Furnace Leaking Water: What It Means, What to Check, and When to Call

A puddle by the furnace in heating season usually means water the furnace made isn't reaching the drain. Only a high-efficiency furnace makes water: the Department of Energy explains that condensing furnaces condense the water vapor out of their exhaust, and Lennox says a standard-efficiency furnace doesn't produce water at all. Bryant lists the common culprits as a clogged condensate drain, a frozen condensate line, a failed condensate pump, or a leaking humidifier. Less commonly but more seriously, Carrier says, it's a heat exchanger problem.

For the step-by-step fix for a clogged line, see our clogged condensate drain guide. If the water is coming from the tank next door, see water heater leaking; if there's ice on the AC coil or its copper lines, see frozen AC coil.

First: Does Your Furnace Make Water?

Look at the exhaust pipe. Trane says PVC exhaust and intake pipes mean a condensing furnace, and a metal exhaust means a standard one. Lennox adds that the yellow EnergyGuide label should show the AFUE rating, 90% or above for high-efficiency and around 80% for conventional, and warns that a conventional furnace's exhaust pipe can get dangerously hot.

A condensing furnace pulls extra heat from its exhaust in a secondary heat exchanger, Trane explains, and water forms there. The water leaves through a trap, a drain line, and sometimes a small pump; Bryant says a few gallons a day during heating is typical. A standard furnace has one heat exchanger and won't make that water, so water around one probably has another source: Lennox points to a humidifier, the AC's drain pan, or plumbing.

Most Likely Causes

On a high-efficiency furnace, check them in this order. On a standard furnace, start at number 4.

1. A clogged condensate drain line or trap. Bryant calls it a common cause, and Lennox says debris in the drain hose, line, or trap makes it leak. In winter, Carrier and Bryant add, a line can freeze, especially where it runs outside. Clear the line outside the cabinet as our condensate drain guide describes. The trap inside the cabinet is the technician's: Carrier's instructions leave everything but filter care to trained service personnel. Bryant says never to thaw an iced line with an open flame.
2. A failed condensate pump, or a blocked or loose pump line. Where gravity can't carry the water away, a small pump lifts it out. Bryant says one that fails, loses power, or can't discharge lets its reservoir overflow; Lennox says pumps typically last three to five years. Many have a high-water safety switch, which Little Giant says can be wired to shut down the unit producing the condensate, so a furnace that quit beside a full pump may have been stopped on purpose.
3. A loose, disconnected, or cracked pipe joint. Lennox says a drain line may simply have come loose, cracked pipes must be replaced, and a loose or poorly fitted exhaust pipe leaks water from the unit. The PVC exhaust carries condensate too: the Department of Energy's installation guideline says it should slope back toward the furnace, and that vent and condensate lines should be leak-tight to keep water and flue gases out of the house. Bryant says drainage or connection problems often show up as a leak soon after installation or service, so call the installer back. Keep the furnace off if the exhaust has separated: the CPSC lists a loose or disconnected vent among the clues to a carbon monoxide problem.
4. A leaking whole-house humidifier. Trane says a clogged humidifier pad, a clogged or cracked drain line, or loose connections can put water on the floor. Look for drips at its fittings or a wet trail down the duct. Some flow is normal: AprilAire says its drain-through models send a steady stream down the drain line to flush away minerals, enough to fill a quart jar in about five minutes on its Models 500 and 600. Bryant says to turn off its water if you can do so safely and call an HVAC professional; on an AprilAire, fully close the saddle valve on its water line.
5. The air conditioner's indoor coil. Trane explains that the AC's evaporator coil pairs with the furnace, and its condensation drains into a pan that can clog and overflow by the furnace; water while the AC runs isn't a furnace leak. A clogged filter can freeze the coil, and Lennox says the ice then drips onto the floor beneath the furnace when the AC shuts off. Turn the AC off and see our frozen coil guide; Trane says a professional should deal with a frozen coil.
6. Something nearby. Trane says to rule out a water heater, washing machine, or wash tub in the same room, and Lennox says a damaged pipe near the furnace can be the culprit, a job for a plumber or HVAC technician. Dry the floor, lay paper towels around each suspect, and see which gets wet first.
7. A leaking secondary heat exchanger. Lennox says a faulty one can cause a condensation leak. Carrier's leak checklist ties water or corrosion inside or around the furnace, or unusual operation, to a heat exchanger or combustion problem. Leave the furnace off and call: Carrier says never to inspect or repair a heat exchanger yourself, because a crack can let carbon monoxide into the airflow. See cracked heat exchanger.

What You Can Fix Yourself

Lennox says to leave repairs to a furnace expert and stick to damage control. Bryant says you can check the filter and the parts outside the cabinet, but not open it.

