Furnace Leaking Water? Causes and What to Check

A furnace leaking water is usually a high efficiency furnace whose condensate, the water it pulls out of its own exhaust, is not reaching the drain: a clogged trap or drain line, a failed condensate pump or a loose line. Water dripping from above is often the AC coil pan or a humidifier.

What should I do first when my furnace is leaking water?

Turn the furnace off, keep the water away from anything electrical, then find the source.

Look before you step. Use a thermostat or switch only if you can reach it without stepping into water near outlets, cords or the electrical panel. Otherwise stay out and call for help.
Set the thermostat to Off. Trane's furnace leak guide says to turn the system off at the thermostat or breaker; Lennox cites the risk of shock or electrocution and says not to try to fix the leak yourself.
Rule out other water. Trane says to make sure the water is not from a nearby water heater, washing machine or laundry tub.
Note where the water is. The table below matches each spot to a likely cause.

A rotten egg smell, a carbon monoxide (CO) alarm or CO symptoms? Get everyone outside first, then read the safety section below.

Why does a high efficiency furnace make water at all?

Because it pulls extra heat out of its own exhaust. A high efficiency (condensing) furnace cools its flue gases until the water vapor in them turns to liquid, and that water has to drain away.

The U.S. Department of Energy's Building America guideline on high efficiency gas furnaces puts them at 90 to 98 percent AFUE, the share of the fuel a furnace turns into heat, against the 70 to 80 percent furnaces they often replace.

A standard furnace does not make water, Lennox says, so a leak near one points to plumbing or the air conditioner's drain pan.

How to tell which furnace you have

Trane's test: a metal exhaust pipe means a standard furnace; a PVC (white plastic) exhaust paired with a PVC intake pipe means a condensing furnace.

Where does the water from a furnace go?

Through a trap and a drain line to a drain. The DOE guideline describes the path:

The condensate trap in the furnace holds water.
The drain line runs down to a drain set lower than the trap, such as a floor drain, which a humidifier and an air conditioner can share.
A condensate pump lifts the water to a higher drain when none sits low enough.
The house drain, not the yard. In cold climates, the guideline says, condensate should drain only into the house sanitary drain, never outdoors.

What causes a furnace to leak water?

Usually a blocked or broken path to the drain.

Condensate line clogged or furnace drain line clogged

Lennox says dirt and debris in the drain hose, drain line or condensate trap cause water to leak.

A failed or unplugged condensate pump

Carrier names a condensate pump or drainage problem as a common cause; a stopped pump lets its tank overflow.

A cracked, loose or disconnected line

Trane names cracked drain lines and loose connections; Lennox adds a loose or badly fitted exhaust pipe. The DOE guideline wants vent and condensate lines leak tight, to prevent water damage and keep flue gases out of the house, so a leaking vent joint is a call, not a tape job.

The AC coil drain pan

Your air conditioner's indoor coil makes water too, and Trane says it drains into a pan that can clog and overflow. Lennox says a clogged filter can make the coil freeze, and the ice then melts and drips onto the floor when the AC shuts off. Trane says a frozen coil is a job for a professional.

A humidifier on the furnace

Trane names a clogged humidifier pad, a clogged or cracked drain line, or loose connections.

A frozen condensate or vent line

The DOE guideline calls for heat tape and insulation on traps and drain lines when a furnace sits outside heated space in a cold climate, and warns that frozen, burst lines leak into the space below and need a service call to restart the furnace.

A heat exchanger problem

Lennox lists a faulty secondary heat exchanger among the causes. Carrier warns that a crack in a heat exchanger can let combustion gases, including carbon monoxide, into the airflow, and says not to inspect or repair one yourself. If anyone in the house has a headache, dizziness, weakness, upset stomach or nausea, vomiting, chest pain or confusion, alarm or no alarm, get everyone outside to fresh air and call 911. A sounding CO alarm means the same. If we find a cracked heat exchanger, we red tag the furnace, show you the evidence and shut it down.

How can I tell where the water is coming from?

Where the water sits usually points to the cause.

