Texas summer hits 100°F outside, and your AC unit has a block of ice on it. It is one of the most counterintuitive things in HVAC — but it is also one of the most common service calls we run from May through September. Here are the five things that cause an air conditioner to freeze up, how to tell which one is happening to you, and what to do in the next hour to keep it from getting worse.
First — Turn It Off
The instinct is to leave it running because the house is getting warm. Don't. Running a frozen AC pushes ice further up the lines, can damage the compressor, and turns a $250 repair into a $2,000 one.
Set the thermostat to OFF (not just higher). Then switch the fan setting to ON. The fan keeps blowing room-temperature air over the frozen evaporator coil and thaws it out faster — usually 1–3 hours. While that is happening, you can diagnose the cause.
How to Spot a Frozen AC
There are two places ice forms:
On the indoor unit (evaporator coil): This is the unit inside the house — usually in the attic, a closet, or a garage. You will see frost or ice on the copper refrigerant lines coming out of the unit, on the metal coil itself if you can see it, and sometimes water dripping or pooling under the unit as the ice melts.
On the outdoor unit (refrigerant lines or condenser): The large copper line going into the outdoor unit will have a sweater of frost or actual ice on it. Sometimes the entire condenser fins are iced over.
Where the ice forms hints at the cause. Indoor coil icing is usually airflow or a dirty coil. Outdoor line icing is usually low refrigerant or a thermostatic issue.
The 5 Causes — And How to Tell Which Is Yours
### 1. Dirty or Clogged Air Filter (Most Common — Easy Fix)
When the air filter is clogged with dust and pet hair, airflow across the evaporator coil drops. With less warm air moving across the coil, the coil temperature drops below freezing, and condensation that would normally drip away freezes instead. Once it starts, it cascades — ice insulates the coil, airflow drops further, and the whole thing locks up in a few hours.
How to tell: Pull the filter and look at it. If you cannot see light through it, that is your cause. If you cannot remember the last time you changed it, that is also your cause.
The fix: New filter. Texas summer means changing 1" filters every 30 days, not every 90 like the box says. We are kicking up dust and pollen all summer. After replacing the filter, leave the AC off and the fan on until all ice is melted (you will hear water dripping into the drain pan), then run normally and watch for 24 hours.
### 2. Blocked Return Air or Closed Vents
Closing the vents in unused rooms feels like an energy-saving move. It is not. AC systems are sized to push air through every vent. Closing vents raises the pressure across the coil, drops airflow, and freezes the coil — the same way a dirty filter does.
How to tell: Walk through the house. Are any vents fully closed? Is furniture or a rug covering a return air grille? Is the door to the room with the air handler closed (which can starve the unit of return air)?
The fix: Open every supply vent. Move anything blocking returns. If you have rooms you genuinely do not want to cool, talk to an HVAC tech about a zoned system instead — closing vents on a single-zone system will keep freezing the coil.
### 3. Low Refrigerant (Refrigerant Leak)
Refrigerant is what absorbs heat from the indoor air and releases it outside. The system is supposed to be a closed loop — refrigerant doesn't get used up. If it is low, there is a leak somewhere. With less refrigerant in the lines, the pressure drops, the boiling point of the refrigerant drops, and the evaporator coil gets colder than it should — below freezing — and the moisture in your indoor air freezes onto it.
How to tell: The system has been running fine for years and suddenly cools poorly. Outdoor copper line is iced over. Hissing or bubbling sounds near the indoor unit. The system runs constantly and never reaches the thermostat setpoint.
The fix: This is not a DIY fix. Refrigerant requires EPA certification to handle, and "just adding more" without finding the leak means you will be in the same place in 6 months. A licensed HVAC tech will pressure-test the system, find the leak (usually at the evaporator coil or a fitting), repair or replace the leaking component, and recharge to the correct level. Cost runs $400–$1,500 depending on where the leak is.
### 4. Dirty Evaporator Coil
Even with a clean filter, dust and gunk eventually build up on the evaporator coil itself. This restricts airflow across the coil — same effect as a dirty filter. It is the slower-developing version of cause #1, and it is what we find on systems where the homeowner has been good about filters but has never had a tune-up.
How to tell: Filter is clean. Vents are open. Refrigerant is at the right charge. But the coil still freezes, and when you can see the coil it looks fuzzy or coated.
The fix: Professional coil cleaning. We do this as part of an annual tune-up. The tech will turn off the system, access the coil, and apply a coil cleaner that dissolves the buildup. Costs $200–$400 if it is the only thing being done; included in tune-up packages.
### 5. Thermostat or Blower Motor Problem
Less common, but if the blower motor is failing or running at the wrong speed, airflow drops and the coil freezes. Same with a thermostat that is calling for cooling when it should not, or that is keeping the system running past the setpoint.
How to tell: Air at the vents feels weak even with a clean filter. The blower fan sounds different from normal — slower, louder, or with a grinding noise. Or the system runs continuously even after the house is at temperature.
The fix: Diagnostic call. Blower motor capacitor replacement is around $150–$300; full blower motor replacement is $400–$800. Thermostat replacement is $150–$400.
What Happens If You Keep Running a Frozen AC
Three things, in order:
You stop cooling. The ice insulates the coil and indoor air can no longer transfer heat to the refrigerant. Vents blow air slightly cooler than room temperature — not the 18–22°F drop you should see across the coil.
You drown the indoor unit. When the ice melts, all that water has to go somewhere. The drain pan and condensate line are sized for normal condensation, not a melting block of ice. Overflow into your attic or closet is common, and we get water-damage drywall calls every summer because of it.
You damage the compressor. Liquid refrigerant — instead of vapor — gets pulled into the compressor when the coil is iced over. Compressors are designed to compress vapor, not liquid. "Slugging" damages valves and seals and is one of the leading causes of compressor failure on systems under 10 years old. Compressor replacement is $1,400–$2,500. Full system replacement, if the compressor failure is on an older system, is $5,000–$12,000.
What to Do This Hour
1. Turn the thermostat to OFF, set the fan to ON.
2. Replace the air filter if it is dirty.
3. Open all supply vents.
4. Wait 1–3 hours for the ice to melt fully — listen for water draining into the pan.
5. Restart the AC and watch it for 24 hours. If the coil refreezes, you have a refrigerant or coil-cleaning problem and you need a tech.
When to Call Us
If the AC is icing up repeatedly even after a filter change, or if you can hear hissing near the indoor unit (refrigerant leak indicator), or if the outdoor copper line is the part icing over — call us. Same-day service for AC freeze-ups across Pflugerville, Bastrop, Taylor, Elgin, Georgetown, Hutto, Manor, Round Rock, Cameron, Rockdale, Brenham, and the rest of our Central Texas service area. (737) 260-7255. Flat-rate pricing on diagnostics and repair. Texas license TACLB00027491E.