Central AC System GuideA frozen evaporator coil turns a cooling system into an ice maker. The coil ices over when airflow across it drops or when the refrigerant charge is wrong, and once ice forms the unit stops cooling. Most freezes trace back to a dirty filter or blocked airflow, both of which a homeowner can address, while charge problems belong to a technician.
Thaw before you troubleshoot
Ice has to go first. Turning the cooling off while leaving the fan running melts the ice on the coil, and running the compressor against a frozen coil in the meantime risks damage.
Give the thaw time. A fully iced coil can take several hours to clear, and forcing the system back on before it drains only rebuilds the ice and hides the underlying cause.
Airflow is the usual culprit
Most freezes start with restricted airflow. A clogged filter starves the coil of the warm return air it needs, so the coil surface drops below freezing and condensation turns to ice. ENERGY STAR advises inspecting, cleaning, or changing the air filter once a month for exactly this reason.
The habit pays for itself. The Department of Energy reports that routinely replacing the air filter can lower an air conditioner's energy consumption by 5 to 15 percent, and the same clean airflow keeps the coil above freezing.
Dirty coils and weak blower airflow
A dirty coil freezes as readily as a dirty filter. ENERGY STAR advises cleaning the evaporator and condenser coils, noting that dirty coils reduce the system's ability to cool and force it to run longer.
The blower matters just as much. ENERGY STAR recommends cleaning and adjusting blower components to provide proper airflow, and it reports that airflow problems can reduce a system's efficiency by up to 15 percent. Closed registers and crushed ducts create the same restriction as a dirty coil.
When to call a professional
If airflow is clean and the coil still freezes, the charge is the suspect. Low refrigerant lowers coil pressure and temperature until the coil ices, and correcting the charge requires a certified technician.
ENERGY STAR advises checking the refrigerant level and adjusting if necessary, and it warns that improper levels reduce efficiency and shorten equipment life. Because a low charge means a leak, this is where the homeowner steps back and the professional steps in.