Frozen AC Evaporator Coil: Causes and What to Do Next

A frozen evaporator coil happens when the coil inside your air handler drops below 32°F and the moisture it pulls from the air freezes on its surface instead of dripping into the drain pan. The usual causes are low airflow, from a dirty filter, blocked vents or a dirty coil, and low refrigerant from a leak. To deal with it, turn cooling off, let the ice melt with the fan running, replace the filter, and call a qualified HVAC technician if the coil freezes again.

What does a frozen evaporator coil look like?

The evaporator coil sits inside the indoor unit, usually in a closet, garage or attic, so you may not see the ice directly. These are the signs homeowners tend to notice first:

• Weak airflow at the vents, even with the fan running

• Air that feels cool at first, then lukewarm

• Frost or ice on the larger, insulated copper line where it enters the outdoor unit

• Water dripping or pooling around the indoor unit, or a drain pan that overflows as ice melts

• A system that runs constantly without bringing the temperature down

In a humid climate, a coil can build a thick layer of ice quickly because there is so much moisture in the air passing over it. That is why a minor airflow problem that might go unnoticed somewhere drier can turn into a no-cooling call fast in a coastal Florida home.

Why does an AC evaporator coil freeze?

Low airflow

Your coil needs a steady stream of warm indoor air to stay above freezing. When airflow drops, the coil gets colder until moisture freezes on it. A clogged air filter is the simplest and one of the most common reasons, but closed supply vents, a blocked return grille, a dirty blower wheel, a failing blower motor or a crushed or disconnected duct can all starve the coil of air.

A dirty evaporator coil

Dust that slips past the filter sticks to the wet coil, and in Florida’s warm, damp conditions that film can grow into a mat of biological buildup. A dirty coil blocks airflow and insulates the metal, so it can’t absorb heat properly. Regular coil cleaning is part of keeping a system healthy here.

Low refrigerant from a leak

When refrigerant runs low, pressure in the coil drops, and so does its temperature. The coil falls below freezing and ice forms. Refrigerant is not consumed during normal operation, so unless the system was undercharged when it was installed, a low charge means there is a leak. Simply adding refrigerant, often called a recharge, treats the symptom. The lasting fix is to locate the leak, repair it and then charge the system correctly, which is work for a qualified HVAC technician. On coastal homes, salt pitting in the coil is a frequent source of these leaks.

Other causes

A thermostat set very low on a mild night, a blower motor or capacitor that runs the indoor fan too slowly, and certain control faults can also contribute. These are less common, which is another reason a freeze that keeps coming back deserves a proper diagnosis.

What should you do when your AC coil is frozen?

• Set the thermostat to Off so the compressor stops.

• Set the fan to On. Moving air across the coil helps the ice melt faster.

• Do not chip, scrape or pick at the ice. The fins and tubing are thin and easy to puncture.

• Check the air filter and replace it if it’s dirty.

• Open any closed vents and clear anything blocking the return grille.

• Put towels around the indoor unit and keep an eye on the drain pan as the ice melts.

• Wait until the coil is fully thawed, which can take several hours, before switching back to Cool.

When you restart the system, watch it for the rest of the day. If airflow is strong and the air is cold, a dirty filter was probably the problem. If ice returns, turn the system off again and call for service.

Is it safe to keep running an AC with a frozen coil?

No. Running a frozen system makes things worse in three ways. First, airflow keeps dropping, so the house gets warmer and more humid. Second, liquid refrigerant can return to the compressor instead of fully evaporating, which can damage the most important part of the system. Third, all that ice has to melt eventually, and a pan full of meltwater on top of a partly clogged drain line can overflow into your ceiling or floor. For the full list of no-cooling causes, see our guide to an AC running but not cooling.

How can you keep your evaporator coil from freezing again?

• Check the filter monthly during the cooling season and replace it on schedule.

• Keep supply vents open, even in rooms you don’t use, so the system moves the airflow it was designed for.

• Keep the condensate drain clear so meltwater and normal condensation have somewhere to go.

• Have the coil inspected and cleaned during routine maintenance.

• Ask about UV lights, which can help limit biological growth on the coil in humid homes, along with other indoor air quality options.

A yearly tune-up catches many of these issues before they freeze a coil. Our guide on how often to service your AC in Florida explains what that visit includes.

Call Mike or Joe about a frozen coil

If your coil keeps icing over, an airflow fault or a refrigerant leak is likely, and both need a proper diagnosis. Owners Mike Hanrahan and Joe Sisto check the airflow, inspect the coil and test for leaks, then explain what they found before any repair. Call Mike at (772) 285-6296 or Joe at (561) 895-0339, or request service from Mike and Joe online. For the bigger picture on protecting your system from salt, humidity and drain problems, read how to care for a Florida AC.