Water Dripping From Your Furnace in Summer? It's Usually the AC Drain

Turn the cooling off at the thermostat, mop up, and use the check below to find which of those it is.

First, confirm where the water is coming from

You can sort this out in about two minutes with a flashlight. Do not open any panel that is screwed shut.

Trane gives a quick way to tell which furnace you have: a metal exhaust pipe means a standard furnace, while PVC pipes, usually an intake and an exhaust, mean a condensing furnace that makes its own water in heating mode. In July, with the heat off, the furnace is almost never the source.

Why an AC makes water at all

As air passes over the cold evaporator coil, its temperature drops below the point where it can hold its moisture, and the water condenses and drips into a pan under the coil. The pan then sends it to a drain line, according to the Building America Solution Center, a U.S. Department of Energy resource run by Pacific Northwest National Laboratory. On a humid day that can be a steady stream, so even a partial clog shows up fast.

The five common causes, most likely first

1. Clogged condensate drain line

The drain pan has to drain continuously or it overflows, and a stagnant pan can grow algae and bacteria, BASC notes. That slime, plus dust that falls off the coil, is what blocks most lines. ENERGY STAR's maintenance checklist tells contractors to inspect this drain at every check-up because a plugged drain can cause water damage and affect indoor humidity.

What you can do: find the drain line's cleanout (often a capped tee near the coil) or its outdoor end. With the power off, use a wet/dry vacuum on the outdoor end to pull the clog out. The Department of Energy also suggests occasionally passing a stiff wire through the unit's drain channels to prevent clogs.

2. Dry, missing or badly built trap

Most drain lines have a U-shaped trap. BASC explains that it stops air from moving through the drain. On systems where the fan pulls air across the coil, a poor trap lets air rush up the pipe, which can stop the pan from draining and make it overflow. BASC describes a typical trap as a 2-inch drop from the pan plus 2 more inches for the seal, with a removable cleanout cap. If your line has no trap at all, or it was just rebuilt and leaks started, that is a job for the installer.

3. Condensate pump failure

If your furnace is in a basement or a spot with no floor drain, the coil drains into a small pump. A float inside turns the pump on as water rises, and pumps tubing to a safe spot, BASC says. Pour a cup of water into the pump's reservoir. If the motor does not start, the pump has likely failed. If it runs but water still backs up, the discharge tube is kinked or clogged.

4. Cracked or rusted drain pan

Older systems used steel pans that pit and rust; newer ones are plastic. BASC warns that mistaking a rusted-out pan for a simple clog is costly, because replacing the pan often means replacing the coil. If the drain flows freely when you pour water in but water still drips from the bottom of the coil box, suspect the pan.

5. Frozen coil that is melting

A coil that ices over drips heavily when it thaws. Trane says a dirty coil can freeze and that a professional should handle a frozen coil. Check your filter first: a clogged filter cuts airflow over the coil. If the filter is clean and the coil keeps freezing, low refrigerant is a likely cause, and only a licensed technician can find and fix a refrigerant leak.

Step-by-step: what to do right now

1. Set the thermostat to Off (not just warmer). If you see ice, set the fan to On for a few hours so the coil thaws faster, and keep towels down.
2. Turn off power to the furnace at the service switch (it looks like a light switch on or near the unit) before you touch the drain or pump.
3. Mop up and move anything stored near the unit. Standing water damages flooring and drywall fast.
4. Check the filter. Replace it if you cannot see light through it.
5. Test the drain. Pour a cup of water into the drain line cleanout or pan. It should flow out the other end or into the pump right away.
6. Clear a slow line from the outdoor end with a wet/dry vacuum, then test again.
7. Check any pump by filling its reservoir as described above.
8. Restore power and cooling, then check the area again in an hour.

Call a licensed HVAC technician if the coil keeps freezing, the pan is cracked, water comes from inside a sealed panel, or you see water near wiring. In Texas, HVAC contractors are licensed by the Texas Department of Licensing and Regulation, and you can look up a license there before you hire.

Why your AC may have shut itself off

If cooling stopped and you found water, a safety switch probably did its job. The International Residential Code requires extra protection wherever an overflowing coil pan would damage the building, such as an attic or a furnace above finished floors. Section M1411.9.1 allows four methods:

The same section sets the drain at no less than 3/4 inch pipe, sloped at least 1/8 inch per foot. If water is coming out of a pipe above a window, that is the overflow telling you the main drain is blocked. Clear the main drain; do not cap the overflow. Local codes can differ, so ask your building department or contractor what applies to your home.

How to keep it from happening again

• Change the filter on schedule. ENERGY STAR tells homeowners to inspect, clean or change air filters once a month.
• Book the spring check-up before the first heat wave. ENERGY STAR's list for that visit includes inspecting the condensate drain and cleaning the coils.
• Flush or vacuum the drain line once early in the cooling season.
• If your unit sits above a finished ceiling and you are not sure it has a float switch or auxiliary pan, ask at the next visit.