Where the water is	Likely source	What to do
Base of a high efficiency furnace, heat on	Trap or drain line	Furnace off, call
Around a small pump, tank full	Pump failed or lost power	Check the plug, then call
Dripping from the box above the furnace	AC coil pan	Check the filter, call
Under a humidifier on the ductwork	Humidifier drain or water line	Furnace off, call
Along the white pipes or a joint	Loose or cracked line	Furnace off, call
Near a standard furnace (metal flue)	Not the furnace itself	Check AC, humidifier, plumbing

What can I safely check myself?

A few things, all with the furnace off: thermostat to Off, then the furnace's power switch. Go near the furnace, the pump or the floor drain only if you can do it without stepping into water near outlets, cords or the electrical panel; otherwise stay out and call for help.

The filter. ENERGY STAR's maintenance checklist says to inspect, clean or change it once a month and to check the condensate drain in your furnace. Sheet metal edges inside the filter slot can be sharp, and the cabinet can still be hot right after the furnace ran. Change a dirty filter; open nothing else.
The pump's plug. If the pump is unplugged and the outlet and floor are dry, plug it back in. If it is plugged in, its tank is full and it is silent, call. Do not reach into the tank or take the pump apart.
The end of the drain line. Check that the tube still reaches the floor drain and has not slipped out. If the floor drain is backing up or the line is cracked or clogged, stop and call.
The humidifier. Look for drips from it or its water line; leave valves and fittings to a technician.
The pipes outside. See the snow section below.

Stop there. Carrier says not to take apart internal components. Leave the trap and its tubes alone, never open the burner compartment, and do not restart a furnace that sat in water.

Why shouldn't I ignore a small furnace leak?

Because water inside a furnace damages the parts that keep it safe. Lennox says a leak can cause rust, damage internal electrical parts until the furnace stops running, and grow mold.

The U.S. Consumer Product Safety Commission says gas control valves, circuit breakers, GFCIs and fuses that have been under water must be replaced. So if water reached the furnace's gas valve or controls, a water heater, or your electrical panel, do not relight, restart or reset anything until a technician has checked it.

A furnace that keeps shutting itself off is on our list of reasons to call the 24/7 emergency line.

Can snow on the furnace pipes outside cause problems?

Yes. A condensing furnace that vents through pipes on an outside wall needs those pipes clear, and after a lake effect snow a drift can bury them.

For snowy climates, the DOE guideline says the vent should end one foot above the maximum snow level, in most cases three feet above the ground.
Enbridge Gas Ohio says to clear appliance intake and exhaust vents after every storm, and warns that a furnace that is not working properly can release CO.
Brush the snow away by hand. If a pipe is iced shut, cracked or pulled loose, leave the furnace off and call.

Carbon monoxide and gas: get out first

The CDC calls CO odorless and colorless and lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion as its symptoms; the U.S. Consumer Product Safety Commission also names nausea and sleepiness. Alarm or no alarm, if anyone has those symptoms, get everyone outside. The CPSC says to get outside to fresh air immediately and then call 911. A sounding CO alarm means the same.

A rotten egg or sulfur smell can be a gas leak. Get everyone out first. Enbridge Gas Ohio's gas leak page says not to use switches, phones or electronics or turn anything on or off, and not to look for the leak. That means no flashlight, lighter or garage door opener on the way out, and no relighting anything. From outside, call 911 or Enbridge's emergency line at 1-877-542-2630. On another gas utility, call its emergency line or 911.

When to call Raber HVAC

Water spreading, a furnace that shut off, or no heat in freezing weather: call (216) 888-4822, our 24/7 emergency line; see how to protect your pipes while you wait.
Staying warm meanwhile: use an electric space heater only. The U.S. Fire Administration says to plug it directly into a wall outlet, never an extension cord or power strip, keep anything that can burn 3 feet away, and turn it off when you leave the room or go to bed. Never use a gas oven, grill, camp stove, patio heater or generator indoors or in the garage: the CDC warns against generators, grills and other gasoline, propane or charcoal burning devices, its CO basics page against gas ovens and portable gas camp stoves, and Enbridge Gas Ohio against patio heaters.
A small, steady drip: book furnace repair during office hours, every day from 8:00 AM to 10:00 PM.
A humidifier that leaks: call the same number; humidifiers are part of our indoor air quality work.
Anything that can wait a day: book online, pick a day and an arrival window, and the office calls to confirm.

Why is my furnace leaking water?