It seems backwards, but an air conditioner can freeze on a warm day. You might see frost on the copper refrigerant line near the outdoor unit, ice on the indoor coil above your furnace, or water pooling around the furnace as ice melts. Meanwhile, the air from your vents feels weak or warm. Ice is a symptom, not the problem itself. It tells you something is keeping the indoor coil too cold.
How an air conditioner ends up frozen
Your air conditioner cools your home with an indoor coil, called the evaporator coil. Cold refrigerant flows through the coil, and your furnace blower pushes warm indoor air across it. The coil absorbs heat from that air and removes moisture, which drains away as water.
The coil is designed to stay just above freezing. If too little warm air moves across it, or the refrigerant pressure drops too low, the coil temperature falls below 32°F. The moisture on it turns to ice. That ice blocks even more airflow, so the problem builds on itself until the coil is a solid block.
Common causes
The Lennox homeowner guide to frozen AC units lists the most common causes. They fall into two groups: airflow and refrigerant.
Airflow problems
- A dirty or clogged air filter. This is the most common cause and the easiest to fix.
- Closed or blocked vents. Furniture, rugs or closed registers reduce the air moving through the system. Blocked return grilles have the largest effect.
- Blocked or damaged ductwork. A collapsed or disconnected duct restricts airflow.
- A dirty evaporator coil. Dust on the coil reduces airflow and its ability to absorb heat. The U.S. Department of Energy's air conditioner maintenance guide recommends checking the evaporator coil every year and cleaning it as needed.
- A blower problem. A failing blower motor, a bad capacitor or a worn belt can slow the fan.
Refrigerant and operating problems
- Low refrigerant from a leak. Refrigerant is not used up during normal operation. If the level is low, there is a leak. Low refrigerant lowers the pressure in the coil, and the coil gets too cold.
- Running the air conditioner in cool weather. Lennox advises against running your AC when it is below 60°F outside. Minnesota has many late-spring and early-fall days that start cool and warm up in the afternoon. If you leave the system set to cool overnight on those days, it can run in conditions it is not designed for.
What to do when you find ice
These steps are safe for most homeowners. They stop the freezing, protect the compressor and help you find simple causes.
- Turn off cooling at the thermostat. Running a frozen system can damage the compressor. Set the system to off.
- Set the fan to "On." With cooling off, running the blower moves room air across the coil and helps the ice melt faster.
- Let it thaw completely. A heavily iced coil can take several hours. Do not chip, scrape or use a heat gun or hair dryer on the coil or lines. You can puncture the coil or create an electrical hazard.
- Watch for water. Melting ice drains through the condensate line. Place towels around the furnace if water overflows, and check that the drain line or condensate pump is clear.
- Check the air filter. If it is dirty, replace it with the size and type your system uses.
- Open vents and clear returns. Make sure supply registers are open and that nothing blocks your return grilles.
- Restart and observe. After the ice is gone, turn cooling back on. Check the refrigerant line and airflow over the next few hours.
If the system freezes again after a new filter and open vents, the cause is something you cannot fix yourself.
What not to do yourself
- Do not add refrigerant. Under the EPA's Section 608 technician certification rules, refrigerant work requires a certified technician, and adding refrigerant without repairing the leak does not fix the problem.
- Do not open the outdoor unit's electrical panel or the furnace blower compartment while power is on. Capacitors can hold a dangerous charge even after power is shut off.
- Do not keep running the system while it is frozen. It can damage the compressor, which is one of the most costly parts to replace.
If you smell burning, see smoke or hear loud grinding, turn the system off at the thermostat and the breaker. If there is fire or smoke you cannot account for, get everyone out of the house and call 911.
When to call a professional
Call for service if any of the following is true:
- The coil or lines freeze again after you replace the filter and open the vents
- You hear a hissing or bubbling sound near the indoor or outdoor coil, which can point to a leak
- The blower does not run, or runs weakly, when the fan is set to "On"
- Water from the thaw is overflowing or damaging floors or walls
- The system is blowing warm air after it has fully thawed
A technician will measure refrigerant pressures, test for leaks, check the blower and clean the coil if it is dirty. Our air conditioning repair service can diagnose the cause. To lower the chance of this happening again, ask about a seasonal AC tune-up, which includes a coil and airflow check.
Common questions
Can I run my air conditioner while it is frozen?
No. Turn cooling off and run the fan only until the ice melts. Running a frozen system can damage the compressor.
How long does it take a frozen air conditioner to thaw?
It depends on how much ice has built up. A heavily iced coil can take several hours with the fan running.
Will a new air filter fix a frozen AC?
It will if a clogged filter was the cause. If the system freezes again with a clean filter and open vents, call a technician to check refrigerant and the blower.