Furnace Leaking Water: Find the Source, Then Fix It

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Water on the floor by the furnace is alarming, but it is rarely an emergency on its own. The first question is what kind of furnace you have, because a high-efficiency condensing furnace makes water every time it runs, while a standard 80% furnace should not make any. Once you know which you have, the likely source narrows to a handful of places you can check in ten minutes. This guide is part of our Bay Area Winter Heating Help Center.

Do this first

1. Turn the heat off at the thermostat. If water is near the furnace's wiring, electrical box or the power cord of a condensate pump, also switch off the furnace service switch (it looks like a light switch on or near the unit) or the breaker.
2. Protect the floor and ceiling. Towels, a shallow pan, and for attic furnaces, a bucket under any ceiling stain.
3. Follow the wet trail uphill. Water runs downhill and along pipes, so the puddle is often a foot or two from the source. Use a flashlight and look at the pipes and hoses coming out of the furnace, not only the floor.
4. Look beside the furnace too. In many Bay Area garages and closets the water heater stands right next to the furnace, and it is a common source of water that gets blamed on the furnace.

Which furnace do you have?

You can usually tell in a minute without opening anything.

• Condensing furnace (about 90% efficiency or higher): one or two white or gray plastic (PVC) pipes leave the furnace and run to an outside wall or through the roof, and a small plastic trap and a thin drain tube or hose come off the side of the cabinet. There may be a small condensate pump with a reservoir on the floor next to it. The label or manual will show an AFUE in the 90s.
• Standard or "80%" furnace: a round metal flue pipe leaves the top of the furnace and goes up into a metal vent or chimney. There is no condensate drain on the furnace itself. Many hall-closet and garage furnaces in older homes are this type.

The difference is by design. The U.S. Department of Energy's consumer guidance described condensing furnaces as "condensing flue gases in a second heat exchanger for extra efficiency", reaching 90% to 98.5% AFUE, and noted that PVC pipe is "safely used in condensing furnaces". DOE's 2023 furnace rule likewise lists "condensate drain systems" and condensate removal among the extra parts a condensing furnace needs (Federal Register). An 80% furnace is built to send its flue gas up the vent hot, without condensing it, so it has no drain of its own.

If you have a condensing furnace

The Carrier condensing furnace owner's manual puts it simply: "some water will accumulate in the unit as a result of the heat transfer process." That water is supposed to go from the furnace, through a trap, down a drain line and into a floor drain, a pipe or a condensate pump. A leak means something along that path is blocked, loose or cracked. In rough order of how often we find them:

1. A clogged condensate trap

The trap is a small plastic fitting (often clear or white) on the side of the furnace that holds a little water to keep flue gas out of the drain. Over a year or two it collects rust flakes and a slimy film until it backs up. Water then overflows inside the cabinet or spills from a fitting. Carrier's manual says the condensate removal system "should be cleaned annually by a qualified service agency".

2. A drain line that is blocked, kinked or pulled loose

Thin vinyl drain tubing gets stepped on, kinked behind the furnace, or pulled off its fitting when someone changes the filter or moves storage. Check that the tube runs downhill all the way to the drain and is pushed firmly onto its fittings. A line that ends at a floor drain can also be blocked where it enters the drain.

3. A condensate pump that has failed or is unplugged

When there is no floor drain nearby, a small pump collects the water and pushes it out through a thin tube. If the pump is unplugged, the outlet it uses has tripped a GFCI, the float is stuck, or the discharge tube is blocked, the reservoir overflows. Many pumps have a safety switch wired to shut the furnace off when the reservoir fills, so water on the floor plus no heat often means the pump did its job by stopping the furnace. Check that the pump is plugged in and the outlet has power. If it still does not run, it needs replacing.

4. Vent pipe leaks

The PVC vent carries warm, wet flue gas, and it should slope so that water that condenses in the pipe runs back to the furnace drain. A sagging section, a cracked joint or a pipe pulled apart can drip. Carrier's manual lists inspecting the PVC pipes "for sags, holes, cracks, water leaks, blockage or disconnections". A disconnected vent pipe is a flue gas problem as well as a water problem, so do not run the furnace until it is fixed.

5. Internal leaks

Less often, the collector box or secondary heat exchanger inside the furnace leaks, or a drain port inside the cabinet is blocked. Water inside the burner or blower compartment, or rust streaks on the cabinet, point this way. It is a technician job.

If you have an 80% furnace

If there is no condensate drain on the furnace, the water is almost always coming from something else. Check these:

• The air conditioner or heat pump coil above the furnace. The indoor coil sits on top of the furnace in most systems and has its own drain pan and line. It makes water when cooling, and warm Bay Area winter and spring afternoons can bring the AC on for a day. A clogged or rusted pan leaks down onto the furnace.
• A whole-house humidifier. These are fed by a small water line and many have a drain. A stuck valve, a split supply line or a clogged drain can drip onto or into the furnace.
• The water heater. Look at the relief valve discharge pipe, the fittings on top and the base of the tank.
• The flue. If the metal vent drips, flue gas may be cooling and condensing in the vent, which can happen when a furnace vents into an oversized or unlined masonry chimney, or rain may be entering through a missing or damaged cap. DOE's consumer guidance noted that furnaces in the 80% range "have a greater risk of depositing acidic condensation in chimneys". Either way, a dripping flue needs a technician, because venting problems can also be carbon monoxide problems.
• The house itself. A roof leak above an attic furnace during a storm, or a plumbing line in the same closet.

If none of these explain it, or you are not sure what type you have, call and we will identify it.

What not to do

• Do not keep resetting a furnace that shut down with water in the cabinet. The shutdown is protecting it.
• Do not remove the inner panels, take the trap apart with the furnace running, or disconnect vent pipes.
• Do not pour drain cleaner into the trap or drain. It can damage plastic parts, and the trap is on the flue gas side of the system.
• Do not run a furnace that sat in floodwater, even if it seems to work.

What a technician does

For a condensing furnace we clear and flush the trap and drain, test the condensate pump and its safety switch, re-slope and secure loose tubing, check the vent pitch and joints, and look inside for leaks from the collector box or heat exchanger. We also check whether water has reached the control board, igniter or burners, since rust there causes the next round of problems. For an 80% furnace we trace the source to the coil, humidifier, flue or plumbing and fix what is ours to fix. If water came from the gas side of a flood, the gas controls get replaced, not dried out.

Because a condensing furnace's drain needs cleaning every year, it belongs on the annual service list; our fall furnace maintenance checklist shows the rest. For typical repair costs, see our furnace repair cost guide rather than a guess here. If the leak has left you without heat, run through our no-heat checklist once the water is sorted, and if the furnace is now cycling on and off, see furnace short cycling. Our residential repair team covers the South Bay, Peninsula, Tri-Valley and East Bay cities we serve, with a 1-year parts and labor warranty on every job.

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Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.