I get this call a lot in Calgary. You walk downstairs and there’s a little puddle by the furnace, or you see damp marks around the indoor unit, and now you’re staring at it like it personally betrayed you. Air conditioners make moisture as they run, that part is normal. The problem is when that moisture doesn’t make it where it’s supposed to go, which is down a drain and out of your house, not onto your floor.
Most of the time, it’s not some mysterious failure. It’s basic stuff that’s been quietly building up. A drain tube gets slimy. A trap dries out. A pan cracks. A filter gets ignored and the coil gets too cold, then you end up with ice, then it thaws, then you get a sudden mess. I’ve seen people panic and shut everything off for a week, then fire it up on a hot day and wonder why it’s worse. Well, usually anyway.
Another thing that trips people up is thinking the issue must be outside, like the condenser. But a lot of the wet surprises happen at the indoor side, where the cold coil is and where your drain piping lives. If you’ve got high humidity in the house, or you’re running the fan nonstop, you can also get extra moisture showing up where you never noticed it before, around vents, near the plenum, even sweating on metal.
In this article I’m going to walk you through the common causes I see on real service calls, what you can check without turning it into a bigger job, and when it’s time to stop poking at it and get someone in. If you’re trying to understand how your air conditioning equipment handles moisture, and where it can go wrong, you’re in the right place.
How to confirm the leak source: condensate line vs. refrigerant lines vs. ductwork sweat
If you’ve got a puddle or damp patch showing up, don’t guess. First thing I do on a call is look at where it shows up relative to the furnace or air handler. If it’s right under the indoor coil area or beside the floor drain, my brain goes straight to the condensate system. If it’s along a copper line run, or dripping off the big insulated suction line, that’s a different story. And if you’ve got wet spots on ceilings or around supply boots, I’m thinking duct sweat or air leaks in the ducting, not the drain at all.
Condensate line issues are the most common, and they leave clues. Check the plastic drain pipe, the trap (if you have one), and the pan under the coil. If the pan is full, that’s not “humidity”, that’s a blockage or a broken connection. I’ve opened plenty of units where someone bumped the drain fitting during a filter change, and it’s been quietly dribbling ever since. You can also do a quick test: shut the system off, pour a cup of water into the pan access (or the cleanout tee if it’s accessible), and watch the outlet point. If nothing comes out outside or at the floor drain, the line isn’t moving flow. If it backs up fast, you found your source.
Refrigerant line moisture looks different. The smaller copper line should be warm and dry. The bigger suction line is cold, and it should be insulated the whole way. If that insulation is torn, missing, or taped up like a hockey stick, you’ll get beads forming and dripping, usually right where the insulation ends or at a low point. Sometimes it’s not just insulation either. Low airflow from a filthy filter or a matted coil can make that suction line colder than it should be, so it sweats more. I’ve seen homeowners crank the thermostat down thinking it will “catch up” faster, then the line turns into a little drip rail. If you suspect the refrigerant side and you’re also dealing with poor cooling, ice on the coil, or the unit short-cycling, that’s when you stop poking around and book a proper check, and if it’s an older system that’s been nickel-and-dimed, you might be closer to air conditioning and refrigeration replacement than you want to admit.
Ductwork sweat is the sneaky one because it can show up far from the furnace room. Look for cold metal surfaces, uninsulated duct sections, or gaps where warm basement air is getting pulled into a cold supply trunk. I’ve walked into houses where the duct seams were basically open, then the AC runs and the duct turns into a cold pop can in July. Touch test helps. If the duct skin is cold and wet but the drain and lines at the unit are dry, you’re chasing duct temperature and air sealing, not a drain problem.
Indoor Drips: Clogged Drain Line, Frozen Evaporator Coil, or a Cracked Drain Pan
The most common reason you get a puddle inside is a clogged condensate drain line. I see it all the time in Calgary, and it is almost always the same stuff: algae slime, dust, a bit of insulation fuzz, sometimes a spider nest because of course. The system makes moisture, it has nowhere to go, and it backs up until it spills out around the coil cabinet or down the furnace. People treat their filters like a suggestion and then act shocked the drain plugs up. If you are comparing setups at different air conditioning places, ask them where the drain will run and how it will be serviced, because hiding it behind finished drywall is a cute idea right up until you are mopping.
