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A furnace leaking water is usually caused by a clogged condensate drain, a failed condensate pump or water from the AC coil or humidifier above it. High-efficiency furnaces make water as they run, so a blocked drain is the most common cause. Turn the furnace off, dry the area, and call our local furnace repair service if the leak returns.
Key Takeaways
- High-efficiency furnaces make water. It is normal, as long as it drains away.
- A clogged drain line or trap is the most common reason for a puddle.
- Standard furnaces should not make water. If one leaks, look at the AC coil, humidifier or venting.
- Water near a furnace is a real risk to electrical parts, floors and indoor air.
- A yearly tune-up includes clearing the drain and trap before they overflow.
Why Would a Furnace Leak Water?
Gas furnaces come in two main types. The type you have tells you a lot about where the water is coming from.
A high-efficiency (condensing) furnace has a second heat exchanger. It pulls so much heat from the exhaust that water vapor turns into liquid. That water runs through a trap and a drain line, often to a floor drain or a small pump. These furnaces usually have white or gray PVC vent pipes. Under the ENERGY STAR furnace criteria, a certified gas furnace in the South must be rated 95% AFUE or higher, a level only condensing furnaces reach.
A standard-efficiency furnace usually has a metal flue pipe. It is not built to make water during normal heating. If one is leaking, the water most likely comes from something attached to it.
Quick Way to Tell Your Furnace Type
| What You See | Likely Furnace Type | Where Water Usually Comes From |
|---|---|---|
| White or gray PVC pipes out the side or roof | High-efficiency (condensing) | Condensate trap, drain line, pump or vent pipe |
| Metal flue pipe into a chimney or vent | Standard efficiency | AC coil, humidifier, or condensation in the flue |
| Water only in summer | Either type | AC evaporator coil drain pan above the furnace |
| Water only in winter | Usually high-efficiency | Furnace condensate system or humidifier |
6 Common Causes of a Furnace Leaking Water
1. Clogged Condensate Drain Line
Dust, sludge and algae build up inside the small drain line. When it clogs, water backs up and spills out of the cabinet. This is the most common cause on condensing furnaces. If your furnace and AC share a drain, the steps for clearing AC drain lines apply here too.
2. Blocked Condensate Trap
The trap is a small loop of plastic on the side of the furnace. It holds water so exhaust gas cannot escape into the drain. Debris settles in the trap over time and blocks the flow.
3. Failed Condensate Pump
If the furnace sits below a floor drain, a small pump pushes the water out. Pumps can lose power, jam, or have a stuck float switch. When the pump stops, its tank overflows onto the floor.
4. AC Evaporator Coil Above the Furnace
In most homes, the AC coil sits right on top of the furnace. In cooling season it makes a lot of water. A cracked drain pan, a frozen coil or a clogged AC drain can drip down onto the furnace. Our guide on different AC leak types covers these in more detail.
5. Whole-Home Humidifier
A humidifier mounted on the furnace uses a water supply line and a drain. A loose fitting, worn pad or stuck valve can leak, mostly in winter when it runs.
6. Vent Pipe or Heat Exchanger Problem
A badly sloped or disconnected PVC vent can send water back into the furnace. In rare cases, a cracked secondary heat exchanger leaks. That is the serious one, because it can also let exhaust gas escape.
Signs the Leak Is Getting Worse
- The puddle comes back after you dry it.
- Rust on the furnace cabinet, burner area or floor.
- A musty smell near the furnace or from the vents.
- The furnace shuts off or will not start. Many units have a safety switch that stops the furnace when the drain backs up.
- Water stains on the ceiling below an attic furnace.
- Gurgling sounds from the drain or trap.
Moisture around HVAC equipment can also feed mold. The EPA mold cleanup guidance says the key to mold control is moisture control, and that wet areas should be dried within 24 to 48 hours.
What to Do Right Now
- Turn the furnace off at the thermostat, then at the furnace switch.
- Soak up the water with towels and place a pan or bucket under the drip.
- Check the filter. A clogged filter can freeze the AC coil in summer and add to drips. Here is how often to change your air filter.
- Look at the condensate pump. Make sure it is plugged in. If the tank is full and the motor is silent, it may have failed.
- Find the drain line exit. If it ends outside, check that it is not crushed, blocked or frozen.
- Note when it leaks. Heating only, cooling only, or both. This helps the technician find the cause fast.
If the leak keeps coming back, or water is near wiring, call Gerald Griffin Heating and Cooling at (980) 277-5122. We are open 24 hours, so you do not have to wait until Monday with a bucket under your furnace.
How Pros Fix a Leaking Furnace
| Cause | Typical Fix |
|---|---|
| Clogged drain line | Flush and clear the line, check slope |
| Blocked trap | Remove, clean and refill the trap |
| Failed condensate pump | Clean or replace the pump, test the float switch |
| AC coil or pan leak | Clear the AC drain, repair or replace the pan |
| Humidifier leak | Tighten fittings, replace pad or valve |
| Vent or heat exchanger issue | Re-slope the vent, inspect heat exchanger, advise on repair or replacement |
Furnace Leaking Water FAQs
Should I worry if my furnace is leaking water?
Yes, take it seriously but do not panic. Most leaks come from a clogged drain or a failed pump, which are simple fixes. The worry is what the water does over time: it can rust parts, damage floors, feed mold and short out electrical controls. Fix it promptly.
How do you unclog a furnace condensate drain?
Turn the furnace off. Find where the drain line ends and use a wet/dry vacuum on it to pull out the clog. Then clean the trap and pour clean water through to test the flow. If water still backs up, call a technician.
Is a furnace ruined if it gets wet?
Not always. A small drip on the cabinet is often harmless once fixed. A furnace that sat in standing water, or had water on the control board or gas valve, should be inspected before you run it again.
Can a furnace be repaired after water damage?
Often, yes. Wet parts like the control board, pump or inducer motor can be replaced. If the heat exchanger or burners are badly rusted, replacing the furnace may make more sense.
Why is my furnace leaking water when the AC is on?
The water is almost always coming from the AC coil on top of the furnace. Check the AC drain line and pan first.
Stop the Leak Before It Spreads
A small puddle is a cheap fix today and an expensive one in a month. Call Gerald Griffin Heating and Cooling at (980) 277-5122, tell us whether the leak shows up in heating or cooling mode, and we will come ready for the likely cause. We serve Belmont, Mount Holly, Gastonia, McAdenville, Lowell and the rest of Gaston County, day or night. You can also book a leak inspection online.
Gerald Griffin Heating and Cooling Services
902 Belmont Mt Holly Rd
Belmont, NC 28012
Phone: (980) 277-5122
Hours: Open 24 hours
NC Heating Contractor License #32307