Why Is My AC Freezing Up?
Your AC freezes up when the evaporator coil drops below 32F, usually caused by restricted airflow from a dirty filter, low refrigerant from a leak, a failing blower fan, or blocked return vents. Turn the AC off, set the fan to ON, and let the ice melt completely (2-4 hours). Replace the filter. If it freezes again after restarting, you need a professional.
THE DETAILS
A dirty air filter is the most common cause of AC freeze-ups. When the filter is clogged, not enough warm air flows over the evaporator coil. The coil temperature drops below freezing and moisture in the air freezes on contact.
Low refrigerant causes the pressure inside the coil to drop, which drops the temperature below freezing. This is always caused by a leak — refrigerant does not get used up. A tech needs to find and fix the leak.
A failing blower fan moves air too slowly over the coil. You might notice weak airflow from the vents even though the system sounds like it is running.
Blocked or closed return vents restrict airflow back to the system. Make sure all return vents (the larger grilles on walls or ceilings) are open and unblocked by furniture or curtains.
Running the AC when outdoor temperatures are below 60F can also cause freezing. This is less common in Pittsburgh summers but can happen on cool nights if the AC runs overnight.
🔧 WHEN TO CALL A PRO
If the AC freezes again after you replace the filter and ensure all vents are open, call a pro. Repeated freezing usually means a refrigerant leak or a mechanical problem with the blower.