A frozen evaporator coil is the next big culprit for indoor dripping, and it confuses homeowners because it looks like “too much moisture” when it is actually the opposite problem. The coil ices over from poor airflow (plugged filter, dirty blower wheel, closed vents) or low refrigerant from a small refrigerant loss. Then the system cycles off or you shut it down, the ice melts fast, and that meltwater can overwhelm the drain or miss the pan entirely. I have opened up coil compartments and found a solid block of ice that was quietly building for days. If you see frost on the copper lines or the indoor unit looks like it is sweating like crazy, shut cooling off and run the fan, then call someone who does furnace and ac repair near me so it gets diagnosed properly, not guessed at.
Then there is the drain pan itself. The pan under the evaporator is supposed to catch the moisture and guide it into that drain line, but pans can crack, rust through, or get knocked out of alignment after years of vibration or a sloppy install. Older metal pans corrode, newer plastic ones can split or warp if things are stressed. The annoying part is it can drip only when the blower is at a certain speed or when the coil is making lots of moisture, so you think it “comes and goes” and you cannot catch it in the act.
If the drain line is clear and the coil is not icing, but you still get indoor dripping, I start suspecting pan damage or a pitch issue right away, because I have seen pans that look fine until you put a flashlight in just the right spot and there it is, a hairline crack sending a steady trickle onto the furnace cabinet. That is the kind of thing local air conditioning repair companies can confirm quickly with a simple water test and a good look inside, and it saves you from throwing vinegar at a problem vinegar cannot fix.
Q&A:
My indoor AC unit is dripping water from the front panel—what usually causes that?
Most front-panel dripping is from condensate that can’t reach the drain. Common causes are a clogged drain line (algae/dust buildup), a dirty air filter that restricts airflow and makes the coil too cold, or a drain pan that’s cracked or out of level. First, turn the system off to prevent more water damage. Check the filter and replace it if it’s dirty. If you can access the condensate line, look for kinks or blockage and clear it (a wet/dry vacuum on the outside drain outlet often works). If water still appears, the pan or internal drain connection may be damaged and needs service.
I see water pooling around the furnace/air handler in the basement. Is that normal during cooling?
No—some condensation is normal inside the system, but pooling on the floor means it isn’t draining correctly. In many setups, the evaporator coil sits above the furnace, and the condensate should flow into a drain pan and out through a PVC drain line (sometimes with a condensate pump). Pooling can happen if the drain line is clogged, the pump failed or lost power, the float safety switch shut the unit off, or the pan is rusted/cracked. Shut off cooling, dry the area, then check: (1) the drain line outlet—does water come out? (2) the pump reservoir—full and not pumping? (3) visible pan damage. If the pump isn’t running or the pan is leaking, call for repair; continuing to run it can damage the furnace and flooring.
Why does my AC seem to “sweat” on the vents and some ductwork?
Sweating on metal vents or ducts happens when humid air hits a cold surface and moisture condenses. That points to high indoor humidity, air leaks, or missing/poor insulation on ducts (especially in attics, crawlspaces, or unconditioned basements). Try setting the fan to “Auto” (constant fan can re-evaporate moisture off the coil and raise humidity), use the bathroom/kitchen exhaust fans, and make sure supply registers aren’t blocked. If the sweating is on exposed duct runs, adding insulation and sealing joints can help a lot. If humidity is consistently high (sticky feeling, musty odors, window condensation), the system may be oversized, airflow may be wrong, or a separate dehumidifier may be needed.
Water is leaking only when it’s very hot outside. Why does it happen on the worst days?
Hot days push the system harder, producing more condensation. If the drain is partially restricted, it may cope on mild days but overflow when water output spikes. Another reason is an evaporator coil starting to ice up from low airflow (dirty filter, blocked return, weak blower) or low refrigerant; when the ice melts, it can flood the pan and spill out. Check the filter and make sure return grilles are open and unobstructed. If you see ice on the refrigerant line or the coil, turn the AC off and run the fan to thaw it, then schedule service—low refrigerant or airflow issues won’t fix themselves.
I just installed a new thermostat / changed settings, and now there’s condensation. Could the settings cause leaking?
Settings can contribute, but they usually expose an underlying drainage or humidity issue. Running the fan on “On” can increase indoor humidity and lead to more condensation on vents, especially in humid climates. Large temperature swings (cooling the house quickly after letting it warm up) can also make surfaces cold enough to condense moisture. Use “Auto” for the fan, aim for steady setpoints, and make sure the condensate drain is clear. If the unit still leaks, the cause is typically mechanical (drain blockage, pan problem, coil icing) rather than the thermostat itself.