The indoor coil of an air conditioner normally runs at around 40°F. Warm house air flowing across it keeps it above freezing. When that balance tips (not enough air, not enough refrigerant, or air that is already cool) the coil drops below 32°F, moisture from the air freezes onto the fins, and the ice blocks even more air. Within hours the coil can become a solid block, and frost creeps out along the larger copper line toward the outdoor unit. You notice weak airflow, rooms that stop cooling and, later, water where the ice melts.
Thaw it the right way
- Switch cooling off at the thermostat.
- Run the fan alone. Set the fan to On so room air moves across the coil. Expect a few hours; heavy ice can take most of a day.
- Watch for meltwater. The drain pan can fill faster than the drain empties it. Lay towels around an indoor unit in a closet, and check the ceiling below an attic unit.
- Leave the ice alone. No scraping, no hair dryer, no heat gun. The fins and tubing are thin, and the wiring nearby is not made for heat.
- Replace the filter while you wait, and reopen any supply registers or returns that were closed or blocked.
When the coil and lines are dry, run cooling again and look at the copper lines after an hour. If frost is back, the cause is still there.
Why coils freeze
| Cause | How it freezes the coil | A clue you might notice |
|---|---|---|
| Clogged filter or blocked returns | Too little warm air across the coil | Grey filter, weak air at every register |
| Dirty evaporator coil | A mat of dust on the coil acts like a second clogged filter | Musty smell, ice that comes back fast |
| Blower trouble | Slow or stopped blower motor, failing capacitor, dirty or loose blower wheel | Weak or no air, a hum at the indoor unit |
| Low refrigerant | Lower pressure in the coil means a colder coil | Cooling fading over weeks, hissing, oily spots at fittings |
| Very low setpoint, running nonstop | The coil never gets a break to shed frost | Runs all night even on mild nights |
| Cooling on a cool day | Pressures fall when outdoor air is cool | Ice only after cool, foggy mornings |
That last row matters near the water. In Larkspur and Sausalito, summer mornings are often cool and grey, and a system left running with a low setpoint can ice up even when nothing is broken, because air conditioners are designed to cool against warm outdoor air. If ice shows up only after cool mornings and never on warm days, a thermostat schedule that lets the system rest until the afternoon may be all it needs.
Turn it off and call when
- Ice is on the copper lines at the outdoor unit. The freeze has spread well beyond the coil, and running the compressor in that state strains it.
- The coil freezes again within a day or two of a new filter.
- The indoor blower does not run at all, or hums without turning.
- Water is coming through a ceiling or spilling over the drain pan.
- The breaker trips when the system starts.
What a technician does
Airflow causes most frozen coils, so that is where we begin: filter, return size, blower speed and motor, blower capacitor, and the condition of the coil itself, which often needs a cleaning that can only be done with panels open. Then the refrigerant side. Pressures and temperatures show whether the charge is low, and low charge means a leak to find and repair; adding refrigerant without fixing the leak only restarts the clock. Our technicians are EPA-certified for refrigerant work, repairs use factory certified parts, and you see an itemized price before anything is replaced.
If the coil sits on a duct system with too few returns or crushed return ducts, the lasting fix is in the ductwork; see duct repair and our page on poor airflow. If the freeze-ups started with gradually weaker cooling, read refrigerant leaks as well.
Heat pumps: winter frost is a different story
Many homes cool with a heat pump, and in heating mode the outdoor coil works in reverse. On cold, damp mornings a thin layer of frost on that coil is normal; every so often the unit switches into defrost to melt it, and you may see steam and hear a whoosh as it does. What is not normal is a thick shell of ice that stays for hours, ice building on the top of the unit, or a fan that stops while the coil is iced. Those point to a defrost control, a sensor or the outdoor fan motor, all covered under heat pump services.
Keeping it from happening again
Check the filter monthly during the cooling season, keep furniture and drapes off return grilles, and avoid setting the thermostat far below what the house needs. A yearly AC tune-up cleans the coil, checks the blower and confirms the charge before the first warm spell.