AC Leaking Water Inside? Causes, Fixes & When to Call a Pro

You walk into your utility room, basement, or hallway and there it is — a puddle of water under your AC unit. Maybe it is a slow drip staining the ceiling below your attic air handler, or maybe it is a full-on pool spreading across your hardwood floor. Either way, an AC leaking water inside your house is never something you can ignore.

At Dimatic Control in Union, NJ, we handle AC water leak calls every week during the summer months — and we see a sharp spike from June through September when New Jersey's brutal humidity pushes condensation levels through the roof. The combination of 70–90% relative humidity, aging HVAC systems in older Union County and Essex County homes, and infrequent maintenance makes this one of the most common AC problems we encounter across Central New Jersey.

The good news: most causes of an AC leaking water are fixable, and some you can even troubleshoot yourself in under 30 minutes. The bad news: ignoring the leak — even for a day or two — can cause thousands of dollars in water damage to your home.

In this guide, we will cover the emergency steps you should take right now if water is actively on your floor, walk through the six most common causes, explain what each repair costs in the NJ area, and help you decide when you can DIY and when you need a pro.

How Your AC Produces Water (And Why It Matters)

Before we get into the specific causes of leaking, it helps to understand why your AC produces water in the first place — because it is supposed to.

Your air conditioner does not just cool the air. It also acts as a dehumidifier. Warm, humid air from your home gets pulled across the cold evaporator coil inside your air handler or furnace. When that warm, moisture-laden air hits the cold coil, the moisture condenses into water droplets — the same way a cold glass of water "sweats" on a humid day.

That condensation drips off the evaporator coil into a drain pan (also called a drip pan or condensate pan) positioned directly beneath the coil. From the drain pan, the water flows through a condensate drain line — a small PVC pipe that carries it out of your home, usually dumping it near the outdoor condenser unit or into a floor drain.

When everything works correctly, you never see any of this water. It quietly drains away. But when any part of this drainage system fails — the line clogs, the pan cracks, the coil freezes — the water has nowhere to go except onto your floor, ceiling, or walls.

How Much Condensation Is Normal?

This is a question we get from homeowners all the time, especially in New Jersey where the humidity is no joke.

A properly functioning central AC system in a humid climate like New Jersey can produce 5 to 20 gallons of condensate per day during peak summer. On an exceptionally muggy day in July or August — when Union County humidity hits 85–90% — your system might produce even more than that.

That is a lot of water, and all of it is flowing through a single drain line that is typically only 3/4-inch in diameter. You can see why even a small partial clog can cause problems when the volume is that high.

6 Causes of an AC Leaking Water Inside Your Home

1. Clogged Condensate Drain Line (The #1 Cause)

This is far and away the most common reason we get called to homes across Union County and Central New Jersey for AC water leaks. If we had to guess the cause before even walking through the door, a clogged drain line would be right at least 60% of the time.

What happens: Over time, algae, mold, mildew, dust, and debris accumulate inside the condensate drain line. The warm, dark, wet environment inside that pipe is a perfect breeding ground for biological growth — especially in New Jersey's humid summer months when your system is running constantly and producing gallons of condensate daily.

As the buildup grows, it narrows the pipe opening. Eventually, the clog either fully blocks the drain or restricts it enough that the water backs up into the drain pan faster than it can drain out. The pan overflows, and water spills onto your floor, into your ceiling, or down the side of your air handler.

Signs to look for:

• Water pooling around the base of your indoor air handler or furnace
• The condensate drain line outside your home is not dripping even when the AC has been running for an hour
• A musty or moldy smell near the indoor unit
• The AC shuts off unexpectedly — some systems have a float switch in the drain pan that kills the system when water reaches a certain level

NJ repair cost: $75–$250 for a professional drain line clearing. Most HVAC technicians use a nitrogen flush or specialized brush to fully clear the blockage and then treat the line with an algaecide to prevent regrowth.

DIY vs. pro: You can often clear a minor clog yourself by attaching a wet/dry vacuum to the outdoor end of the drain line and running it for 2–3 minutes. Pouring a cup of distilled white vinegar into the drain line access point (usually a T-fitting near the indoor unit) once a month also helps prevent clogs. However, if the clog is deep in the line or the pipe has partially collapsed, you will need a professional with proper equipment. This issue affects all major brands — whether you have a Carrier, Goodman, or Rheem unit, the drain system works the same way. For a complete walkthrough, see our step-by-step guide to unclogging your AC drain line.

