Why is my furnace leaking water? Condensate drains, the AC coil and other sources

Does a furnace make water on its own?

It depends on the type. A high-efficiency furnace, also called a condensing furnace, makes water as it heats. Lennox says a conventional furnace, also known as a standard efficiency furnace, does not produce water.

Here is why. Combustion, meaning the burning of fuel, creates hot exhaust gases that contain water vapor, which is water in the form of a gas. The Building America Solution Center, a DOE resource, says Category IV furnaces, the high-efficiency condensing kind, have two heat exchangers. The second one cools the exhaust gases enough for their water vapor to condense, or turn back into liquid.

That liquid is called condensate. The DOE resource says the condensate line can be routed to an existing sewer or septic system. Ideally that is a simple gravity line, it says, but many installations will require a condensate pump. The same resource says the condensate is fairly acidic, and some local codes require neutralizing it.

How to tell which kind of furnace you have

Trane and Lennox both point to the exhaust pipe. If the pipe is metal, Trane says it is a standard furnace. If the exhaust pipe is polyvinyl chloride (PVC) and paired with a second PVC intake pipe, Trane says you have a condensing furnace.

Lennox adds that most newer homes have a high-efficiency furnace, while older homes that have not been updated have a conventional one. It says the furnace's efficiency rating should be printed on a yellow EnergyGuide label on the unit.

This matters because the list of likely causes changes. With a condensing furnace, the furnace's own drain is a suspect. With a conventional furnace, Lennox says standing water is probably coming from another appliance. Our guide to furnace types and efficiency explains the kinds in more detail.

Causes when the heat is running

Trane says that if a puddle shows up by your furnace while the heat is on, you likely have a high-efficiency condensing furnace. Its list of causes covers the condensate line, the condensate pump if you have one, the humidifier, and the filter.

A contractor quoted in Carrier's guide names a clogged condensate drain as one of the most common causes. If the drain line becomes clogged or blocked, Carrier says water may back up and leak from the furnace. Lennox names dirt and debris in the drain hose, drain line or condensate trap.

Lennox also says the line may simply have come loose. It says broken condensate pumps and cracked pipes must be replaced. The installation manual for one Carrier condensing furnace warns that if a condensate pump is installed, a plugged condensate drain or a failed pump may cause the furnace to shut down.

Cold is a third cause in some places. Carrier says a furnace may leak in winter because of a frozen condensate line. The Carrier installation manual calls for special precautions when a furnace is installed in an area that may drop below freezing.

Causes when the air conditioner is running

HVAC stands for heating, ventilation and air conditioning, and AC is short for air conditioner. Trane is direct about this case. If you think your furnace is leaking water when the AC is on, it says, it's not your furnace that is leaking, but another part of your HVAC system.

A split system is a central air conditioner with an outdoor unit and an indoor cabinet. Penn State's energy course says that in many split systems, the indoor cabinet also contains a furnace. The evaporator coil is installed in the cabinet or main supply duct of this furnace.

Trane says the evaporator coil produces condensation during normal operation. That water drains into a drain pan and then to a drain. The pan can become clogged and overflow, causing a puddle of water near the furnace. Carrier says dust on the coil mixes with the moisture the AC removes from the air and forms a sludge that can block the drain line.

Carrier lists signs of that clog: musty smells, standing water near the indoor unit, and a system that shuts down. It says the shutdown is often triggered by a safety float switch, a sensor that turns the unit off when the drain pan fills with water.

A frozen coil is the other summer cause. Lennox says a clogged filter restricts airflow, which can make the coils too cold and freeze. The ice melts and drips onto the floor beneath the furnace when the air conditioner shuts off. Trane says a professional should deal with a frozen evaporator coil. For more on the cooling side, see why an air conditioner leaks water.

Other sources: humidifiers, exhaust pipes and plumbing

A whole-house humidifier is a device connected to your furnace and your home's water lines. Trane says a clogged humidifier pad, a clogged or cracked drain line, or loose connections could put water on the floor. Lennox says that in a conventional furnace, the built-in humidifier empties excess water through the furnace drain line. If that line clogs, Lennox says, water can build up and leak from the humidifier into the furnace.

Lennox also names a loose or poorly fitted exhaust pipe, which can let water that is meant to be expelled leak from the unit. It warns that a conventional furnace exhaust pipe can become dangerously hot, so do not touch it. Lennox says to call an HVAC professional to repair or replace the pipe.

The water may not be from the furnace at all. Trane says to rule out a water heater, washing machine or wash tub in the same room. Lennox says a damaged or clogged plumbing pipe near the furnace could be the source.

What to do now, and the Florida picture

Trane's first steps are to turn the system off at the thermostat and mop up the water. Carrier says to place a bucket or towel under the leak and contact an HVAC contractor. Carrier says repairs involving the gas supply, electrical components or the heat exchanger belong with professionals, because improper repairs can create fire, electrical or carbon monoxide hazards.

Act quickly. The Environmental Protection Agency (EPA) says to fix the source of the water problem or leak to prevent mold growth. It says to clean and dry damp or wet building materials and furnishings within 24-48 hours. Our guide to mold in HVAC systems covers moisture inside the equipment.

Carrier calls a heat exchanger problem the less common but more serious cause. If a leak is related to a damaged heat exchanger, Carrier says combustion gases such as carbon monoxide may become a concern. Carbon monoxide is one of the gases that burning fuel makes. See our guide to furnace heat exchanger cracks for what safety agencies say about it.

Now the Florida picture. The Energy Information Administration (EIA) counted far fewer Florida homes heated mainly by natural gas than by electricity in 2020. The Building America Solution Center, a DOE resource, says that in warm climates, central forced-air systems are typically sized to the cooling load, not the heating load.