AC drain line clogs: why they happen and what to do

The short answer

A clogged condensate drain is one of the most common problems we see on South Florida air conditioners. The drain carries away the water your AC pulls out of the air, and because it stays wet nearly all year, slime builds up inside it. When the line blocks, water backs up into the pan. Then one of two things happens: the pan overflows into a closet, garage or ceiling, or a float switch shuts the system off to prevent that.

There are a few safe things you can do before calling. Turn the system off, clear the drain's access point and flush the line. If water is near electrical parts, the ceiling is stained or sagging, or the clog keeps coming back, it's a job for a technician.

Why drains clog here

As warm, humid air crosses the cold indoor coil, moisture condenses on it, the same way water beads on a glass of iced tea. That water drips into a pan under the coil and leaves through a plastic drain line, usually to the outside or into a household drain.

In a Broward summer a system may run for most of the day, and it keeps running through much of the winter. The pan, the trap and the line are rarely dry. Anything that grows in standing water has time to grow, and anything that gets past the filter has somewhere to stick. Over weeks and months the line narrows until the water can't keep up. Trane describes the same pattern, with growth forming in the standing water of the drain pan, and ENERGY STAR notes that a plugged drain can cause water damage in the house and affect indoor humidity.

Zoogloea: the slime in the line

When we clear a clogged drain here, what comes out is usually slime: gray, tan or brown, sometimes in soft, jelly-like clumps. A good part of it is biofilm, a layer of bacteria living on the wet inside of the pipe. One of the bacteria behind it is Zoogloea. In a drain that stays warm and wet, it forms a slimy film that clings to the pipe walls and the trap and catches whatever else washes past.

South Florida gives it what it needs. Long cooling hours produce a lot of condensate, and a lot of condensate means the line never dries out. The film builds a little at a time, which is why a drain that was cleared in the spring can be backing up again by late summer.

Zoogloea is one important contributor to that slime. It isn't the only cause of clogs, and most of the drains we clear have more than one thing going on:

• Dust and debris that got past a dirty or loose filter and settled in the pan and trap.
• Algae and mold growing in the pan and along the line.
• Poor slope. A line that runs flat or sags holds water and sludge instead of draining.
• A missing or wrong trap. Without the right trap, the blower can pull air back up the drain and hold water in the pan.
• Venting problems that keep water from moving freely down the line.
• Pipe damage, such as a crushed, cracked or disconnected section.

The biology and the plumbing work together. Slime grows fastest where water sits, so a line with a sag or a bad trap keeps clogging no matter how often it's flushed.

What maintenance does about it

A line that's wet every day won't stay perfectly clean, but the buildup can be kept from turning into a clog. On a maintenance visit, the technician:

• flushes the drain line,
• treats the drain to slow new growth,
• inspects the pan for standing water, rust and cracks,
• checks the float switch, and the condensate pump where there is one.

If the same drain keeps clogging between visits, tell us. That usually points to the slope, the trap or the venting rather than to how often the line gets cleaned.

What a float switch does

Florida's mechanical code requires extra protection wherever an overflow could damage the building, such as an air handler in an attic or above a finished ceiling. Depending on the installation, that means a secondary drain pan with its own drain, a separate overflow line, or a water-level switch that shuts the equipment off before the water overflows.

That last device is the float switch. When it trips, the system stops cooling. It looks like a breakdown, but the switch is doing its job, and the clog behind it is the real problem.

Signs of a clog

• No water dripping from the drain outlet outside on a hot, humid afternoon while the system runs.
• Standing water in the drain pan, or water in the secondary pan under the air handler.
• The system stopped cooling and the thermostat shows it's calling for cooling. See AC not draining.
• Water on the closet or garage floor, or a stain on the ceiling below the air handler. See AC leaking water.
• A musty smell near the air handler or from the vents. See AC smells.

What you can safely do

These steps follow what manufacturers such as Lennox and Trane tell owners, adapted for a typical Broward system:

1. Turn the system off at the thermostat before you start.
2. Find the access point. Many drain lines have a capped vent or cleanout near the air handler, often a short pipe with a removable cap. Take the cap off and look for standing water or buildup.
3. Clear the line. A small brush works at the access point. Many homeowners also use a wet/dry vacuum at the outdoor end of the line to pull the clog out.
4. Flush it. Pour in diluted vinegar, or a cleaner approved for PVC, let it sit for 30 minutes or longer, then flush with water. If you're not sure how, ask your technician to show you on the next visit.
5. Check the outlet outside. Make sure the end of the line isn't buried in mulch or soil and that water drips freely once the system runs again.
6. Repeat every few months, especially through the summer.

Flushing slows the buildup. It won't fix a line with a sag, a bad trap or a crushed section. Manufacturers also warn that apart from filter changes and general drain-pan care, leak problems should be handled by a professional, and if your equipment's instructions differ from these steps, follow them.

When to call a technician

• The clog comes back within weeks of clearing it.
• You can't find an access point, or the line runs inside a wall or ceiling.
• The drain pan is rusted or cracked, or the secondary pan has water in it.
• A condensate pump is part of the system and isn't running.
• The float switch keeps tripping after the line is clear.
• There's water damage, or you see or smell mold. EPA advises drying water-damaged areas within 24 to 48 hours to prevent mold growth, and not running a system you suspect is contaminated with mold.

How it's fixed properly

Clearing a line fixes today's clog. A proper repair also looks at why it clogged and whether the drain is built so it can be maintained:

• Slope. Florida's mechanical code calls for condensate drains to slope at least an eighth of an inch per foot. A sag holds water and sludge.
• Cleanouts. The code requires drains to be arranged so blockages can be cleared without cutting the line.
• The trap and venting, so water leaves instead of sitting in the pan.
• Secondary protection that works: an auxiliary pan, an overflow line, or a float switch in the right place for that equipment.
• Condensate pumps. In attics, the code requires the equipment to shut off if the pump fails.
• The pan itself. A rusted or cracked pan leaks no matter how clean the line is.