Ice on the coil at 95°F.

BLUF: switch the thermostat to fan-only, let the coil thaw 2 to 4 hours, replace the air filter. If ice returns within 24 hours the cause is not a dirty filter — it is refrigerant charge, a blower motor, or a restricted coil, and those need a licensed technician.

What an Evaporator Coil Actually Does

The evaporator coil is the indoor side of your AC system. Refrigerant enters the coil as a high-pressure liquid, passes through a metering device (TXV or fixed-orifice) that drops the pressure dramatically, and evaporates into a cold low-pressure vapor. The evaporation pulls heat out of the air passing across the coil surface — that is the cooling. Warm humid air enters, cold drier air exits, and water condenses on the coil fins to drain out through the condensate line.

The coil surface sits at roughly 32 to 42 degrees Fahrenheit during normal operation. That is cold enough to condense moisture but warm enough to let it drain as liquid water. Push the coil colder than 32°F and the moisture freezes on the fins instead of draining. Once ice forms, it insulates the coil from the airstream, reducing heat absorption and making the coil even colder. The freeze accelerates until the coil is a solid block of ice and airflow stops entirely.

Cause 1: Restricted Airflow (The Most Common)

Airflow across the coil controls coil temperature. Less airflow = colder coil. The engineered minimum is typically 350 to 400 CFM per ton. Below that, the coil drops below 32°F.

Airflow restrictions we see across the east corridor, in descending order of frequency:

• Dirty air filter. A neglected 1-inch filter can drop airflow 30 to 50 percent. This is the single most common freeze cause and the reason spring filter replacement matters.
• Dirty evaporator coil. Dust and biological matter accumulate on the coil fins over years. A coil that has never been cleaned can drop airflow 20 to 40 percent.
• Closed or blocked supply vents. Homeowners closing vents in unused rooms reduces total airflow. If more than 30 percent of vents are closed, the system can freeze.
• Collapsed or disconnected return duct. A return plenum that came loose in a crawlspace or attic cuts airflow dramatically.
• Failed or degraded blower motor. ECM motors degrade and lose output capacity. PSC motors fail suddenly. Either way, airflow drops.
• Undersized return grille. Common in 1990s and early 2000s builder-grade homes across Clay and Moody. The system was installed with inadequate return area and limps along until a hot summer pushes it past the line.

Cause 2: Low Refrigerant Charge

AC systems are factory-charged to a precise refrigerant quantity. When the charge drops below specification — almost always from a slow leak, not from normal operation — the low-pressure side of the system runs at lower pressure than designed. Lower pressure means lower boiling temperature, which means the coil runs colder. Push it below 32°F and ice forms.

Common leak points we find during diagnostic calls:

• Service valve Schrader cores — pinhole leaks develop over years at the valve stems.
• Brazed joints at the coil — factory brazing fails from vibration and thermal cycling.
• Evaporator coil itself — pinhole corrosion from formaldehyde-related formicary corrosion. Common on 8 to 15 year old systems.
• Line set rubbing — line sets that run against a duct or stud eventually rub through.
• Service port caps missing — we see this on systems that had a DIY recharge attempt. Caps should be tight with Teflon tape.

Per EPA Section 608 federal law, only certified technicians may handle refrigerants. DIY refrigerant addition is illegal and unsafe. A licensed technician with EPA 608 certification can find the leak with electronic leak detection, repair it, evacuate the system, and recharge to manufacturer specification.

Cause 3: Condensate Drain Failure

When the condensate drain clogs, water pools in the drain pan. Eventually the water rises high enough to reduce airflow across the bottom of the coil, or to trip the float safety switch that shuts the system down. Before the float switch trips, you often get partial freezes as airflow drops unevenly across the coil.

Alabama humidity loads drain lines fast. A biofilm builds up in the drain over 12 to 24 months and eventually blocks the flow. Vinegar treatment every 60 days during cooling season prevents this. Our AC Summer Failure Prevention Checklist covers the full drain maintenance schedule.

Cause 4: Low Outdoor Temperature

Less relevant in Alabama summer than in spring or fall, but worth noting: AC systems are designed to operate above about 65°F outdoor temperature. Running them below that — which Alabama homeowners occasionally do on cool May evenings — can freeze the coil. If the freeze happens on a 60°F May morning, the cause is likely ambient, not mechanical. Wait for warmer weather.

Cause 5: Thermostat Set Too Low

Setting the thermostat to 68°F in 95°F weather forces the system to run continuously. Continuous operation with high humidity accelerates condensate load and, if airflow is marginal, pushes the coil below 32°F. We recommend 72 to 74°F as a comfortable Alabama summer setpoint — lower than that forces the system into territory where marginal issues become freeze issues.

The Diagnostic Walk — What a Technician Does on the Call

1. Switch system to fan-only, let coil thaw if not already done.
2. Check filter — if restricted, replace and note.
3. Inspect coil visually — dirt accumulation and biological matter.
4. Check static pressure across the air handler — confirms blower and return adequacy.
5. Measure refrigerant superheat and subcooling — confirms charge is within manufacturer specification.
6. Check condensate drain flow and float switch function.
7. Inspect blower motor current draw and ECM output voltages.
8. Verify supply and return temperature split (should be 15 to 20°F for properly operating AC).

Within 30 to 60 minutes, a technician can typically identify which of the five causes triggered the freeze. The repair runs from free (replace filter, open closed vents) to $150 (coil cleaning) to $250 to $600 (refrigerant diagnosis, leak repair, and recharge) to $400 to $900 (blower motor replacement). See our emergency AC repair cost guide for the full breakdown.

When to Call

Call a licensed technician immediately if:

• Ice is visible on refrigerant lines outside the house.
• The coil re-freezes within 24 hours of thawing and filter replacement.
• The system has not had a professional tune-up in the last 18 months.
• You hear hissing or bubbling from the outdoor unit — likely refrigerant leak.
• Utility bill has spiked dramatically compared to last summer.

Our AC repair service dispatches 24/7 to Leeds, Moody, Pinson, Clay, and Springville. The diagnostic call is the first step toward a written estimate before any wrench turns.