AC Frozen? What Causes Ice on the Coil and What to Do

It seems backwards: ice forming on an air conditioner in the middle of a hot Southern summer. But a frozen evaporator coil is a common problem, and it's a clear signal that something isn't right. Here's what causes it, what to do right away, and when to call a technician.

How to tell if your AC is frozen

• Frost or ice on the larger copper refrigerant line near the outdoor unit or at the indoor air handler.
• Weak or no airflow from the vents, even though the system is running.
• Air that feels only slightly cool.
• Water around the indoor unit as ice melts.
• A hissing or bubbling sound (which can point to a refrigerant leak).

What to do right now

1. Turn cooling off at the thermostat. Running a frozen system can damage the compressor.
2. Set the fan to On. Running just the blower helps thaw the coil faster. Some technicians prefer the system fully off if there's a lot of water risk; either way, don't run cooling.
3. Protect the area. Melting ice can overflow the drain pan. Put towels down and check that the drain line isn't clogged.
4. Wait. Thawing can take a few hours or longer. Don't chip at ice or use a heat gun; the fins and tubing are easily damaged.
5. Check the filter while you wait, and replace it if dirty.

The common causes

Restricted airflow

The indoor coil needs a steady stream of warm air passing over it. If airflow drops, the coil gets colder and colder until moisture on it freezes. Common culprits are a dirty filter, closed or blocked supply vents, blocked return grilles, a dirty indoor coil, collapsed or disconnected ducts, or a failing blower motor.

Low refrigerant

When refrigerant is low because of a leak, pressure in the coil drops and so does its temperature, often below freezing. This needs a licensed technician to find and repair the leak; refrigerant handling requires EPA certification. Simply adding refrigerant without fixing the leak is a temporary patch.

Running in cool weather

Running the AC when it's cool outside, especially at night in spring or fall, or setting the thermostat very low, can sometimes cause icing in systems that aren't designed for it.

Drain problems

A clogged condensate drain can let water stand near the coil. Humid climates make drain clogs especially common.

Mechanical or control issues

A faulty blower relay, a fan that runs too slowly, or problems with the metering device can all contribute. These need a technician to diagnose.

After it thaws

Once the ice is gone and you've replaced the filter and opened vents, you can try cooling again. Watch it over the next several hours. If ice returns, the problem probably isn't just the filter. Turn the system off and call for service.

Preventing it next time

• Change or check filters monthly in peak season.
• Keep supply vents and return grilles open and unblocked.
• Don't set the thermostat dramatically low to cool faster; it doesn't work that way.
• Schedule annual maintenance, which includes checking refrigerant pressures, coils and airflow.

When to call a pro

Call if ice comes back after a thaw, you hear hissing, you see oily residue on refrigerant lines, the blower doesn't seem to move much air, or there's water damage. A licensed HVAC contractor can measure refrigerant pressures and airflow and pinpoint the cause.

Safety first

Some HVAC checks are safe for any homeowner: the thermostat, the air filter, a single breaker reset, and keeping the outdoor unit clear. Others are not. Leave refrigerant, capacitors, wiring inside the unit, and anything involving gas to a licensed professional. If you ever smell gas or see smoke or sparks, leave the home and call 911 or your gas utility from a safe place.

More guides

• Why Is My AC Blowing Warm Air? 8 Causes and What to Check
• AC Repair vs. Replace: How to Decide
• How Often Should You Change Your AC Filter?
• Heat Pump Basics for Southern Homeowners

This article is general information, not professional advice or a diagnosis. Always consult a licensed HVAC contractor about your specific system.