How do I clear a clogged AC condensate drain?
Turn the cooling off, find where the water comes from, then flush the line through the cleanout tee or vacuum the clog out at the far end of the pipe. Most clogs are slime in the trap. Rinse the pump float too. The $79 trip charge is waived when you approve a repair.
Key points
- Water by the furnace in summer is usually a plugged trap, drain line or condensate pump.
- A tripped float switch can shut the AC off completely, so check the drain before calling it dead.
- A wet/dry vacuum on the end of the line clears most clogs.
- On a 90%+ furnace the same drain often carries furnace water all winter.
- National cost guides put drain clearing at $75–$250 and pump replacement at $100–$450.
A clogged AC drain is cleared by flushing warm water through the cleanout tee or by pulling the clog out with a wet/dry vacuum at the end of the line, after you turn the cooling off. The AC wrings a lot of water out of humid Minnesota air; it condenses on the indoor coil above the furnace, drips into a pan and leaves through a small PVC drain. When that drain plugs, the water lands on the basement floor, in the furnace cabinet, or on a ceiling below an attic coil. A 90% or higher furnace makes water all winter, often into the same pump, so one clog in a shared line can cause an AC leak in July and a furnace shutdown in January. Work through the steps below in order.
How is an AC condensate drain put together?
Follow the white pipe from the coil box on top of your furnace. You'll usually see:
- The primary drain fitting on the coil's drain pan
- A trap, a U-shaped bend that holds water so the blower's air can't move through the drain
- A cleanout tee with a cap, often right next to the trap
- A float switch on some systems, either in the pan, on a second drain fitting, or clipped to the line
- The discharge point: a floor drain, a laundry tub, or a condensate pump, a small box with a reservoir that pumps water up and over to a drain
How do you find and clear the clog?
- Turn the cooling off at the thermostat. Leave the fan off too. Put down towels.
- Find the source. Look at the coil pan, the trap, the pump and the furnace itself. Water from a humidifier drain or the furnace's own condensate line can look like an AC leak.
- Check the float switch. If the AC went dead along with the leak, a tripped float is likely. It will reset once the water level drops.
- Open the cleanout. Pull the cap and look down. Standing water in the tee means the blockage is downstream, usually in the trap or the next few feet.
- Flush it. Pour warm water slowly into the cleanout. If it drains freely, you're done with the line.
- Pull the clog out. If it won't drain, go to the far end of the line (at the floor drain or pump inlet) and hold a wet/dry vacuum hose over the pipe, sealed with a rag, for a minute or so. The slime usually comes out in one gray-green slug.
- Run the AC and watch it for 20 minutes. Water should trickle steadily out the end of the line, with nothing collecting in the pan.
If a musty smell came with the leak, the same slime may be growing in the pan. Our guide on why an AC smells like dirty socks covers that side of it.
What does water in each spot mean?
| What you see | Likely cause | What to do | Who handles it |
|---|---|---|---|
| Water under the coil box or around the furnace base in summer | Clogged trap or drain line | Flush the cleanout or vacuum the line | You |
| AC dead, water standing in the pan | Float switch tripped by a clog | Clear the drain and let the switch reset | You |
| Water around the condensate pump | Stuck float, plugged check valve or a worn-out pump | Rinse the reservoir and float | You, or a technician to replace the pump |
| Ice on the coil or the refrigerant lines, then water when it melts | Frozen coil from a clogged filter, low airflow or low refrigerant | Cooling off, fan on to thaw, new filter | A technician if it freezes again |
| Water inside the furnace cabinet | Overflowing pan, or the furnace's own condensate trap | Turn the system off | A technician |
| Water by the furnace in winter | The furnace's condensate line, often frozen where it leaves the house | Check the line through cold spaces | A technician if it won't clear |
| Stain on a ceiling below an attic unit | Primary drain clogged and secondary pan full | Turn the cooling off now | A technician, right away |
A coil that ices up and then melts is the most common false alarm; our guide to an AC that's not cooling covers the airflow side.
What goes wrong with an AC condensate pump?
