Quick answer
Water around a furnace almost always comes from something attached to it: a clogged drain on the AC coil above it in summer, a blocked condensate line on a high-efficiency furnace in winter, a humidifier on the ductwork, or a water heater in the same closet. Switch the furnace off, soak up the water and use the season to narrow it down. If the water keeps coming with the furnace off, close the humidifier valve or the water heater's cold inlet. Call (888) 840-6512 to reach a certified water damage restoration company in your area; the line is answered 24/7.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed September 30, 2026· Next review Mar 2027
A furnace that burns gas does not normally leave water on the floor. When one does, the water is almost always coming from something attached to it: the air conditioner coil that sits on top, the condensate drain of a high-efficiency furnace, a whole-house humidifier, or a water heater sharing the same closet. Finding which one matters, because two of them keep leaking whether or not the heat is on.
First, Make It Safe
- 1Switch the furnace off. Use the switch on or next to the unit, or its breaker. Water and the furnace's electrical controls do not mix.
- 2Soak up the water and find the wet edge. Towels or a wet vacuum, then look at how far the water has spread under the cabinet, into carpet, and against the walls.
- 3Work out which part is dripping. The season and where the water collects narrow it down fast, using the table below.
- 4Check the floor and walls before you call it fixed. Furnaces sit in basements, closets and garages, often on wood and next to drywall. Press the baseboards and look under any flooring the water touched.
Where Furnace Water Comes From
| When it leaks | Likely source | What it means |
|---|---|---|
| Summer, while the AC runs | AC coil above the furnace, clogged drain or cracked pan | The coil drain is backing up into the furnace cabinet |
| Winter, heat on, high-efficiency furnace | Condensate line, trap or pump | These furnaces make water; the drain is blocked or disconnected |
| Winter, a humidifier on the ductwork | Humidifier valve or its drain line | Supplied from a water line, so it can leak with the furnace off |
| Any time, puddle spreading from the side | Water heater in the same closet | Often mistaken for the furnace; check the tank first |
| Rust streaks inside the cabinet | Long-running slow leak | Parts may be corroded; have it inspected |
A high-efficiency furnace (a white plastic vent pipe instead of a metal flue) cools its exhaust until water condenses out of it. That water goes down a small drain, sometimes through a pump. When the drain freezes, clogs or comes loose, the water lands on the floor. The furnace itself is doing its job; the drain is not.
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Humidifiers and Water Heaters Keep Going
A humidifier is plumbed to a water line and a water heater is connected to the main. Either can leak with the furnace switched off. If the water keeps coming after you shut the furnace down, close the small valve feeding the humidifier, or the cold-water inlet on top of the water heater. The water heater leaking guide covers the tank.
The Damage Around the Furnace
Most furnace leaks are slow, and the unit hides the water under its own cabinet. By the time a puddle shows, the water has often wicked into the bottom plate of the wall, the carpet pad in the next room, or the subfloor under a first-floor closet. Wood and drywall that stay wet start growing mold in about 24–48 hours. A furnace room also feeds air into the ductwork, which is how a musty closet becomes a musty house. If you already smell it, mold in air ducts and vents explains what the next step looks like.
Two Different Jobs
The HVAC technician fixes the drain, the pan, the pump or the humidifier. A restoration company measures what the water reached, dries it to a dry reading, and removes what cannot be dried. Clean-water drying runs around $3–$7 per square foot. It stays small when it is started while the drywall is still solid.
If the floor around the furnace is soft, or the water has run under a wall, (888) 840-6512 is answered 24/7 and connects you with a certified restoration company that serves your area. The company gives you an arrival window for your address on the call, and there is no charge for the referral.
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Frequently Asked Questions
Why is water leaking from my furnace?
In summer it is usually the air conditioner coil sitting on top of the furnace, with a clogged drain or cracked pan. In winter a high-efficiency furnace makes its own water, and a blocked, frozen or disconnected condensate drain puts it on the floor. A humidifier or a nearby water heater are the other usual sources.
Is a furnace leaking water dangerous?
The water itself is not an emergency, but it can reach the furnace's electrical controls and rust parts inside the cabinet. Switch the unit off at its switch or breaker until the source is found.
Why is my furnace leaking water when the AC is on?
The AC's evaporator coil sits in or on top of the furnace cabinet. When its condensate drain clogs or the pan cracks, water runs down through the furnace and pools under it.
Can a furnace leak with the heat off?
The furnace itself stops making water when it is off, but a whole-house humidifier is connected to a water line and a water heater is connected to the main. Either can keep leaking. Close the humidifier's feed valve or the water heater's cold-water inlet.
Does water under a furnace cause mold?
It can. Furnaces sit on wood floors and next to drywall in closets and basements, and material that stays wet grows mold in about 24–48 hours. The furnace then moves that air through the ducts.
Who do I call right now for water damage?
Call (888) 840-6512. The line is answered 24 hours a day, seven days a week, and connects you with an IICRC-certified water damage restoration company that serves your area. Describe what you can see — where the water is coming from, how long it has been there, and whether it smells — the more specific you are, the faster the right company is on the line.
If the source is a pipe or an appliance, shut the main water valve first. Have four things ready and the call goes quickly: the address, when the water started, what is wet now, and whether you have filed an insurance claim.
Still not sure which of these is your situation? (888) 840-6512 · toll-free · answered 24/7
Sources
- IICRC S500 Standard for Professional Water Damage Restoration
- EPA — A Brief Guide to Mold, Moisture and Your Home
- U.S. Department of Energy — Furnaces and Boilers
Methodology: How we source and verify data · Report an error
Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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