Furnace leaking: where the water is coming from
Water on the floor near a furnace is rarely the furnace "breaking". It is usually a drain, a humidifier or the air conditioner sharing the same cabinet.
Start with what kind of leak it is
If the leak is water, it is usually one of three things: furnace condensate, a whole-house humidifier. Or the AC evaporator coil sitting on top of the furnace. Most leaks are slow and fixable. A few are not.
First, the one case that is not about water at all. If you smell gas, hear a hiss, or a carbon monoxide detector is sounding, leave the house now. Call your gas utility or 911 from outside. Do not look for the source.
| Where the water shows up | Most likely source | Who handles it |
|---|---|---|
| Under a high-efficiency furnace, with a small plastic drain line nearby | Blocked or disconnected condensate drain | You can check the visible line; technician for the rest |
| Under the humidifier or on the duct beside it | Humidifier water line or drain | You can shut off its water valve; technician to repair |
| Under the coil box above the furnace, mostly in summer | AC condensate drain pan or line | Technician |
| Rusty water inside the cabinet | Condensate trap or a corroded component | Licensed HVAC technician |
High-efficiency furnaces make water on purpose
A condensing furnace pulls so much heat out of the exhaust that the gases turn to water. That water leaves through a small drain line, usually white PVC tubing, into a floor drain or a pump. If the line clogs, kinks or pops off, the water goes onto the floor instead.
You can look at the visible part of the line. Is it attached? Is it sagging? Is the pump, if you have one, plugged in? That is where your part ends. Cleaning or reworking the trap inside the cabinet is a technician's job, because it sits next to the burner and exhaust.
A humidifier is the usual suspect in winter
A whole-house humidifier mounted on the furnace has its own water supply. A stuck valve or a clogged drain can drip for weeks without anyone noticing. If the water is near the humidifier, close its water supply valve and see whether the leak stops. If it does, you have your answer.
In summer, think about the air conditioner
If the furnace has a cooling coil above it, that coil collects water every time the AC runs. A clogged drain line backs that water up into the pan and then onto the furnace. A leak that only appears when the AC is running points here, not at the heating side.
Rust or water inside the cabinet is a bigger flag
Water inside the furnace cabinet is worth a call. Moisture near burners, the control board or the exhaust path can cause corrosion and failed starts. Turn the furnace off at the switch and let a licensed technician look before you run it again.
What not to do
- Do not open the furnace cabinet to chase the leak. Combustion parts and live wiring are in there.
- Do not keep running the furnace with standing water around the base.
- Do not seal or cap a drain line to "stop" the leak. The water has to go somewhere.
- Do not ignore it. Even a slow leak can rot the floor and grow mold.
What you can do safely
Put a towel or shallow pan under the leak. Check the filter and the humidifier valve. Look at the drain line for kinks. Then note when the leak happens (heating, cooling, or both) and tell the technician. That one detail often saves a visit.
Frequently asked questions
Is it normal for a furnace to leak water?
A high-efficiency furnace makes condensate and sends it down a drain, so a little water in the drain is normal. Water on the floor is not. It means the drain is blocked, loose or backed up.
Can I keep using the furnace if it is leaking?
If it is a small drip clearly from a humidifier or AC drain, the heating itself may still be fine. If water is inside the cabinet or pooling at the base, turn it off and call a licensed technician.
Could the leak be gas?
Gas does not drip. If you smell rotten eggs, hear hissing or a CO alarm sounds, leave the house and call your gas utility or 911 from outside.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics