Your air conditioning is probably the cause

If we had to name one cause for most of the mold in Bryan-College Station homes, it would not be roofs, plumbing or flooding. It would be air conditioning condensate.

Here is why, and here is how to check yours in about fifteen minutes.

What your system is actually doing

An air conditioner cools by passing warm humid indoor air over an evaporator coil that is well below the dew point. Water condenses out of the air onto the coil, runs down into a drain pan, and leaves through a condensate line.

In a Central Texas summer that is a substantial volume of water — gallons per day on a hot humid afternoon — produced continuously inside your house for most of the year.

The system is designed to handle it. When any part of that chain fails, the water goes somewhere else.

The five failure points

1. Blocked primary drain line

The most common by a wide margin. Biological slime develops in a line carrying condensate at ambient temperature. It builds gradually, restricts the flow, then blocks it. Water backs up into the pan and over the edge.

2. No float switch, or one that is disconnected

A float switch sits in the pan or in the drain line and shuts the system down when water rises. It is inexpensive. It is also, on a great many installations here, either absent or disconnected because it tripped once and somebody found that annoying.

Without it, a blocked drain becomes a ceiling instead of an inconvenience.

3. Rusted or cracked drain pan

Older galvanised pans corrode from the inside. A pan that leaks does so slowly and continuously, which is the worst pattern for growth.

4. Secondary pan that is not monitored

Attic units should have a secondary pan underneath with its own emergency drain, usually terminating somewhere visible outside — over a window or under an eave. That drip is a warning signal, and it is meant to be seen.

If yours has ever dripped and nobody investigated, the primary system was failing at that moment.

5. Sweating ductwork

Not the drain at all. Flex duct carrying air near 55°F through an attic at 130°F condenses on its outer surface wherever the insulation has failed, torn or compressed. The water drips onto the ceiling below and produces exactly the same stain as a roof leak.

How to tell it from a roof leak

The diagnostic that saves people from re-roofing unnecessarily.

It is the air conditioning if: the stain appears or worsens in hot weather with no rain; it is near the air handler or along the condensate line's route; it grows slowly and steadily; there is a supply register or duct run nearby; the secondary drain outside has been dripping.

It is the roof if: it appears or worsens during and after rain and dries between events; it is near a penetration such as a vent boot, chimney or skylight; there was recent wind or hail; you can see daylight through the deck from inside the attic.

The seasonal pattern is the strongest single clue. Summer staining with no rainfall is not a roof.

The fifteen-minute check

Worth doing before the hot season rather than during it.

1. Find the air handler. Attic, closet or garage.
2. Look in the drain pan. It should be damp when running, not holding standing water. Standing water means the drain is slow.
3. Check for a float switch. A small device in the pan or spliced into the drain line, with wiring to the unit. If there is none, that is the cheapest improvement available to you.
4. Follow the condensate line. It should fall continuously. Sagging sections hold water and encourage blockages.
5. Look at the secondary pan under an attic unit. It should be dry and clean. Rust staining means it has been catching water.
6. Find the emergency drain outlet outside and look for staining below it.
7. Look at the duct insulation in the attic for tears, gaps and compressed sections, particularly at the boots where ducts meet the ceiling.

Maintenance that actually helps

Clear the condensate line annually before the cooling season. A wet-dry vacuum on the outside end of the line pulls the blockage out and takes a few minutes.

Pan treatment tablets slow biological growth in the pan. They are a supplement to clearing the line, not a substitute.

Change the filter on schedule, and check that it seats properly. A gap around the filter lets unfiltered air bypass onto the coil, which is how coils load up with the dust that eventually blocks everything downstream.

Have the coil cleaned periodically. A coil that has never been touched in fifteen years is the most likely source of a musty smell.

The bigger version of this problem

Beyond leaks, there is a systemic version worth knowing about.

An air conditioner dehumidifies as a side effect of running. A system that is oversized for the house — very common, because larger felt safer to whoever specified it — satisfies the thermostat quickly and shuts off before it has run long enough to remove much moisture.

The result is a house that is cool and clammy, with indoor relative humidity above 60%. At that level, growth is supported on surfaces throughout the building without any leak anywhere.

Put a cheap hygrometer in the hallway for a week. If it sits consistently above 60%, you have a mechanical problem rather than a cleaning problem, and the answers are a correctly sized system, a variable-speed blower that runs longer at lower capacity, or a whole-house dehumidifier tied into the return.

See HVAC and air duct mold removal and ceiling leak water damage.