Why Is My AC Leaking Water? (And When to Worry)

Every July I get a version of the same call, usually a couple times a week: "There's water around my furnace and the air conditioning is on. Is my furnace leaking?" It's a fair question. In most homes around Jenison and Grand Rapids, the AC's indoor coil sits in a cabinet directly on top of the furnace, so when water shows up, the furnace gets blamed. In summer, it's almost never the furnace. It's the air conditioning side, and specifically, it's the drain path that carries away all the water your AC makes.

And it makes a lot of water. That surprises people. The good news is that most AC water leaks are among the cheapest problems we fix all summer. The bad news is that a small minority of them are a warning sign for something bigger, and a leak in the wrong spot can wreck a ceiling or a control board while you're deciding whether to call.

Here's where the water comes from, the causes in order of how often I actually find them, and how to tell a shrug from a shut-it-down.

Where All That Water Comes From

Your air conditioner is a dehumidifier that happens to cool. As humid indoor air passes over the cold evaporator coil, moisture condenses out of it the same way it beads up on a glass of iced tea in July. That water drips off the coil into a drain pan, and from the pan it flows down a drain line, usually a white PVC pipe, to a floor drain, a condensate pump, or outside.

On a sticky West Michigan afternoon, when the dew point is up in the 70s and the system is running long cycles, a whole-house AC can pull several gallons of water out of your air in a day. Sometimes a lot more. That's not a malfunction. That's half the job, and it's a big part of why a properly running AC makes a house feel so much better here — I went into that in why humidity control matters so much in West Michigan.

So the question is never "why is my AC making water." It always makes water in summer. The question is why the water stopped going where it's supposed to go.

The Usual Causes, in Order of Likelihood

Start at the top and work down. This is the order I check things in on a real call.

1. A Clogged Condensate Drain Line

This is the winner by a mile. The drain line carries cool, mineral-rich water through a dark pipe all summer, which is basically a petri dish. Algae and bacterial slime build up inside until one day the pipe closes off, the pan backs up, and water goes over the edge.

This one has a legitimate DIY fix. Find where the line terminates, which around here is usually at a basement floor drain or a stub of PVC outside near the foundation. Seal a wet/dry vac hose over the end, run it for a minute or two, and you'll often pull out a slug of gray sludge followed by a rush of trapped water. If the pan drains down and stays down, you fixed it.

You'll see advice online about pouring bleach down the line. I'm cautious about that one. Bleach poured against an existing clog mostly just sits on top of it, and repeated bleach doses are hard on metal drain pans, pump seals, and anything the line drains near. For routine upkeep, a cup of diluted white vinegar in the drain line's access tee a couple times a season does the job gently. For an actual clog, suction beats chemicals.

2. A Clogged or Rusted Drain Pan

The pan under the coil catches every drop before it heads for the drain. Two things go wrong with pans. The drain opening at the bottom clogs with the same gunk that clogs the line, which is an easy cleaning fix. Or, on older systems, a metal pan rusts through, and then water leaks out the bottom no matter how clear the drain is.

A rusted pan is worth an honest conversation, because on many coils the primary pan is built into the coil assembly itself. If a 15-year-old system has rusted through its pan, we'll talk about whether it makes sense to put money into that coil at all before you spend it.

3. A Failed Condensate Pump

Here's a very West Michigan one. Most of our houses put the furnace and coil in the basement, and if there's no floor drain nearby, or the drain sits higher than the pan outlet, the condensate collects in a small plastic pump box that pushes it up and over to a drain or outside. Basement installs are everywhere around Hudsonville, Grandville, and Wyoming, so I see a lot of these pumps.

They're hardworking little devices and they fail two ways: the float switch sticks or the motor quits. Either way, the box fills, overflows, and you get a slow, steady puddle next to the furnace. Quick test: pour a cup of water into the pump reservoir. If the pump doesn't kick on and empty it, you've found your leak. Replacement is a routine, same-visit repair.

4. A Frozen Coil That's Thawing

This is the sneaky one, because the leak is a symptom of a different problem. If airflow through the coil drops, usually from a filter that hasn't been changed in months, or if the refrigerant charge is low from a leak, the coil gets too cold and ices over like a freezer that needs defrosting. Then the system shuts off, the ice melts fast, and the sudden flood overwhelms a pan designed for a steady drip.

The tell is the pattern. Drain-line clogs leak steadily while the AC runs. A thawing coil dumps water in a rush, often right after the system stops, and it usually comes with weak airflow or warm air beforehand. If that sounds familiar, the water isn't your real problem. Read why your AC freezes up, shut the system off, and let it thaw before anyone works on it. If a low charge is the cause, that refrigerant went somewhere, and it needs a real AC repair visit, not just a mop.

5. An Install or Pitch Problem

Water only flows downhill. A drain line with a sag in it, a pan that's not level, or a trap that was never primed can cause chronic, low-grade leaking from day one. If your system has leaked a little every summer since it went in, this is my first suspicion. It's also the cause I find most often behind "we've had three companies out and it still drips."

Water in the Right Place vs. Water in the Wrong Place

Not all water is equal, and this distinction matters more than the cause.

Water in the right place: a trickle from the drain line into the basement floor drain, damp concrete right under the drain termination, condensation sweat on an uninsulated line on a brutal humid day. That's the system doing its job, or at worst a small problem on a forgiving surface. Concrete dries.

Water in the wrong place: a brown stain spreading on a ceiling below an attic or upstairs air handler. Water inside the furnace cabinet, dripping toward the control board, the gas valve, or the blower motor. Water reaching wood framing, drywall, carpet, or anything stored next to the furnace. I was in a basement in Kentwood earlier this month where a condensate pump had been quietly overflowing behind some storage bins for what looked like weeks. The pump was a routine swap. The moldy carpet and boxes of family photos were the expensive part.

Ceilings deserve special mention. An attic air handler sits in a secondary drain pan, and by the time you see a stain, the primary drain has failed and the backup pan may be full too. That's exactly the setup where a float switch earns its keep.

When It's Urgent and When It Can Wait

My honest triage, the same one I'd give you on the phone:

Notice that "shut it off" shows up a lot, and that's on purpose. Turning off the AC stops the water at the source, because no cooling means no condensate. You lose comfort for a day. You don't lose drywall.

What the Repair Visit Looks Like, and What It Runs

When we come out for a leak, the visit is mostly detective work: confirm where the water's coming from, clear or repair the drain path, test the pump and float switch if there is one, check the coil and filter for the freeze-up pattern, and check the refrigerant charge if the coil shows signs of icing. Then we run the system and watch the water actually go where it should before we leave.

On cost, I'll give you honest wide ranges for our market rather than a fake precise number. Clearing a clogged drain line is usually plain service-call territory, roughly $100 to $300. A condensate pump replacement typically lands around $150 to $450 installed. A float switch added while we're there is usually well under $250. Pan and coil problems are the wide-open end: cleaning a pan is cheap, but a rusted-through pan built into the coil, or a refrigerant leak behind a freeze-up, can run from several hundred dollars to well over a thousand depending on what the system needs and whether it's worth putting into an aging unit. We'll tell you which side of that line you're on before any work starts.

And I'd be leaving something out if I didn't say this: nearly every drain-side leak on this list is preventable. Flushing the drain line, cleaning the pan, and testing the pump are standard items on a spring AC tune-up, and Comfort Plan members get that visit built in every year. The leak calls I run in July are overwhelmingly on systems nobody has looked at since the sticker on the cabinet was new.