A clear line can still overflow if the pump fails. Common pump problems:
- Float stuck. Slime in the reservoir holds the float down, so the pump never kicks on. Unplug it, remove the lid, rinse the reservoir and float, and try again.
- Check valve plugged. The pump runs but water doesn't move up the discharge tube.
- Pump runs constantly. It may be worn out or the discharge tube may be kinked or frozen where it passes an outside wall.
- Safety switch not wired. Many pumps have an overflow switch that should shut the AC off before the reservoir spills. If yours isn't connected, the pump can overflow silently.
A quick test before the first humid week of June: pour water into the pump's reservoir until the float lifts. The pump should kick on and empty it in a few seconds.
Why does an AC condensate drain keep clogging?
One clog a summer is normal. One every few weeks means something is wrong with the drain, not the maintenance:
- Poor pitch. The line has to slope down the whole way. A sag holds water and grows slime.
- A missing or poorly built trap. The trap keeps the blower's air pressure from fighting the water. Without one, air can blow out through the drain, or, on an air handler where the blower pulls air through the coil, hold water up in the pan until it spills.
- Long runs with many elbows in a finished basement.
- A dirty coil. Dust that gets past a poor filter ends up as sludge in the pan. A good filter and an annual maintenance visit cut down on it.
If a clog has sent water into the furnace cabinet, don't just dry it off and move on. Water on the control board or burners should be checked before the heating season, and our furnace repair visit covers that.
Can a furnace condensate drain freeze in a Minnesota winter?
A high-efficiency furnace drains water every time it runs, and in a cold snap that's most of the day. Where the pump's discharge tube runs through an unheated garage or out a rim joist to the yard, it can freeze solid, back the water up and shut the furnace down on its pressure or float switch. Our guide to a frozen furnace condensate line walks through thawing it and keeping it from happening again.
Older city basements have their own version of the problem. The floor drain is often across the room from the furnace, so the line either runs a long way on the floor or goes to a pump that gets forgotten. Our page on AC repair in Minneapolis covers what those houses bring.
How much does condensate pump replacement cost?
National cost guides like HomeGuide put condensate pump replacement at $100–$450 and professional drain-line clearing at $75–$250, depending on the pump and how the drain is routed. With us, you get a written price before any repair starts. An AC tune-up is $129, or $219 with the furnace, and is a good time to have the line flushed and the pump and float switch tested.
When should you call about a condensate drain problem?
- Water inside the furnace cabinet or on the control board
- Rust in the bottom of the furnace or on a secondary heat exchanger
- The AC won't restart after the water is gone, which may mean the problem wasn't the float switch after all; our checklist for an AC that won't turn on covers the other causes
- A pump that needs replacing, or a line that needs re-pitching
- A leak that comes back within days
What to do next
Clearing a drain is a common AC repair. If you've cleared what you can and water keeps showing up, or the system won't restart, call 612-429-3465. It also belongs on your spring AC checklist so the first humid week of June doesn't find it for you.
Common questions about cooling
Can a clogged AC drain cause mold?
A clogged drain leaves water standing in the pan, and slime growing there is the usual source of a musty smell. Standing water can also support mold, so clear the line promptly, rinse the pan, and keep the line flushed each spring.
Can I pour bleach down my AC drain line?
We'd skip it. Strong chemicals can do the coil and drain pan no favors, and they don't fix a trap that's badly pitched or a pump with a stuck float. Plain water flushes most lines. A cup of white vinegar in the cleanout in late spring is a common, gentle way to slow slime growth. Check your equipment manual before using anything stronger.
Where does the AC condensate drain go in a Minnesota basement?
Usually from the coil on top of the furnace through a PVC trap to a floor drain, a laundry tub or a condensate pump that lifts it to one of those. Outside discharge is less common here because a line running outdoors freezes in winter.
How often should the condensate drain be cleaned?
Once a year, before cooling season, is enough for most homes. An AC tune-up is the natural time to check the trap, flush the line and test the pump and float switch. Homes with pets, a lot of dust or a basement coil that runs hard in humid Julys may need it more often. Repeat clogs every few weeks point to a pitch or trap problem.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com