2. Frozen Evaporator Coil

A frozen evaporator coil is the second most common cause of AC water leaks inside your home, and it is closely connected to several other issues on this list. We wrote an in-depth guide on AC freezing up that covers this topic in full detail, but here is how it relates to water leaks specifically.

What happens: When your evaporator coil gets too cold — from restricted airflow, low refrigerant, or other causes — the moisture condensing on the coil does not just drip off as water. It freezes solid on the coil, building up a layer of ice that can grow to several inches thick.

The problem comes when the system cycles off or the ice starts to thaw. That thick ice layer melts all at once, producing a massive volume of water that overwhelms the drain pan. The pan was designed to handle a steady drip of condensation — not a sudden rush of melted ice. Lennox and Trane systems use similar drain pan designs that are prone to the same issues in NJ humidity. The result: water overflows the pan and leaks into your home. Learn why this happens and how to fix it in our AC freezing up guide.

Signs to look for:

• Ice or frost visible on the refrigerant lines (the copper pipes running from indoor to outdoor unit)
• AC running but blowing warm air — the ice insulates the coil and prevents it from cooling
• A sudden large leak that coincides with the AC shutting off (the ice is melting)
• Reduced airflow from your supply vents

NJ repair cost: Depends on the root cause. A dirty air filter (the most common cause of a freeze-up) is a $5–$30 DIY fix. A professional evaporator coil cleaning runs $150–$400. If the freeze was caused by low refrigerant from a leak, expect $200–$1,500 for leak detection and repair plus a refrigerant recharge.

DIY vs. pro: If the freeze-up was caused by a dirty filter, replace the filter and let the ice thaw completely (1–4 hours with the fan set to "On"). If the system freezes again after restarting, the cause is something more serious and you need a professional to check refrigerant levels, coil condition, and blower motor performance.

3. Damaged or Rusted Drain Pan

This cause is especially prevalent in older New Jersey homes where the HVAC system is 10 years old or more. We see it frequently in Union, Hillside, Elizabeth, and other communities where a significant portion of the housing stock was built in the 1950s through 1980s.

What happens: The drain pan sits directly under your evaporator coil and catches all the condensation. Most air handlers have two drain pans — a primary pan that is welded or molded into the air handler, and a secondary (auxiliary) pan that sits underneath the entire unit as a backup.

Over years of constant contact with water, metal drain pans rust through and develop cracks or pinholes. Plastic pans can also crack from age, heat cycling, or physical damage during filter changes or other maintenance. Once the pan has a hole or crack, water drips straight through it instead of flowing to the drain line.

Signs to look for:

• Visible rust stains or corrosion on the drain pan
• Water dripping from the bottom of the air handler rather than from the drain pan overflow
• The drain line outside is dripping normally, but you still have water under the unit — this means the pan is leaking, not the drain
• Water stains or discoloration on the ceiling below an attic-mounted air handler

NJ repair cost: Replacing a drain pan typically costs $150–$600 in the Union County area, depending on whether it is the primary or secondary pan and how accessible the air handler is. Primary pans that are welded into the unit are more expensive and labor-intensive to replace. Secondary pans are usually a straightforward swap.

DIY vs. pro: You can inspect the drain pan yourself with a flashlight — look for visible rust, cracks, or standing water that is not draining. A temporary fix for a small crack is water-resistant epoxy or sealant, but this is a short-term patch. A cracked or rusted pan should be replaced professionally to avoid ongoing leaks and potential water damage.

4. Dirty Air Filter

We already mentioned dirty filters in the emergency steps section, and there is a reason — this single maintenance item is responsible for an outsized number of AC problems, including water leaks. If you have not been keeping up with regular air filter changes, this could easily be your culprit.

What happens: Your air conditioner needs a continuous flow of warm air across the evaporator coil to work properly. The warm air is what keeps the coil from getting too cold. When a dirty filter restricts that airflow, the coil temperature drops below 32°F and the condensation on the coil freezes into ice instead of dripping into the drain pan.

When the system eventually cycles off, that ice melts — and the sudden volume of water overwhelms the drain pan, causing an overflow and an indoor water leak. This creates a frustrating cycle: the system runs, ice builds up, the system shuts off, the ice melts, water floods, and the process repeats.

Signs to look for:

• Your air filter is visibly dirty — you cannot see light through it
• Reduced airflow from supply vents even though the system is running
• The water leak seems to come and go — worst after the system has been running for a long stretch
• Ice on the refrigerant lines or evaporator coil when you check

NJ repair cost: $5–$30 for a new air filter. Standard 1-inch filters cost $5–$15. Higher-quality pleated MERV 11–13 filters run $15–$30. This is the cheapest possible AC repair and one of the most impactful.

DIY vs. pro: This is 100% a DIY fix. Pull out the old filter, note the size printed on the frame (e.g., 16x25x1), and buy a replacement at any hardware store. In New Jersey, we recommend changing your filter every 30–60 days during summer, especially if you have pets, allergies, or live near construction areas like the Route 22 corridor in Union.

5. Low Refrigerant

Low refrigerant creates a chain reaction that ends with water on your floor. This is a cause that a homeowner cannot diagnose or fix themselves — it always requires a licensed HVAC technician.

What happens: Refrigerant is the chemical that cycles through your AC system, absorbing heat from indoor air at the evaporator coil and releasing it outside at the condenser. When the refrigerant level is low — almost always because of a leak somewhere in the system — the pressure inside the evaporator coil drops.

Lower pressure causes the remaining refrigerant to expand more than normal and get abnormally cold. The coil temperature plummets well below freezing, and ice forms on the coil. When the system cycles off or the ice accumulates enough to fully block airflow, it begins to thaw — dumping a large volume of water that overwhelms the drain pan. Low refrigerant from a leak can also cause freezing and water overflow — see our AC refrigerant leak guide for details.

Signs to look for:

• AC blowing warm air or not cooling the house to the set temperature — read our guide on AC not blowing cold air for more detail. If the leak is causing your AC to blow warm air instead of cold, see our guide to AC running but not cooling.
• Ice on the refrigerant lines or outdoor unit
• A hissing or bubbling sound near the indoor unit (refrigerant escaping through a leak)
• Higher than normal electricity bills — the system runs longer trying to compensate for reduced cooling capacity
• The system freezes up repeatedly even after replacing the air filter

NJ repair cost: $200–$1,500 depending on the location and severity of the leak. Simple repairs like tightening a connection or replacing a Schrader valve are on the lower end. Repairing a leak in the evaporator coil or replacing a section of refrigerant line is on the higher end. The refrigerant recharge itself adds to the cost, especially for older R-22 systems that use a phased-out refrigerant.

DIY vs. pro: This is always a professional repair. Refrigerant handling requires EPA Section 608 certification, and the leak must be found and repaired before recharging the system. Simply "topping off" the refrigerant without fixing the leak is a waste of money — it will leak out again.

6. Improper Installation or Sizing

This cause is less common than the others, but when it is the problem, it is one of the most frustrating because the leak will never fully stop until the installation issue is corrected.

What happens: Several installation-related issues can cause chronic water leaks:

• Oversized AC unit: A system that is too large for your home cools the air too quickly, causing short cycling. The system runs in brief bursts, pulling down the temperature fast but not running long enough to properly dehumidify. This leads to excessive moisture on the coil that can overwhelm the drain system.
• Improper drain line slope: The condensate drain line must slope downward continuously from the air handler to the exit point. If the line was installed flat, with a dip, or with an uphill section, water pools in the low spots instead of draining out, eventually backing up into the pan.
• Disconnected drain line: Vibration, settling, or a poor initial connection can cause the drain line to separate from the drain pan fitting, letting water drip freely out of the gap.
• Missing or improperly installed insulation: The refrigerant lines and sometimes the drain line need proper insulation to prevent condensation from forming on their exterior surfaces. Missing insulation causes "sweating" that drips water in areas where it should not be.

Signs to look for:

• The leak started shortly after a new AC installation or a major repair
• The system short-cycles frequently — running for only 5–10 minutes before shutting off
• Your home feels clammy or humid even though the thermostat shows the temperature is at the set point
• Water appears to be leaking from the refrigerant lines or connections rather than the drain pan

NJ repair cost: Varies widely. Reconnecting or re-sloping a drain line is typically $100–$300. Replacing insulation on refrigerant lines runs $150–$400. However, if the system is significantly oversized, the real fix is replacing it with a properly sized unit — which is a major investment. A good HVAC installer will perform a Manual J load calculation to properly size the system for your home before installation.

DIY vs. pro: Installation issues should always be addressed by a professional. If you suspect the problem started after a recent installation, contact the company that did the work — reputable installers will come back and correct their work.

AC Water Leak Repair Costs: Quick Reference

Here is a summary of typical NJ-area repair costs for each cause. Actual costs may vary depending on your system type, accessibility, and time of service.

Water Damage Prevention: Protecting Your NJ Home

The water leak itself is a problem, but the secondary water damage is often far more expensive than the AC repair. Here is what to watch for and how to protect your home.

Immediate Water Damage Risks

• Hardwood floors: Can begin warping within hours of water exposure. In older Union County homes with original hardwood, replacement is costly — typically $8–$15 per square foot installed.
• Drywall and ceilings: Water-saturated drywall loses structural integrity and can sag, crack, or collapse. Attic-mounted air handlers are especially risky because a leak can saturate the ceiling below before you notice it.
• Mold growth: Mold can begin forming on damp surfaces within 24–48 hours in New Jersey's humid climate. Professional mold remediation typically costs $500–$6,000 depending on the extent.
• Carpet and padding: Once carpet padding gets soaked, it rarely dries properly and becomes a breeding ground for mold and mildew. Most water damage professionals recommend replacing saturated padding.
• Electrical hazards: Water near electrical components, outlets, or wiring creates a shock and fire hazard. If water is near any electrical connection, turn off the breaker for that area.

Preventive Measures

• Install a condensate overflow switch: Shuts off the AC automatically if the drain pan overflows — prevents damage even if a clog happens while you are away.
• Add a secondary drain pan: For attic or ceiling installations, a secondary pan with its own drain line catches any overflow from the primary pan.
• Install a water leak detector: Battery-powered sensors ($10–$30) placed near the air handler will alarm when they detect water. Smart models can send alerts to your phone.
• Schedule annual maintenance: A professional spring AC tune-up includes a full drain line flush, pan inspection, and coil cleaning that catches problems before they cause leaks.
• Flush the drain line monthly: Pour a cup of distilled white vinegar into the drain line access point every month during cooling season to prevent algae buildup.

When to Call a Professional: Decision Guide

Not every AC water leak requires a service call. Here is a straightforward decision guide to help you determine when you can handle it yourself and when it is time to call a pro.

Handle It Yourself If:

• The air filter is dirty — replace it and monitor the system
• You can clear the drain line with a wet/dry vacuum and water starts flowing normally
• The system had a minor freeze-up that resolves after a filter change and 2–4 hours of thawing
• The leak was a one-time event and does not recur after basic troubleshooting

Call a Professional If:

• The drain line clog will not clear with a wet/dry vacuum
• The system freezes up repeatedly even after replacing the air filter
• You see ice on the refrigerant lines or outdoor unit
• The drain pan is visibly cracked, rusted, or damaged
• You hear a hissing or bubbling sound from the indoor unit (possible refrigerant leak)
• The leak started after a new AC installation or major repair
• Water is leaking near electrical components
• You have already tried the DIY steps and the leak continues

Seasonal Maintenance to Prevent AC Water Leaks in NJ

Most AC water leaks are preventable with regular maintenance. Here is a seasonal checklist specifically for New Jersey homeowners dealing with our high-humidity climate:

Spring (March–May):

• Schedule a professional AC tune-up before the cooling season starts
• Replace or clean the air filter
• Flush the condensate drain line with vinegar
• Inspect the drain pan for cracks or rust
• Test the condensate overflow switch (if you have one)

Summer (June–September):

• Change the air filter every 30–60 days
• Flush the drain line with vinegar monthly
• Check that the outdoor drain line exit is dripping when the system is running
• Keep the area around the outdoor condenser clear of debris
• Watch for any signs of moisture near the indoor air handler

Fall (October–November):

• Flush the drain line one final time before shutting down the AC for the season
• Inspect the drain pan before winter storage
• Schedule a fall HVAC maintenance checkup to transition from cooling to heating

Keeping up with this schedule — especially the monthly drain line flushes during summer — will prevent the vast majority of AC water leaks. It takes 5 minutes and costs less than a dollar in vinegar. Compare that to the cost of a water damage repair and it is a no-brainer.

Last updated: April 13, 2026. Cost estimates reflect current NJ-area pricing and may vary based on system type, accessibility, and time of service.