Frozen Evaporator Coil: What Causes It and When to Call

Your AC coil is frozen. Ice has built up on the indoor unit, airflow has dropped, and the system probably stopped cooling. In most cases you didn’t break anything, and there’s a safe way to thaw it while you figure out what to do next.

What actually causes a coil to freeze

The evaporator coil sits inside your air handler and gets very cold as refrigerant expands through it. Two things keep ice from forming: enough airflow across the coil, and enough refrigerant to absorb heat properly. Take away either one and the coil drops below freezing, moisture condenses on it, and you get ice.

Restricted airflow is the most common cause by a wide margin. A dirty or collapsed air filter is usually the first thing to check. The coil itself can get coated in dust over time (especially if you’ve run the system with a thin filter or none at all), and that layer of grime insulates the coil and cuts airflow just as much as a clogged filter. Closed or blocked supply vents, a failing blower motor, and collapsed flex duct follow the same logic.

Low refrigerant is the second most common cause. If the system is low, pressure in the evaporator drops, the coil gets colder than it should, and ice forms. Low refrigerant means there’s a leak somewhere. It doesn’t “run out” on its own; it escapes. This one you cannot fix yourself, and handling refrigerant without EPA 608 certification isn’t legal.

Running the AC in cold weather will also freeze the coil. If outdoor temps drop into the low 60s or below and the AC is still running, low-side pressure falls and the coil freezes. Not common in the Bay Area in summer, but it happens in spring and fall.

A dirty blower wheel, a stuck metering device, or a failing TXV (thermal expansion valve) can also cause freezing. Those are less frequent and require instruments to diagnose properly.

How to thaw it safely

Turn the system off at the thermostat first. Don’t leave the compressor running; if the problem is refrigerant-related, running it against a frozen coil can damage the compressor.

Switch to fan only after you’ve confirmed the compressor is off. This circulates warm air across the coil and speeds things up. Expect a few hours, sometimes longer, depending on how much ice built up.

Put towels around the base of the air handler and near the drain pan. A heavily frozen coil releases a surprising amount of water as it thaws, and the drain pan can overflow if the drain line is partially clogged.

While you wait, check the filter. If it’s gray and solid with dust, that’s probably your cause. Replace it with the correct size and MERV rating for your system (most residential units do fine with MERV 8; check the label on the unit if you’re unsure).

Once the coil is fully thawed (no ice visible, drain pan empty), turn the system back on and watch it for an hour. If it cools normally and stays that way, you likely had an airflow issue and replacing the filter fixed it. If it freezes again within a day or two, there’s an underlying problem and you need a tech.

What a technician actually does

When I send a tech out for a frozen coil call, the first thing they do is check static pressure across the coil with a manometer. That tells you right away whether airflow is the problem. Then they check suction line temperature and suction pressure to see where refrigerant charge sits. If pressure is low, they’ll leak-check the system with a detector and UV dye.

A coil that’s dirty enough to restrict airflow needs to be chemically cleaned: pull the access panel, apply coil cleaner, let it dwell, flush it out. It takes 1 to 2 hours and makes a real difference. A vacuum and a brush from the outside doesn’t do it.

If refrigerant is low, they find and fix the leak before adding any refrigerant. Adding refrigerant to a leaking system without repairing the leak first is a waste of money and, depending on the refrigerant type, an EPA violation.

When to call us

If the coil freezes again after you’ve replaced the filter and let it fully thaw, that’s your cue. It almost always means a refrigerant leak or a coil that needs a proper chemical cleaning. Neither is a homeowner repair.

Also call if the compressor is short-cycling (turning on and off every few minutes), if there’s water damage around the air handler, or if you’re not comfortable opening the unit yourself.

In the Bay Area, a frozen coil in June or July is usually a neglected filter or a slow refrigerant leak. Both are fixable, and neither means you need a new system.

Call us at (925) 999-4095 or schedule at bayareahvacservice.com. We cover the Tri-Valley and surrounding areas. We’ll get you on the schedule fast, often same or next day when we can.

Key takeaways

• Restricted airflow (dirty filter, blocked vents, dirty coil) is the most common cause of a frozen evaporator coil.
• Turn the system fully off, then switch to fan-only to thaw safely. Running the compressor on a frozen coil can damage it.
• If the coil refreezes after you've replaced the filter, you likely have a refrigerant leak that needs a licensed technician.
• Refrigerant work requires EPA 608 certification. It's not a DIY repair.
• A proper evaporator coil cleaning requires chemical treatment and takes 1 to 2 hours. Surface vacuuming doesn't fix it.

Related questions

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA 608 certification, and completed factory training at the Daikin/Goodman plant in Houston in 2025. He writes from direct field experience, not marketing copy.

Further reading

• Mitsubishi Hyper-Heat (H2i): How It Keeps Putting Out Heat When It Gets Cold in the Bay Area , Most people think heat pumps quit when it gets cold. Mitsubishi's Hyper-Heat (H2i) line is built to keep working as temperatures drop, which matters more here than folks expect, especially on cold mornings and up in the East Bay hills.
• Mitsubishi Mini-Split Blinking a Fault Light? What It Means and What to Do , A blinking light on your Mitsubishi indoor head is the system telling you something is wrong. Here is what the blink pattern means, the checks you can safely do yourself, and when to bring in a factory-trained tech.
• Why Your Mitsubishi Mini-Split Is Leaking Water Inside (and How to Fix It) , A Mitsubishi mini-split dripping water inside almost always comes down to condensate that can't drain. Here's what causes it, the safe checks you can do yourself, and when to call a tech.