Why Is My AC Dripping Water Outside? (Denver HVAC Expert)

You walk outside and notice water dripping from your AC unit. Is that normal? The short answer: a small drip from the outdoor unit on a hot, humid day is completely expected. A steady puddle, water dripping inside your home, ice formation on the unit, or water near the indoor air handler are different stories entirely.

Understanding where the water is coming from — and what it means — can save you from water damage inside your home, mold in your ductwork, or an expensive refrigerant repair that got worse by being ignored.

Normal vs. Abnormal AC Water Dripping

The key to understanding AC water is knowing where it’s coming from and how much there is. Let’s break down each scenario.

Normal: Small Drip From Outdoor Unit Base

On a warm, moderately humid day, it’s normal to see a small amount of water dripping from the base of your outdoor unit. This is water that condensed on the indoor evaporator coil, drained through the condensate drain system, and exits through a drain port near the outdoor unit in some configurations. Alternatively, in high-humidity conditions, some moisture condenses on the outdoor unit itself.

Abnormal: Water Inside Your Home

Any water dripping, puddling, or staining near your indoor air handler, furnace, or in your attic (if that’s where your air handler lives) is not normal. This almost always indicates a clogged condensate drain line or overflowing drain pan — and it can cause serious water damage and mold growth quickly.

Abnormal: Ice on the System

Ice forming on the outdoor unit, refrigerant lines, or visible through the indoor air handler grille indicates the evaporator coil is freezing. This is caused by restricted airflow (dirty filter, blocked registers, low refrigerant) and will cause the system to stop cooling while appearing to “work.” Turn off the AC and let it thaw before calling for service.

Condensate Basics: Where Does the Water Come From?

Your air conditioner doesn’t just cool your home — it also removes moisture from the air. As warm, humid indoor air passes over the cold evaporator coil, moisture condenses on the coil’s surface — exactly like a cold glass sweating on a warm day. This condensate collects in a drain pan beneath the coil and exits through a condensate drain line, typically a white PVC pipe that drains outside or to a floor drain.

On an average summer day in Denver, a typical home AC system removes 10–20 pints of water from the air. During Denver’s July–August monsoon season, that can climb to 25–30 pints per day. All of this water needs to drain reliably through a system that most homeowners never think about — until it backs up.

Indoor Drain Pan Overflow: The Most Common Cause of AC Water Damage

The most frequent cause of water dripping inside a Denver home from the AC system is a clogged condensate drain line. Here’s how it happens:

Your condensate drain line is a PVC pipe, typically 3/4″ to 1″ diameter. It collects condensate from the drain pan and routes it outside or to a floor drain. Over time, algae, mold, dust, and mineral scale build up in the line — especially in attic air handlers where the line runs horizontally for a long distance. When the line clogs, water backs up into the drain pan.

Most systems have a primary drain pan (under the coil, usually fiberglass or metal) and a secondary/overflow pan (below the primary, usually plastic, designed to catch overflow). When the primary pan fills and overflows into the secondary pan, most modern systems have a float switch that shuts down the AC to prevent water damage. If your AC suddenly stopped working on a humid day for no apparent reason, this is a common cause.

How to Clear a Clogged Condensate Drain (DIY)

For simple clogs, homeowners can often clear the line themselves:

1. Turn off the AC at the thermostat
2. Locate the condensate drain line access port (usually a threaded cap on the PVC near the air handler)
3. Pour a cup of white vinegar into the access port — this dissolves algae buildup
4. Wait 30 minutes, then flush with water
5. For more stubborn clogs, use a wet/dry vacuum at the exterior drain outlet to suction the blockage out
6. If water still backs up after this, call a technician — the clog may be deep in the line or at an internal trap

Refrigerant Leak: When Water + Poor Cooling = Bigger Problem

Sometimes homeowners notice AC dripping combined with poor cooling performance and assume the dripping is the problem. In some cases, the root cause is actually a refrigerant leak. Here’s why refrigerant leaks cause water issues:

When refrigerant is low, the evaporator coil gets abnormally cold — cold enough to freeze the moisture condensing on it instead of letting it drain away as liquid. This creates an ice buildup on the coil that blocks airflow and reduces cooling. When the system cycles off (or when you turn it off), the ice melts rapidly and can overwhelm the drain pan, causing overflow and water dripping inside the home.

Signs that your water issue is actually a refrigerant leak:

• Ice visible on the outdoor unit or refrigerant lines (the large insulated copper line)
• Poor cooling performance — the AC runs but the house isn’t getting cold
• Water dripping inside only when the system cycles off (melting ice)
• Hissing or bubbling sound from the refrigerant lines
• Warm air from supply registers despite the system running

Denver Monsoon Season (July–August): When Normal Dripping Increases

Denver homeowners are sometimes confused when their AC starts dripping more than usual in late July. The culprit is almost always Denver’s monsoon season — a period of increased humidity as moisture flows north from the Gulf of Mexico.

The combination of Denver’s 5,280-foot altitude and monsoon moisture creates unique conditions: the dry air coming down from the mountains meets the humid air tracking north, creating rapid humidity swings. Your AC system that handled a 40% humidity day without issue may suddenly produce overwhelming condensate on a 70% humidity day.