You set the thermostat, the AC kicks on, and then you notice it — a puddle forming near the air handler or water dripping from the ceiling below the unit. An AC unit leaking water inside your home isn’t something to put on the back burner. Water and building materials are a bad combination. Even a small, slow leak from your air conditioner can cause significant damage to drywall, insulation, flooring, and ceiling joists if it’s left unaddressed. And once moisture sits long enough, mold enters the picture. This guide explains why your AC unit leaking is happening, what to do about it right now, and how to prevent it from becoming a much bigger and costlier problem.
Why Is Your AC Unit Leaking Water?
Your air conditioner doesn’t just cool the air — it also removes humidity from it. That moisture has to go somewhere. Normally, it drains out of your home through the condensate drain system. When something in that system fails, you end up with an AC unit leaking water where it shouldn’t be. There are four main causes.
Clogged Condensate Drain Line
This is by far the most common cause of this problem. The condensate drain line collects moisture from the evaporator coil and carries it out of the house. Over time, algae, mold, and dust buildup create blockages in the line. When it clogs, the water backs up into the drain pan and eventually overflows. In San Diego’s warm months, when your AC runs frequently, this problem can get out of hand quickly. The U.S. Department of Energy recommends periodically flushing the drain line to prevent exactly this kind of buildup.
Frozen Evaporator Coil
The evaporator coil absorbs heat and moisture from the air in your home. If airflow across the coil is restricted — usually because of a dirty filter or low refrigerant — the coil can freeze over. When the ice eventually melts, it produces more water than the drain pan can handle, and the overflow spills onto the floor. A frozen coil is a signal that something else is wrong with the system. Just mopping up the water won’t solve it.
Damaged or Corroded Drain Pan
Your air handler has one or two drain pans to catch condensate water. The primary pan sits directly below the evaporator coil. If that pan has rusted through, cracked, or is misaligned, water falls through rather than draining properly. Older systems in San Diego homes — especially those exposed to coastal humidity — are particularly susceptible to pan corrosion. If your system is more than 10 years old, the drain pan condition is worth checking when you’re tracking down this type of water issue.
Low Refrigerant
Refrigerant leaks cause the system pressure to drop. Low pressure means the evaporator coil gets too cold, leading to ice formation and eventually the same melt-and-overflow problem described above. You might also notice your home isn’t cooling as efficiently as it used to. If your AC unit is leaking water and you’ve ruled out other causes, low refrigerant may be the culprit. An HVAC technician will need to find and seal the leak before recharging the system — you can’t just top it off and call it fixed.
How Much Damage Can an AC Unit Leaking Cause?
More than most homeowners expect. The location of your air handler determines where the water goes when the leak starts. In many San Diego homes, the air handler is installed in the attic or in a closet on an upper floor. When the problem goes unnoticed — or is spotted but ignored — water can saturate insulation, wick into ceiling drywall, and drip down into the room below.
We’ve responded to water damage calls where a slow condensate drip had been going undetected in an attic for weeks. By the time the homeowner saw the ceiling stain, the damage extended well beyond what was visible. Saturated insulation, warped ceiling joists, and water-damaged drywall all had to be addressed. And once moisture sits for 24 to 48 hours, mold growth can start. That turns a plumbing repair into a full water damage restoration project.
Read more about why untreated water damage compounds so quickly. What starts as a minor drip becomes structural damage, mold, and an insurance claim — often within just a few days.
What to Do Right Now If You See an AC Unit Leaking
Speed matters. Here’s what to do immediately when you find an AC unit leaking water inside your home:
Turn off the system. Don’t let it keep running while water is actively leaking. Continued operation can freeze the coil further or push more water into the overflow. Turn the system off at the thermostat.
Protect the area below. If water is dripping into a room below the air handler, move furniture, electronics, and valuables out of the affected area. Place buckets or towels to limit floor contact.
Check and replace the air filter. A clogged filter is one of the easiest problems to fix yourself. If the filter is grey and matted with dust, it may be restricting airflow enough to cause a frozen coil. Replace it and see if that resolves the issue once the coil thaws.
