Why Is My AC Freezing Up? A California Homeowner's Guide

If you've discovered ice forming on your air conditioning unit, you're experiencing one of the most common HVAC problems in California. Understanding why AC freeze-ups occur helps you address the issue effectively.

Understanding AC Freeze-Ups

When your AC "freezes up," ice forms on the evaporator coil, refrigerant lines, or even the outdoor unit. This occurs when the coil temperature drops below 32°F while moisture in the air condenses and freezes on the cold surfaces.

Common Causes of AC Freeze-Up

1. Restricted Airflow

The most common cause of freeze-ups is insufficient airflow across the evaporator coil:

Dirty or clogged air filters (check monthly!)

Blocked or closed supply vents

Dirty evaporator coil

Collapsed or blocked ductwork

Malfunctioning blower motor

When airflow is restricted, the coil gets too cold because there's not enough warm air passing over it to absorb the cold.

2. Low Refrigerant Levels

Refrigerant (commonly called Freon) is the substance that absorbs heat from your home's air. When levels are low:

The remaining refrigerant expands more than normal

This causes excessive cooling of the evaporator coil

Ice begins forming on the coil

Low refrigerant always indicates a leak somewhere in the system—refrigerant doesn't "run out" like fuel.

3. Thermostat Issues

Running your AC when outdoor temperatures are too cool can cause freeze-ups:

Operating AC below 60°F outdoor temperature

Thermostat set too low (below 68°F)

Faulty thermostat reading incorrect temperatures

4. Mechanical Problems

Various component failures can lead to freeze-ups:

Malfunctioning expansion valve

Faulty metering device

Damaged blower motor

Collapsed suction line

What to Do When Your AC Freezes

Step 1: Turn Off the AC

Switch your thermostat from "Cool" to "Off" immediately. Running a frozen AC can damage the compressor.

Step 2: Keep the Fan Running

Set the fan to "On" to help thaw the ice faster by circulating room-temperature air over the coil.

Step 3: Check Your Filter

While the system thaws (2-4 hours), replace the air filter if it's dirty.

Step 4: Inspect Vents and Returns

Ensure all supply vents are open and unobstructed. Check that return air grilles aren't blocked by furniture or drapes.

Step 5: Test After Thawing

Once fully thawed, turn the system back on. If it freezes again, call a professional.

Bay Area Specific Considerations

California's diverse climate affects freeze-up risk:

**Coastal areas**: Higher humidity increases ice formation potential

**Inland valleys**: Temperature swings stress the system

**Older homes**: Ductwork issues more common in vintage properties

Preventing Future Freeze-Ups

Replace filters monthly during cooling season

Schedule annual professional maintenance

Keep vents open and unobstructed

Don't set thermostat below 68°F

Address refrigerant leaks promptly

If your AC freezes repeatedly, don't ignore it. Continued operation with freeze-up issues can cause compressor failure—an expensive repair that's often more cost-effective to replace than fix.