Ice on an air conditioner's indoor coil or refrigerant line stops it from cooling. Here is what causes freezing, how to thaw the system safely and which causes need a technician.
The evaporator coil sits inside the air handler or on top of the furnace. Refrigerant passes through it cold, and indoor air blows across it, giving up heat and moisture. The coil is meant to be cold but not frozen, and the moisture it removes should drip into a pan and drain away.
A coil freezes when it gets too cold or when too little warm air flows across it. Moisture then freezes on the surface, and a layer of ice builds up. The ice acts as insulation and blocks airflow, which makes the problem worse. The result is weak airflow, warm or lukewarm air and, eventually, water everywhere when the ice melts.
Look for these signs, and treat any of them as a reason to check the system.
Most causes fall into two groups: too little airflow and a refrigerant problem.
A pattern in the symptoms can hint at the cause. Ice that appears only after the system has been running for many hours, or only when the outdoor weather is cool, points toward airflow or operating conditions. Ice that returns quickly after thawing, even with a fresh filter and open vents, points toward refrigerant or a component fault.
Turn the cooling off at the thermostat first. Leave the fan setting on so the blower pushes room air over the coil and speeds up thawing. If the blower itself seems to be the problem, switch the system off completely instead.
Give it time. Thawing can take hours, depending on how thick the ice is. Place towels or a shallow pan under the unit, because meltwater can overflow the drain pan. Never chip, scrape or use a heat source on the ice, since coil tubing is thin and easy to damage.
While waiting, check the simple things: replace the filter, open vents and clear the area around returns. Make sure the condensate drain is not blocked, as described in our guide on why an AC blows warm air.
If the system has a heat pump, the situation differs in winter. Heat pumps form frost on the outdoor coil and have a defrost cycle to clear it, which is normal. Heavy ice that stays on the outdoor unit through repeated defrost cycles is a different issue and is worth a service request.
Once the ice has melted and the coil is dry, switch the thermostat back to cool and watch for a few hours. If the system cools normally and no ice returns, the cause was probably airflow, and the new filter or open vents solved it. If ice returns, stop running the compressor and request service. A system that refreezes has an underlying problem you will not fix by repeated thawing.
Keep notes on when the freeze-up happens, what the weather was, the filter's age and whether any vents were closed. These details help a technician narrow the cause. Take a photo of the ice, if you can, for the same reason.
Refreezing after you have addressed airflow is a strong sign of low refrigerant, a restriction or a component fault. A technician can measure the system's operating conditions, inspect the coil, check the blower and locate any leak. Adding refrigerant without finding the leak is not a repair, and handling refrigerant takes certification. Background on how refrigerant types are changing is in our refrigerant guide.
Running a frozen system can damage the compressor, which is the most expensive component. The AC repair and replacement page explains how Mr. Air approaches diagnosis, and the question of whether an old unit is worth repairing is discussed in how long an HVAC system lasts.
Prevention is mostly airflow. Change the filter on schedule, keep vents open, and do not close off many rooms to save energy, since that raises resistance in the system. A yearly coil inspection and cleaning is part of a tune-up, as listed in what is included in an HVAC tune-up.
Smart thermostats and some systems can alert you when the indoor coil temperature or run times look wrong. If yours offers that, pay attention to it, since an early warning can mean the difference between an inconvenience and a damaged compressor.
A dirty air filter or other airflow restriction is the most common. Low refrigerant is the other major cause.
No. Turn the cooling off and run only the fan to thaw it. Running the compressor can damage it.
It depends on how much ice there is. Allow several hours and keep towels or a pan ready for meltwater.
If airflow was the cause and you fixed it, probably not. If it freezes again, request service to look for a refrigerant or component problem.
Tell us what your system is doing and we will follow up with next steps.