Dirty AC Coils and Clogged Drain Lines: The Water Damage Nobody Sees Coming

An air conditioner pulls a surprising amount of water out of the air, and all of it has to leave the house through one small drain line. When that line clogs, or when a dirty coil sends water where the drain pan cannot catch it, the result is a slow leak that can soak ceilings, utility rooms, and finished basements before anyone notices.

Where the water comes from

The indoor half of your air conditioner is the evaporator coil, which sits in a metal cabinet on top of or beside the furnace. Warm, humid air from the house passes over the cold coil, and moisture condenses on it.

That water drips into a pan under the coil and flows out through a plastic pipe, usually to a floor drain. Where there is no drain nearby, a small condensate pump pushes it to a sink or outside. During a humid Nebraska July, a system can remove several gallons of water a day.

How the drain gets blocked

The pan and drain line are dark, wet, and constantly fed with whatever dust gets past the filter. Over time a slime of algae, bacteria, and mold forms and combines with dust and rust flakes. It collects in the trap or at an elbow, and eventually the line plugs. The pan fills and overflows.

Condensate pumps add their own failure points: a float that sticks, a small discharge tube that kinks or clogs, or a pump motor that simply wears out.

What a dirty coil adds to the problem

A clogged drain is the most common cause, but the coil itself plays a part.

• It feeds the clog. A coil matted with dust holds moisture and grows the same slime, which then washes down into the pan and line.
• It can ice over. Dirt restricts airflow, and a coil without enough air moving across it can drop below freezing. Ice builds up, and when the system shuts off it melts faster than the pan can handle, or drips outside the pan altogether.

A neglected furnace filter causes the same airflow problem.

Where the damage shows up

Basement utility rooms

Most homes around Omaha and Lincoln have the furnace in the basement. Overflow runs across the concrete to the nearest low spot, and in a finished basement that is often the edge of the carpet or the bottom of a framed wall. Carpet pad and drywall wick water upward, so the damage spreads out of sight. The first clue is frequently a musty smell.

Ceilings

Some two-story homes, townhomes, and condos have an air handler in the attic or in an upstairs closet. When that pan overflows, water comes through the ceiling below as a stain, a sagging patch of drywall, or a drip from a light fixture.

It is a winter issue too

High-efficiency furnaces produce condensate all through the heating season, and they often share the same drain or pump as the air conditioner. Whole-house humidifiers, which are common here because winter air is so dry, have their own water supply and drain. A blocked drain in January can leak just as steadily as one in July.

Warning signs

• Water, dampness, or rust stains on the floor around the furnace
• A musty odor near the utility room or when the air conditioner starts
• The air conditioner shutting off unexpectedly, which can mean a safety switch has tripped
• Ice on the copper refrigerant line or the coil cabinet
• A condensate pump that runs constantly, hums without pumping, or never runs
• Weak airflow or a house that feels more humid than usual

Preventing it

• Stay on top of the filter. Check it monthly during peak cooling and heating months.
• Have the coil, pan, and drain line cleaned. An AC and HVAC cleaning from Air Force Restoration removes buildup from the coil and blower and clears the drain before it plugs.
• Ask about a float safety switch. It shuts the system off when the pan or drain line backs up.
• Watch it work. Early in the cooling season, confirm that water is actually coming out of the drain line or pump while the air conditioner runs.
• Leave space around the furnace. If boxes are stacked against it, you will not see a leak until it has traveled.

If you find water

1. Turn the system off at the thermostat.
2. If water is near wiring or outlets, stay out of it and shut off the breaker if you can reach the panel safely.
3. Take photos of the water, the source, and anything damaged.
4. Blot or extract what you can, and move belongings out of the wet area.
5. Find out how far it went. Water travels under carpet and behind baseboards, and it takes moisture meters to know for sure.
6. Have the cause repaired before you run the system again.

Materials that stay wet for more than a day or two can begin to support mold growth, so drying should not wait for a convenient weekend. Professional water damage restoration includes checking inside walls and under flooring. If the leak has been going on for a while, mold remediation may be part of the job, and damaged finishes in a lower level may lead to basement reconstruction.

A word on insurance

Many policies draw a line between a sudden, accidental discharge of water and seepage that continued over a long period, so report a leak promptly once you find it. Coverage depends on your policy, and our insurance claims page explains how the process usually works.

The takeaway

Once or twice a season, walk down to the furnace and look at the floor, the drain line, and the filter. If you find water, shut the system off, document what you see, and get the area dried properly before the damage spreads.