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Cooling · MaintenanceAC Drain Line Cleaning & Unclogging
AC drain line cleaning and unclogging clears the trap and the pipe that carry condensate from the indoor coil to the floor drain, and it runs $150–$350 including a rebuilt trap where one is needed. If water is on the floor by the furnace or a ceiling is staining under an attic unit, switch cooling off now; the water stops when the coil stops. A float switch added at the same visit shuts the system down before the next clog becomes a spill.
A cooling coil pulls litres of water out of the air on a humid day near Lake Ontario, and all of it leaves through one small pipe. The trap on that pipe stays wet, dark and at room temperature all summer, which is what algae and bacterial slime need. The growth narrows the trap until the pan fills faster than it drains. A soft vinyl hose makes it worse: any sag holds standing water and a second plug forms there.
The trap itself can be the fault. On an air handler where the blower pulls air through the coil, the drain pan sits under suction, and a missing or dried-out trap lets air rush up the pipe and hold the water back in the pan until it spills over. That arrangement is common on attic air handlers, hydronic air handlers and heat pump systems. On a coil above a gas furnace the pan is under pressure instead, and the trap is there to stop conditioned air blowing out through the drain.
Overflow goes straight down. On a furnace-top coil that means into the furnace cabinet: onto the control board, the blower motor and the heat exchanger, where it shorts electronics in July and shows up as rust in January. On an attic unit it means the secondary pan, and when that is blocked too, the ceiling below. A control board runs $400–$800, against $150–$350 for the drain cleaning that would have kept it dry.
Clearing is done from the outlet end with a wet vacuum, or with a short burst of nitrogen from the clean-out toward the drain, never back at the coil. The pan and trap are flushed until water poured in at the coil runs freely at the far end. A trap that is glued solid with no way in is cut out and rebuilt with a clean-out tee and a proper slope, and a float switch is wired into the cooling circuit. If you are still working out where the water is coming from, start with why an AC leaks water.
Signs the condensate drain is blocked
- Water on the floor around the furnace in summer, or rust streaks down the front of the cabinet
- A wet secondary pan or a ceiling stain under an attic or closet air handler
- The AC stops cooling and the float switch on the drain line or pan is found tripped
- Gurgling at the indoor unit, or water standing in the clear section of the trap
- A musty or sour smell from the registers when cooling starts
How it works
Set the thermostat to off and put towels or a tray under the drip. Tell us where the indoor unit sits: basement, closet or attic. A homeowner with a wet vacuum can try the outlet end first; how to unclog an AC drain line shows the safe method.
The line is vacuumed or blown clear, the pan cleaned, and the reason identified: slime, a sag, a trap with no water seal, a line with no slope, or a failed pump. The furnace compartment below the coil is opened and checked for water.
Where needed the trap is rebuilt with a clean-out, the hose replaced with rigid pipe on a steady slope, and a float switch fitted and tested by filling the pan. You see the system shut off on the switch before the technician leaves.
Common questions
How often should the AC drain line be cleaned?
Once a year, in spring, before the first humid stretch. It belongs in any proper spring cooling visit, where the pan, trap and line are flushed and the float switch is tested. Systems that share the drain with a high-efficiency furnace and a humidifier collect more debris and are worth a second look in the fall, before the furnace starts making its own condensate.
Is a float switch worth adding?
Yes, on any coil above a furnace, over a finished basement, or in an attic or closet. It is a small switch in the drain line or pan that opens the cooling circuit when the water rises, so a clog gives you a warm house and a service call, not a wet ceiling. It needs testing each year; a switch nobody has tripped in a decade may not trip when it counts.
The drain runs to a pump, not a floor drain. Does that change the job?
It adds a second place to fail. The pump's reservoir grows the same slime, its check valve sticks, and its thin discharge tube kinks or clogs. Cleaning covers the reservoir and tube as well as the trap. A pump that hums without emptying, or runs constantly, is replaced; condensate pump replacement runs $250–$450 installed.
Can I pour bleach or vinegar down the line?
Check the equipment manual first. Where it is allowed, a cup of plain white vinegar poured into the clean-out a few times a summer slows the slime and does not harm PVC. Skip bleach near the coil and in any drain that feeds a condensate pump, and never mix products. Neither liquid clears a line that is already blocked; that takes suction or pressure from the right end.
Who handles a blocked drain on a condo fan coil?
It depends on the declaration: many corporations own the riser and drain stack while the owner is responsible for the fan coil and its pan. A blocked pan in one suite leaks into the suite below, so report it to management as well as calling for service. Who is responsible for a condo fan coil explains the usual split, or contact us to book.