furnace leak but the most serious, because the same crack can let carbon monoxide into the house.
Flat rule: on an 80% furnace, water is a call to a licensed HVAC technician, not a project. Don't keep running it while you wait. The fix involves the vent, the chimney liner, or the heat exchanger, and none of that is homeowner work.
Repair or replace
The drain, the pump, and the humidifier are cheap fixes on any furnace worth keeping. The decision only gets hard when a technician says the water is coming from the secondary heat exchanger itself. Lennox acknowledges that a failing secondary heat exchanger can leak condensate, but it's far less common than a blocked drain. Ask the tech to show you that the trap and drain were cleared and the leak continued anyway.
Trane recommends a second opinion on any furnace repair quoted over $500. A heat exchanger at $1,000 to $3,000 clears that easily. If the heat exchanger is out of warranty, get a replacement quote before you approve the repair. Our HVAC quote guide covers what that bid should include.
Research sources
- Trane: Furnace Leaking Water? Find Out Why and What to Do
- Trane: What Is a Condensing Furnace?
- Trane: What Is a Heat Exchanger in HVAC?
- Trane: Average Furnace Repair Cost
- Trane: Cracked Heat Exchanger, What It Means
- Carrier: Furnace Leaking Water? Causes, Signs, and Repair Steps
- Lennox: Furnace Leaking Water, Causes and Solutions
- Lennox: What Is a Condensing Furnace?
- Lennox: ML193UH Installation and Maintenance Instructions
- Nu-Calgon: pH-Treat Condensate Neutralizer
- AprilAire: Frequently Asked Questions
- AprilAire: When to Change Your Water Panels
- ICC: 2018 IRC Section M1411.3, Condensate Disposal
- ICC: 2018 IRC Chapter 24, Fuel Gas (Section G2404.10)
- EPA: A Brief Guide to Mold, Moisture and Your Home
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