What “freezing up” actually means

Your AC has two coils — one outside (condenser, where heat leaves), one inside (evaporator, where heat is absorbed from your air). When the indoor coil drops below 32°F, water vapor from your air freezes on it. Ice insulates the coil, which means it can’t absorb heat anymore, which means cooling stops. Then the ice slowly melts, drips into the drain pan, and the cycle starts over.

So the question isn’t “why is there ice” but “why did the coil get below 32°F.” There are exactly 5 causes.

Cause 1 — Restricted airflow (90% of cases)

Without enough warm air moving across the evaporator, the refrigerant inside it doesn’t pick up enough heat — and gets way colder than design. The fix list:

• Clogged filter — replace it. Should be done every 2–3 months in FL.
• Dirty evaporator coil — needs professional cleaning, usually $250–$400.
• Closed registers or blocked returns — open vents, move furniture.
• Failing blower motor — listen at the air handler; reduced airflow with normal noise = motor capacitor weak. Tech repair, $250–$350.
• Crushed flex duct — common after roofers or pest control work in the attic.

Cause 2 — Low refrigerant from a leak (8%)

Less refrigerant means the coil pressure drops, which lowers boiling point, which means colder coil. You’ll often see frost on the larger copper line outside the unit too. Adding refrigerant is not the fix — refrigerant doesn’t get used up; if it’s low, it’s leaking, and the leak needs to be found and sealed before re-charging.

Cause 3 — Failing thermostatic expansion valve (1%)

The TXV controls how much refrigerant enters the indoor coil. When it sticks closed or partially closed, you get a similar effect to low refrigerant. A tech with gauges can diagnose in 15 minutes; replacement runs $400–$700.

Cause 4 — Outdoor temp too low for cooling mode (rare in FL)

Below 60°F outdoor, residential ACs in cooling mode lose pressure regulation and the coil can freeze. This almost never happens in South Florida outside of cold-snap weeks in January–February. Don’t run cooling mode below 60°F outdoor.

Cause 5 — Oversized system (chronic, not acute)

An AC that’s too big for the space short-cycles — runs hard for 8 minutes, shuts off, runs for 8 minutes. The coil never reaches steady-state temperature, and on humid days it can dip below freezing during the run cycle. The only fix is replacing the system with one sized properly via Manual J load calculation.

Emergency steps when you see ice

1. Switch thermostat from COOL to OFF immediately. Continuing to run it pushes ice into the blower and risks a $1,500 motor.
2. Set thermostat fan setting to ON (not AUTO). This circulates warm air across the coil and melts ice in 1–2 hours.
3. Replace the air filter while waiting.
4. Once melted, run cooling for 30 minutes and check the larger copper line outside. If frost returns, you’re leaking refrigerant — call us.

FAQ

Will running my AC with a frozen coil break it?
Yes — short term, you risk water overflow into the drain pan and ceiling damage; long term, ice can warp blower fins or stress the compressor.

How much does refrigerant leak repair cost?
Leak detection: $250–$400. Repair varies wildly — pinhole at a fitting: $300. Coil leak: $1,500–$2,500 (often replacement makes more sense). Reach the right diagnosis before the work; don’t accept “we’ll just top it off” as a fix.

How often should I have the indoor coil cleaned?
Every 2 years for most homes; annually if you have pets, dust, or live within 3 miles of the ocean. Included in our Comfort Plan.

Need it diagnosed today?

AtlantFlow runs same-day service across Palm Beach, Broward, and Miami-Dade. Licensed FL CAC1824422.

Call (561) 510-9414