Try to clear the drain line. If you can locate the condensate drain line access port (usually a PVC pipe near the air handler), you can try using a wet/dry vacuum to suction out the clog. This works in mild blockage cases and is a legitimate DIY fix.
Call an HVAC technician. If the problem persists after those steps, the issue likely requires a professional — especially if you suspect a refrigerant leak, a cracked drain pan, or a damaged coil.
Check for Hidden Water Damage After an AC Unit Leaking Event
Once the mechanical issue has been fixed by a technician, the job isn’t finished. You need to assess whether water has penetrated building materials beyond the immediate drip zone. Ceiling stains are the most obvious sign, but water travels — it follows insulation batts, runs along joists, and pools in low spots that aren’t visible from below.
Check for signs of water damage behind walls and in adjacent ceiling areas. If the attic was affected, inspect insulation for saturation and look for any discoloration on wood framing. If water reached a lower-floor ceiling, press gently on the drywall — soft or sagging areas indicate deeper damage. A professional moisture inspection can identify exactly how far the water traveled using equipment that measures moisture levels inside walls without opening them up.
If there’s been any ceiling water damage, it needs to be properly dried and assessed before it’s repaired. Closing up wet drywall creates a hidden mold factory inside the wall cavity.
When to Call a Water Damage Restoration Professional
Some of these situations are straightforward HVAC problems. Others cross over into water damage territory that requires more than a technician with a wrench. Call a water damage professional when:
Water has reached ceiling drywall in a room below the unit. Drywall that has absorbed condensation water can begin supporting mold growth within 24 to 48 hours — especially in warm, closed attic spaces. Don’t assume it will dry out on its own. If you need water damage repair in San Diego, getting professional drying equipment in quickly is what prevents a minor incident from becoming a major claim.
You see or smell mold near the air handler or in the affected area. If the leak has been a slow, ongoing problem — even intermittently — there may already be mold growing inside the air handler, in the drain pan, or in the surrounding structure. Our article on mold after water damage timelines explains how fast this can develop. Professional mold remediation may be needed before the area is safe to close up again.
Insulation in the attic or walls was affected. Wet insulation is not salvageable in most cases. It compresses, loses its R-value, and retains moisture that promotes mold. A restoration team can remove and replace it properly.
5 Ways to Prevent Your AC Unit Leaking in the Future
Once you’ve resolved this kind of event, prevention becomes the priority. These five habits will dramatically reduce your chances of seeing it happen again:
1. Change your air filter every 1–2 months. A clean filter is the single most effective way to prevent frozen coils and the leaks they cause. In San Diego’s dusty inland areas or coastal homes where air quality varies, monthly changes are often appropriate during peak cooling season.
2. Schedule annual HVAC maintenance. A certified HVAC technician should inspect the coils, check refrigerant levels, clean the drain pan, and flush the condensate drain line once a year — ideally in spring before the hot months hit. The Department of Energy’s AC maintenance guidance outlines exactly what that annual visit should include.
3. Pour a cup of diluted bleach into the condensate drain line access once a month. This kills algae and mold before they can clog the line. It takes about 30 seconds and can save you a major headache.
4. Install a condensate overflow shutoff switch. This is a small sensor that cuts off the AC before the drain pan overflows. It’s inexpensive and can be added to any system. It won’t fix the underlying problem, but it prevents water damage from occurring while you wait for a repair.
5. Know where your air handler is and check it seasonally. Homeowners who don’t know where their air handler is located can’t notice the early signs of an AC unit leaking problem. A quick visual check near the unit at the start of summer and mid-season takes five minutes and can catch issues before they cause damage.
Don’t Wait on Water Damage From an AC Leak
An AC unit leaking water inside your home is a system problem — but the water damage it causes is a structural problem. The HVAC technician fixes the first part. A restoration team handles the second. Both matter. If you’ve already dealt with the mechanical issue and now need to assess or repair water damage in your San Diego County home, American Response Team is available 24/7. We serve Vista, La Jolla, and surrounding communities with fast, IICRC-certified leak detection and full water damage restoration services. Contact us today to get the situation assessed and dried out the right way.
