Furnace Dripping Water When AC Is On: What It Means and How to Fix It

Noticed a furnace dripping water when AC is on? Don’t panic—it’s one of the most common homeowner HVAC headaches. This usually isn’t your furnace “leaking” at all, but rather condensation from your air conditioner traveling through or around your furnace area. If ignored, though, it can cause rust, mold, or even electrical issues. 💧

Here’s the quick guide to why it happens, how to troubleshoot, and when to call in the pros.

Why does water drip from the furnace when AC is running?

When your AC cools warm air, it pulls moisture out of the air (humidity) and sends it down a condensate drain line. If that line is clogged, cracked, or improperly routed, the water often shows up as leaks around your furnace. That’s why you see water dripping from furnace when AC is on.

The furnace itself isn’t the culprit—it’s just sitting under the AC coil where condensation collects.

Wide angle view of a well-ventilated basement with a furnace

Common causes of a furnace dripping water

1. Clogged condensate drain line. Dirt, algae, or debris clogging the drain hose forces water to back up and overflow near your furnace.

2. Cracked or loose drain pan. The pan under the evaporator coil catches condensation. If it’s cracked or rusted, water escapes onto your furnace.

3. Improper installation or slope. If the drain line isn’t angled correctly, gravity can’t move the water away.

4. High-efficiency furnace issue. If you see a furnace exhaust pipe dripping water, that may be condensation from a high-efficiency furnace vent, not your AC.

5. Frozen evaporator coil. Restricted airflow or low refrigerant can freeze the coil, and when it melts, water overwhelms the drain system.

According to the U.S. Department of Energy, poor maintenance of HVAC systems is a leading cause of leaks, airflow problems, and reduced efficiency.

Step-by-step: how to fix furnace dripping water when AC is on

1. Shut off the system

Turn off both the AC and furnace at the thermostat and breaker for safety.

2. Check the drain line

Locate the condensate line—usually a small PVC pipe—and inspect for clogs. A wet/dry vacuum can often clear blockages. The Energy Star program recommends seasonal checks of drain lines to prevent leaks.

3. Inspect the drain pan

Look under the evaporator coil for cracks or rust. Replace if needed.

4. Verify line slope and connections

Make sure the drain hose slopes downward and is tightly fitted.

5. Watch for recurring water

If leaks return quickly, you may have a frozen coil or refrigerant problem—this requires professional repair.

A leak that comes back after basic cleaning usually signals a deeper issue—don’t ignore it. Small leaks today can mean big damage later.

When to call a professional

• Water pooling around your furnace continues after clearing the line

• You notice rust or electrical damage inside the furnace

• Your AC freezes up frequently

• You see persistent furnace exhaust pipe dripping water in cold weather

FAQ

Why is my furnace dripping water when AC is on?

Because condensation from your AC isn’t draining properly, often due to a clogged drain line or cracked drain pan.

What does it mean if my furnace exhaust pipe is dripping water?

That’s normal condensation in high-efficiency furnaces, but it should drain through a condensate line—not onto your floor.

Why is water dripping from furnace when AC is on during summer?

Likely a blocked drain line or damaged pan; clearing the line usually fixes it.

Can I fix this myself?

Yes, if it’s a simple clog in the condensate line. More complex issues like frozen coils or refrigerant problems need a pro.

Is water around the furnace dangerous?

It can be—water near electrical components can cause short circuits and long-term damage. Always address leaks quickly.

Eye-level view of a humidifier attached to a furnace

Final verdict: furnace dripping water when AC is on

If you see a furnace dripping water when AC is on, it’s almost always a condensate drainage problem, not a broken furnace. Clear the drain line, inspect the pan, and check for slope issues. If the problem persists—or if you notice exhaust pipe condensation—call a professional. Addressing it early prevents costly damage and keeps your HVAC running safely. ⚡