What Causes an AC to Freeze in Summer
Ice forming on an AC system in 110°F summer heat sounds backwards. But it makes physical sense once you understand what is happening at the evaporator coil.
The evaporator coil (inside your home) absorbs heat by keeping the refrigerant inside it at a very low temperature, typically around 40-50°F. Warm air from your home passes over the coil, the refrigerant absorbs the heat, and the air exits cooled. If the coil gets too cold (below 32°F), the moisture in the air freezes on contact with the coil instead of draining away.
Two things cause the coil to get too cold: not enough warm air moving across it, or too much refrigerant pressure drop from a leak.
Cause 1: Restricted Airflow
If airflow is restricted, less warm air reaches the coil. The refrigerant inside the coil keeps absorbing what little heat it can, but without enough warm air to moderate the coil temperature, the coil gets colder and colder until it freezes.
Airflow restriction sources:
- Clogged air filter (most common)
- Closed or blocked supply/return vents
- Dirty evaporator coil
- Failing blower motor running below rated speed
- Collapsed or crushed flex duct
Cause 2: Low Refrigerant
When refrigerant is low due to a leak, the pressure in the evaporator coil drops. Lower pressure means a lower boiling point for the refrigerant, which means the coil gets colder than it should. The result is the same: coil temperatures drop below freezing and ice forms.
Low refrigerant always means there is a leak. Refrigerant does not evaporate or get used up normally. If you have a refrigerant issue, it needs to be found, repaired, and the system recharged. Adding refrigerant without fixing the leak is a temporary fix.
Why Las Vegas AC Systems Are Especially Prone to Coil Icing
Vegas systems run nearly year-round. That means more total filter hours, more coil exposure to dust and debris, and less downtime for the system to be inspected.
Las Vegas air carries fine particulate matter from desert dust and construction. These particles load air filters faster than in most cities. A filter that would last 60-90 days in a humid climate may need replacement every 30-45 days in Vegas during summer, when the system runs continuously.
Evaporator coils also accumulate dust in Vegas homes faster than average. A coil that looks clean to the eye can still have enough fine dust coating to meaningfully restrict heat transfer. Professional coil cleaning every 1-2 years prevents this.
What to Do When Your AC Freezes Up
Follow these steps in order.
- 1
Turn the system off at the thermostat
Switch the thermostat to OFF or switch to fan-only mode. Do not leave it in cooling mode. Running the system frozen forces the compressor to work under abnormal pressure conditions and accelerates compressor failure.
- 2
Switch the fan to ON to help thaw
Set the thermostat fan switch to ON (not AUTO). This runs the indoor blower without engaging the compressor. Warm room air moving across the frozen coil speeds the thaw. Leave it running until the ice is fully melted.
- 3
Check and replace the air filter
Locate your air filter (usually at the return air vent or in the air handler cabinet). Pull it out and inspect it. If it is grey, clogged, or you cannot see light through it, replace it with a fresh filter. A clogged filter is the most common cause of AC freezing.
- 4
Check all supply and return vents
Walk through the home and make sure every supply vent is open and unobstructed. Closed or blocked vents restrict total airflow through the system and can cause coil temperatures to drop below freezing.
- 5
Wait 2-4 hours for a full thaw
Give the system at least 2-4 hours before restarting in cooling mode. Check the indoor unit for signs of ice. If you see any remaining frost or ice on the refrigerant lines or coil, wait longer.
- 6
Restart and monitor
After the thaw, switch the thermostat back to COOL. Check the air coming from vents after 15-20 minutes. It should be noticeably cool. Watch the system for the first hour. If the lines start frosting again, turn it off and call a technician.
What Happens If You Keep Running a Frozen AC
Running a frozen system forces the compressor to work against abnormal refrigerant pressure. The compressor is designed to compress gas, not liquid. When refrigerant backs up in liquid form due to a frozen coil blocking flow, the compressor can receive liquid refrigerant, which causes what technicians call liquid slugging.
Liquid slugging damages internal compressor valves and can cause complete compressor failure. A compressor that might have lasted another 5 years can be destroyed in a single extended run on a frozen coil. Compressor replacement: $1,500-$2,800. Turning the system off when you notice freezing: free.
Call Immediately vs. Handle Yourself
| Situation | Action |
|---|---|
| First time freezing, filter was clogged | Handle yourself: thaw, change filter, restart |
| Froze again after fresh filter | Call a technician |
| Ice on outdoor unit or refrigerant lines | Call a technician |
| Grinding noise from outdoor unit while frozen | Call immediately, system off |
| Water pooling or dripping heavily indoors | Call a technician |
| Temperature above 105°F, vulnerable persons in home | Call for emergency service |
How to Prevent Future Freezing
- Change the air filter every 30-45 days in summer (shorter cycle than the package says for Vegas conditions).
- Keep all supply and return vents open, including in unused rooms.
- Schedule annual maintenance that includes evaporator coil inspection and cleaning.
- Keep the area around the indoor air handler clear of stored items that could block the return air intake.
- Have refrigerant levels checked if the system is 5+ years old and has never been inspected.
For more on what causes warm air after a freeze-thaw cycle, see our guide on why your AC is blowing warm air.