How do I clear a clogged AC condensate drain line?

Many clogged AC drain lines can be cleared by a homeowner with a wet/dry vacuum, a flashlight and a cup of vinegar. Turn the cooling off first, clear the line from the outside end, flush it, and dry any pan you can reach without opening the unit. If water has already reached a ceiling, or the line clogs again within weeks, it is time for a technician.

What the drain line does and where it is

When your air conditioner or heat pump cools, the indoor coil pulls moisture out of the air. That water drips into a pan under the coil and leaves the house through the condensate drain line, usually a ¾-inch PVC pipe that carries condensation from the evaporator coil to a drain outside the home.^[1] Trane notes that it may be visible in a crawl space or attic, but it may also run under a concrete basement floor.^[3]

Most lines have a capped tee or open stub near the indoor unit. That is the access point you pour into. The other end runs to a drain or outside the home, usually near the outdoor unit.^[2]

Why it clogs, and how you can tell

The clog is usually a slow build-up. Carrier explains that dust and airborne particles the coil collects mix with the moisture the AC pulls out of the air and form a sludge that travels into the drain line.^[1] Trane adds mold and mildew growing in standing water in the drain pan, ordinary dust, dirt and dander, and even a spider's web.^[3]

The common signs are musty smells from the indoor unit or vents, water stains on ceilings or floors near the unit, weak cooling, and a system that shuts off or will not start.^[1] Trane also lists rust on the bottom of the indoor unit.^[3]

That last sign, a system that simply stops, is often a safety feature doing its job. Some systems have a float or safety switch that shuts the indoor unit down when water rises in the drain pan.^[1][3] Without that switch, the system keeps running, the pan fills and spills over, and the water can damage ceilings or drywall.^[3]

Many systems also have a secondary drain pan and a backup line that usually ends above a door or window.^[2] If water is coming out of that backup line, the main line is likely blocked. Water dripping from the main line outside is a sign the system is working properly.^[2]

Before you start

Check whether the system is still under the manufacturer's warranty. Trane warns that if your unit is under warranty, going the DIY route could void the warranty,^[3] so read the warranty terms or call the installer first.

Gather a wet/dry vacuum, a flashlight, a few old towels, a small wire or bottle brush, distilled white vinegar and a jug of clean water. Wear gloves: what comes out of the line is slimy and smells.

The steps on this page clear the line itself. They do not involve opening the indoor unit's panels, which is where the coil, blower and electrical parts are. The main drain pan usually sits inside the unit under the coil; a pan you can see without removing a panel is the only one to touch.

Choosing what to pour in

Distilled vinegar is the usual choice. Carrier and Trane both suggest a cup of it down the line between service visits,^[1][3] and Lennox suggests diluted vinegar or a cleaner made for PVC pipe left to sit before a water flush.^[2] Hot water, not boiling, can help loosen minor buildup.^[1]

Keep expectations modest. Carrier's guide notes that vinegar and bleach may not fully prevent blockages because many clogs are debris that is not organic.^[1] That is why the steps vacuum the clog out first and use the vinegar to clean up what is left.

Keeping it from clogging again

A clear line stays clear longer with a little routine care. Lennox suggests flushing the line every few months^[2] and changing air filters about every three months so dust does not build up on the evaporator coil.^[2] See how often to change your furnace filter for the details.

ENERGY STAR lists checking the condensate drain as part of a typical annual check-up and warns that a plugged drain can cause water damage in the house and affect indoor humidity levels.^[4] If you already pay for yearly service, ask the technician to show you the access point and how they clean it. What a tune-up costs in Virginia covers what that visit should include.

Stop and call a pro if

Lennox's list of signs that the job has gone past a simple fix: extensive water damage or rust, clogs that keep coming back, a musty odor from the vents, a clog high up in the line that you cannot reach, and a system that makes unusual noises, stops cooling or shuts off.^[2] Add these:

• The line runs under the slab or inside a finished wall and you cannot get to either end.
• Water is coming through a ceiling below an attic unit. Turn the system off at the thermostat and get it looked at the same day.
• The system will not restart after the line is clear and the pan is dry. Something other than the drain is wrong.
• The fix means opening the indoor unit to reach the pan, the coil or the float switch.
• Water is pooled around the air handler or furnace, or has reached its electrical parts, a nearby outlet or an extension cord. Lennox turns the AC off first because "working with water and electricity can be dangerous."^[2] Keep away from the wet area, do not reach into it, and call a technician.

Virginia's Code defines a tradesman as anyone who does electrical, plumbing or heating, ventilation and air conditioning work for the general public for compensation,^[6] and no one may do that work without a tradesman licence unless they work under the supervision of a tradesman licensed in that specialty.^[5] Loclis lists DPOR-licensed HVAC businesses with their licence number and class: look up HVAC businesses in Blacksburg or another town, browse HVAC maintenance, or read more heating and cooling answers.

If it is more than a clog, describe the problem once and licensed local HVAC businesses quote it.