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Furnaces

Furnace Leaking Water? Why It Happens and What to Check

A furnace burns gas, so water seems out of place. But a high-efficiency furnace makes water on purpose every time it runs, and most leaks are that water failing to drain.

By Household Atlas Editors Updated 3-minute read 3 sources

Quick answer

A white plastic (PVC) exhaust pipe means a high-efficiency condensing furnace, which makes water as it runs; a leak usually means its condensate line, trap or pump is clogged or failed. If it only leaks with the AC on, the coil’s drain pan is overflowing. A furnace-mounted humidifier is another source. Turn the system off, mop up, change the filter, clear the drain, and call a technician if it continues.

Difficulty
Moderate DIY
Makes water on purpose
Condensing furnaces, 90% AFUE or higher
Most common cause
Clogged condensate line, trap or pump
Leaks only when cooling
AC coil drain pan, not the furnace
Turn off first
Water and furnace electronics do not mix
On this page

Burning natural gas produces water vapor. A standard furnace sends that vapor up a metal flue while it is still hot. A high-efficiency condensing furnace squeezes extra heat out of the exhaust with a second heat exchanger, cooling it until the vapor condenses into liquid. That water, slightly acidic, has to drain away through a trap and a line to a floor drain or a small condensate pump. When it does not, you get a puddle.

First, stop and protect

  1. Turn the system off

    at the thermostat, then at the furnace service switch. Water near the control board and wiring is a shock and damage risk.

  2. Mop up and dry the area

    with towels or a wet/dry vacuum, so you can see where new water appears. Standing water around a furnace also invites mold.

  3. Replace the air filter

    if it is dirty. A clogged filter can freeze the AC coil in summer and makes any furnace work harder.

Find the source

When it leaks Clue Likely source
Heating, high-efficiency furnace (white PVC vent) Water near the drain trap, hoses or pump Clogged or disconnected condensate line, trap or pump
Heating, pump on the floor Pump full, not running, or its tubing kinked Failed condensate pump or its float switch
Heating, very cold weather Drain line runs through a cold garage, crawl space or outside Frozen condensate line backing up
Heating, humidifier on the furnace Drips from the humidifier or its water line Clogged pad, drain line or loose connection
Cooling only Water down the furnace sides, wet pan above AC coil drain pan overflowing, or a frozen coil thawing
Heating, standard furnace (metal vent) Rust or drips at the vent pipe Poorly fitted or sloped vent, or humidifier
Any time, water from inside the cabinet No obvious external source Internal drain or secondary heat exchanger: technician
Where water comes from in a furnace and AC system An AC evaporator coil sits on top of the furnace and drips into a drain pan. Inside a condensing furnace, a secondary heat exchanger produces condensate that runs through a trap to a drain line or condensate pump. A humidifier mounted on the ductwork has its own water supply and drain. AC evaporator coil Drips into its own drain pan Primary heat exchanger Secondary exchanger Condensate forms here Humidifier Pump or drain PVC vent = condensing furnace
Plate 1Three separate water sources share one corner of the basement: the AC coil above the furnace, the condensing furnace’s own condensate, and a humidifier on the ductwork. Each has its own drain, and each can overflow.

Fixes you can usually do

  • Clear the condensate line. With the furnace off, disconnect the external drain line where it leaves the trap or enters the pump and clear it with a wet/dry vacuum or by flushing it. Lennox suggests white vinegar and hot water for clogs. Reconnect it so it slopes steadily down to the drain.
  • Check the condensate pump. Lift the pump’s float gently or pour water into its tank: it should start and empty. If it hums without pumping, or never starts, it needs replacing. Make sure its outlet tubing is not kinked or frozen.
  • Thaw a frozen line. Lines run through unheated spaces can freeze in cold snaps. Warm the line gently and insulate or reroute it so it stays above freezing.
  • Service the humidifier. Replace a crusted pad, clear its drain line and tighten its water connection.
  • Clear the AC drain in cooling season, and replace the filter if the coil froze. The AC freezing guide covers this.

Call a technician when water comes from inside the cabinet, the leak returns after you clear the drain, or you see rust, corrosion or water near the burners or vent connections. A failed secondary heat exchanger or a leaking vent connection needs professional repair, and any furnace problem is a good moment to confirm your carbon monoxide alarms work.

Prevent it

  • Have the furnace serviced each fall; the technician should clean the condensate trap and check the pump.
  • Change filters on schedule, which protects the AC coil from freezing and overflowing.
  • Insulate or reroute condensate lines that pass through cold spaces.
  • Add a water sensor next to the furnace so a slow leak is caught early.
Bottom lineA white plastic vent means your furnace makes water by design, and a leak means its drain, trap or pump is blocked or failed. Leaks only in summer come from the AC coil above. Turn the system off, dry the area, clear the drain, and call a technician if water comes from inside the cabinet.

Frequently asked questions

Is it normal for a furnace to leak water?

It is normal for a high-efficiency (condensing) furnace to produce water, but not for that water to end up on the floor. It should run through a condensate drain to a floor drain or a small pump. A puddle means something in that drain path is clogged, cracked, disconnected or failed.

How do I know if I have a high-efficiency furnace?

Look at the exhaust pipe. Condensing furnaces vent through white PVC plastic pipe and have a small drain line; standard furnaces vent through a metal flue. The yellow EnergyGuide label or the model plate shows the AFUE; Trane describes high-efficiency furnaces as 90 percent AFUE or higher.

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

The furnace is not leaking. The air conditioner’s evaporator coil usually sits on top of the furnace and drips condensation into a pan with its own drain. If that drain clogs or the coil freezes and then thaws, the pan overflows and water runs down the furnace cabinet.

Can I clean the furnace condensate line myself?

Often, yes, on accessible lines: with the furnace off, disconnect the line at the trap or pump, clear it with a wet/dry vacuum, and flush it. Lennox suggests white vinegar and hot water for clogged condensate lines. Do not pour anything into the furnace’s internal trap or collector box, and call a technician if the leak is from inside the cabinet.

Can a cracked heat exchanger cause a water leak?

A failed secondary heat exchanger in a condensing furnace can leak condensation, and it is an expensive repair. Any suspected heat exchanger problem needs a technician, and a working carbon monoxide alarm nearby.

Sources

  1. Furnace Leaking Water? Find Out Why and What to Do Trane
  2. Furnace Leaking Water: Causes & Solutions Lennox
  3. Furnace Leaking Water? Causes, Signs, and Repair Steps Carrier

First published October 7, 2026. Last reviewed October 7, 2026. Spot an error? Tell us and we will check it against the source.

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