If your AC is leaking water in the attic, the most common cause is a clogged condensate drain line. Algae and dust block the drain, and water backs up into the pan and over the edge. A frozen evaporator coil or a failed float switch can also be to blame. Shut the system off at the thermostat and call a licensed HVAC technician before the ceiling takes on damage.
Hey y’all — this is Mr. Cool AC & Heating, your hometown HVAC team here in Cypress and Greater Houston. We have been keeping local families comfortable since 1979, and if there is one call that spikes every single August, it is this one: water showing up where water has no business being.
Sometimes it is a brown ring on the hallway ceiling. Sometimes it is a drip coming out of a supply vent. Sometimes a homeowner climbs into the attic for a storage bin and finds a pan full of standing water sitting under the air handler. Whatever version you are dealing with, the good news is that the cause is usually simple and the fix is usually affordable — as long as you catch it early.
Here is what is actually happening up there, what you can safely check yourself, and when it is time to pick up the phone.
Before you troubleshoot anything, go to your thermostat and switch the system from COOL to OFF. Do not leave it running while you figure things out.
Here is why that matters. Your air conditioner is not just cooling your home, it is pulling moisture out of the air. On a humid Houston afternoon, a typical residential system can produce five to twenty gallons of condensate in a single day. That water is supposed to drain outside through a PVC line. If the line is blocked, every minute the system runs adds more water to a pan that is already overflowing.
Turning it off stops the flood. It is uncomfortable for a few hours, but it is a whole lot cheaper than replacing drywall, insulation and a ceiling fan.
This is not a problem homeowners in Denver or Phoenix deal with the same way, and there are three very local reasons for that.
From June through September, our dew points sit in the mid to high 70s. Every hour your AC runs, it is stripping that moisture out of the air and sending it down the drain line. More water moving through the system means more chances for something to back up.
Across Cypress, Bridgeland, Fairfield, Copperfield and most of Northwest Houston, the indoor unit sits above your living space instead of in a basement. That means gravity is not your friend. When the pan overflows, the water goes into your ceiling, your insulation and your light fixtures before you ever see a drop.
A Houston attic can hit 130 degrees in August. That heat, combined with a cold coil and standing moisture, creates the perfect environment for algae and biological slime to grow inside the drain line. That slime is what plugs the pipe. It is not dirt or debris from outside — it grows in there.
This is roughly eight out of ten calls we run. The three quarter inch PVC line that carries water from the drain pan to the outside gets choked with algae, dust and sludge. Water has nowhere to go, so it fills the primary pan and spills over the side.
What you will notice: a water stain on the ceiling, a musty smell near the return, or water dripping from the secondary drain line outside near a window — that outside drip is actually the system warning you.
If your system is twelve to fifteen years old, the metal pan under the evaporator coil has been sitting in water for a decade. Rust eats through it, and water starts dripping through the hole instead of draining out. Plastic secondary pans can crack from age and attic heat too.
What you will notice: a slow, steady leak that keeps coming back even after someone has cleared the drain line.
A clogged filter restricts airflow across the evaporator coil. Without enough warm air moving over it, the coil drops below freezing and builds up a block of ice. When the system shuts off and that ice melts, it melts all at once — far more water than the pan and drain were designed to handle.
What you will notice: weak airflow from the vents in the days before the leak, plus visible ice on the copper refrigerant lines. We covered this one in detail in our post on why AC systems freeze up.
Condensate lines drain by gravity alone. If a joint separates, or if the line sags between supports, water pools inside instead of flowing out. We see this most often after another trade has been working in the attic — an insulation crew, a plumber, a cable installer — and the line got bumped.
Most properly installed systems have a safety float switch in the pan or the drain line. When water rises, the switch cuts power to the system so it stops making condensate. It is your last line of defense against ceiling damage. When that switch fails or was never installed, nothing stops the overflow.
Worth knowing: if your AC has been shutting itself off on hot days and you cannot figure out why, do not bypass it. That switch is probably doing its job and telling you the drain is backing up.
A few of these you can handle from the ground, no ladder required.
✕ What not to do
Do not pour bleach down the drain line. It corrodes the metal drain pan and the coil over time. Distilled vinegar is the safer maintenance option, and only into a line that is already flowing.
Do not bypass or tape down the float switch to keep the system running. That switch is the only thing standing between a clogged drain and your ceiling.
Do not run the system to dry out the attic. It makes more condensate, not less. Do not go walking around an attic in August looking for the leak. Between the heat, the ductwork and the ceiling joists, this is where people get hurt or put a foot through the drywall.
A clogged drain line is a small repair. What it turns into is not.
The repair is usually a same day visit. The remediation is not. That gap is the whole reason we tell people not to wait it out over a weekend.
When you call us out for a leak, here is what actually happens. No mystery, no upselling.
Every technician is working under Texas license TACLB24041E, and we run same day and next day service throughout our service area.
Condensate leaks are one of the most preventable failures in the whole system. Three habits handle almost all of it.
Yes, distilled white vinegar is a safe maintenance option for a line that is already draining. Pour about a cup into the access port near the air handler every couple of months during cooling season. Never use bleach — it corrodes the metal drain pan and the coil. If the line is already fully clogged, vinegar will not clear it and you need it vacuumed out.
Water coming out of a supply vent usually means condensate is overflowing the drain pan above and running along the ductwork, or the duct itself is sweating because of poor insulation and high attic humidity. Both need a technician. Shut the system off and get it looked at before the drywall around the vent gives out.
A straightforward drain line clearing is one of the least expensive repairs in HVAC — typically a single service visit with no parts. Cost goes up only if the drain pan is rusted through, the float switch has failed, or a frozen coil caused the backup. We give you the number before we start the work, and our $49.99 First Time Service Call applies to new customers, with conditions.
If your system has a working float switch, yes — that is exactly what it is designed to do. If your AC keeps shutting itself off on hot days and then running again later, a backing up drain line is one of the first things to check. Systems without a float switch, or with a failed one, will keep running and keep overflowing.
Twice a year for most Houston area homes. Our climate produces far more condensate than most of the country, and attic heat accelerates algae growth inside the line. Homes with pets, heavy dust or an older system benefit from checking it more often during the summer.
Mr. Cool AC & Heating has been the family owned HVAC team for Cypress and Greater Houston since 1979. We are licensed in Texas (TACLB24041E), BBB accredited, and we do not charge overtime rates for showing up when you actually need us.
We serve Cypress, Katy, Jersey Village, Tomball, Copperfield, Bridgeland, Fairfield, Bear Creek, Willowbrook, Spring and Northwest Houston.
