Why is my AC freezing up?

Your evaporator coil gets cold as refrigerant absorbs heat from your indoor air. Normally a steady stream of warm return air keeps it above freezing. When something drops the coil temperature too far, or starves it of that warm airflow, the moisture condensing on it freezes into ice. That ice then insulates the coil so it can’t absorb heat, cooling fades to warm air, and when it finally thaws you can get water around the air handler. Two root causes explain nearly every freeze-up: airflow and refrigerant.

This is the most common cause, and the most preventable. Anything that reduces the warm air moving across the coil can let it ice over:

• A dirty or clogged air filter (the usual suspect).
• A dirty evaporator coil or a buildup inside the ductwork.
• Closed, blocked, or furniture-covered supply registers.
• A weak or failing blower, or crushed/leaky ducts.

Restore the airflow and you usually solve the freeze-up. Clean ductwork and a fresh filter are the foundation.

If airflow is fine, the next cause is a low refrigerant charge, which means a leak, because the system holds a sealed amount. Low pressure makes the coil run colder than designed, so it freezes even with good airflow. A system that keeps refreezing after you’ve changed the filter and opened the vents is signaling a leak. This isn’t a DIY fix: a technician has to find the leak, repair it, and recharge to the correct level.

If you find your AC iced over, here’s the safe sequence:

• Turn the AC OFF at the thermostat to stop making more ice.
• Set the fan to ON to push warm air over the coil and speed thawing, expect a few hours.
• Don’t chip or scrape the ice, you can puncture the coil.
• Replace the filter and open any closed or blocked registers.
• Mop up melt water so it doesn’t damage flooring.
• Once fully thawed, restart cooling. If it refreezes, shut it off and call, it’s likely refrigerant.

Running a frozen system can let liquid refrigerant reach the compressor and damage it, the most expensive part of the system, so don’t force it to keep cooling on ice.

Most freeze-ups are an airflow story, which makes them preventable: change the filter on schedule, keep supply and return vents open, and have the coils and ducts cleaned so air moves the way the system was designed to. An annual tune-up checks the refrigerant charge and airflow together, catching a small leak or a dirty coil before it leaves you without cooling on the hottest day of the year.