Why Is My AC Freezing Up? Causes, What to Do, and Is It Ruined?

Ice on an air conditioner seems backward in an Alabama summer, but it is one of the most common calls we get. A frozen AC stops cooling, and if it keeps running, it can damage the compressor. The good news: most freeze-ups are caught early and fixed without major parts.

What a frozen AC looks like

• Ice or frost on the larger copper line at the outdoor unit
• Ice on the indoor coil (often hidden inside the air handler)
• Weak airflow from the vents, and air that is not cold
• Water around the indoor unit or in the drain pan as ice melts

Why it happens

Your indoor coil runs cold on purpose. Warm house air blowing across it keeps it above freezing. When too little air moves across the coil, or the refrigerant pressure drops, the coil falls below 32 degrees and the moisture in the air freezes on it. The ice blocks more air, and the problem snowballs.

The most common causes

1. A dirty air filter. This is the number one cause. Less air across the coil means a colder coil.
2. Closed or blocked vents and returns. Closing a lot of vents, or furniture over a return grille, cuts airflow.
3. A dirty indoor coil. Dust and grime act like a second filter stuck to the coil.
4. Blower problems. A weak blower motor or failing capacitor moves less air.
5. Low refrigerant from a leak. Lower pressure means a colder coil. This needs a technician.
6. Running the AC on cool nights. Most systems are not designed to cool when it is below about 60 degrees outside. In spring and fall, North Alabama nights often drop that low.
7. Collapsed or disconnected ductwork. A crushed return duct in the attic can starve the system of air.

What to do right now

1. Set the thermostat from Cool to Off.
2. Set the fan to On. Moving air melts the ice faster.
3. Replace the air filter.
4. Put towels down near the indoor unit. If it is in the attic, check the drain pan under it. Melting ice adds a lot of water.
5. Wait until all the ice is gone. That often takes 1 to 3 hours, sometimes longer for heavy ice.
6. Turn cooling back on and watch it. If it freezes again within a day, call us.

Please do not chip or scrape the ice. The coil fins and tubes are thin and easy to puncture.

Is my AC ruined if it freezes up?

Usually not, if you shut it off soon. The risk is running it frozen for a long time. Liquid refrigerant can flow back to the compressor, which is built to pump vapor, not liquid. The compressor is the most expensive part of the system, so turning the AC off at the first sign of ice protects it.

At what temperature will an AC freeze?

The coil itself freezes when its surface drops below 32 degrees. That can happen on a hot day if airflow is poor or refrigerant is low. Running the AC when it is below about 60 degrees outside raises the risk, because the system cannot build normal pressure.

How to keep it from happening again

• Change the filter every 1 to 3 months during cooling season
• Keep supply vents open and return grilles clear
• Do not run cooling on cool nights; open a window or use the fan instead
• Get a yearly AC tune-up that includes a coil check, airflow check and refrigerant check

When to call

Call if the AC freezes again after you replaced the filter, if you hear hissing, if the outdoor unit is iced over, or if water is overflowing the drain pan. Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our AC repair service.

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