1. Power off, as in the box above.
2. Mop up. Trane says to soak up water with towels, a sponge, or a wet/dry vac, then dry the area with a fan. The EPA says areas dried within 24 to 48 hours usually won't grow mold.
3. Check the filter. Lennox says to replace it if it's dirty or wet; Bryant notes a new one won't clear a blocked drain. Our filter guide shows the swap.
4. Check the condensate pump. Bryant says to see whether it has power and whether its reservoir is full. Little Giant, a pump maker, says never to handle a pump with wet hands or while standing on a wet surface, and to unplug it before touching its water. Its manual says to make sure the floats move freely, clean a dirty float, set the pump level, and clear kinked or blocked outlet tubing. If a pump with power and a clean, free float won't start with a full tank, the manual says the switch or motor is defective and the pump should be replaced; Carrier's instructions call for one approved for acidic furnace condensate.
5. Walk the drain line. Bryant says to look for blockages, kinks, disconnected sections, and ice where it runs outside. Lennox suggests flushing a clog with equal parts white vinegar and hot water; use it only on the line outside the cabinet.
6. Shut off a leaking humidifier's water and leave it off until it's repaired.

Carrier says not to run the furnace if that could expose electrical parts to water. Restore power once everything is dry; if the water comes back or the furnace won't restart, call.

When to Call a Pro

• Water from inside the cabinet or the heat exchanger area. Carrier says to contact a qualified HVAC professional immediately.
• Rust or corrosion. Lennox says rust can cause holes and cracks in the heat exchanger that can expose you to carbon monoxide, and the CPSC lists rusting or water streaking on a vent among the clues to a CO problem.
• A shutdown with a pressure switch code. On one Carrier condensing furnace, a blocked condensate drain is among the causes of a pressure switch fault, and water trapped in the vent pipe can trip the switch. On Rheem's upflow 90-plus furnaces, a blocked-drain switch cuts the gas valve, and four trips in one heating call lock the furnace out. Trane says to write down the blink pattern for the person scheduling the repair; see furnace won't turn on.
• Any gas smell. Leave the house without switching anything on or off or using a phone inside, and call 911 and the gas utility once you're a safe distance away. NW Natural, a gas utility, says to go even if the smell seems mild.
• Soot on the furnace or its vent. The CPSC lists sooting among signs of improper appliance operation, and says only a trained service technician can detect and correct hidden problems.
• Water on wiring or the control board. Lennox says water on internal electrical parts can stop the furnace working altogether. Call, too, for a frozen line or a leak that keeps coming back.

Prevent It

• A yearly tune-up. ENERGY STAR's fall heating check-up includes the furnace's condensate drain and heat exchanger, and Trane suggests a furnace tune-up each fall and the AC's each spring.
• Condensate line care. Lennox says to have the furnace inspected and cleaned at least once a year to avoid a clogged drain. Our condensate guide explains why makers point to vinegar, not bleach.
• The pump. Little Giant says to inspect and test it every six months, clean it with a 50/50 mix of water and vinegar once or twice a year, and keep chlorine out, which damages the pump.
• The humidifier. AprilAire says to replace the water panel yearly on most models, or twice a heating season on the Model 400, and to close the saddle valve for summer.
• Winter trips. Carrier warns that a plugged drain or failed pump can shut the furnace down, and says to protect the pipes before leaving the house in freezing weather. Keep the heat no lower than 55°F when away, as the Red Cross advises.
• A leak sensor by the furnace and pump. Little Giant says that where a leaking or stopped pump could cause damage, a backup switch or alarm should be used and monitored. See our leak sensor setup guide and smart leak detector picks.

Frequently Asked Questions

Is it normal for a furnace to leak water?

Making water is normal for a high-efficiency furnace, Bryant says, but Carrier says it should flow through a drain line, not pool on the floor; a puddle means the drainage is likely blocked or broken. A standard furnace shouldn't make water at all.

Is a leaking furnace an emergency?

Usually not, Bryant says, but fix it quickly. Carrier says it can be one if a failing heat exchanger is the cause, because of the carbon monoxide risk, and that even a minor leak can create electrical hazards. A gas smell, a CO alarm, or soot means act now.

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

Trane says that's not the furnace but the AC's indoor coil, whose drain pan can clog and overflow. Ice on the coil or its lines means the AC has frozen; turn it off.

Why does my furnace leak more in winter?

Carrier says a furnace runs more in winter and makes more condensate, so drain problems show up then, and Bryant says a line exposed outdoors can freeze.

The Bottom Line

Turn the furnace off, dry the floor, and work outward: the drain line and trap, the pump, the pipe joints, the humidifier, the AC coil, then anything nearby. Water from inside the cabinet, rust, soot, a pressure switch lockout, or a gas smell means a technician, and a leak sensor by the furnace catches the next leak early.

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