Cause one: not enough air across the coil
The evaporator coil is designed to have a specific volume of warm indoor air moving across it. That airflow is what keeps the coil above freezing. Restrict it and the coil's temperature falls.
The usual culprits, in the order we find them:
- A clogged air filter — by far the most common.
- Supply vents closed or blocked by furniture, or too many rooms closed off.
- A dirty blower wheel, which quietly loses capacity over years.
- Collapsed, crushed, or disconnected flex duct in the attic.
- A failing blower motor or capacitor running the fan below its rated speed.
Cause two: low refrigerant charge
Refrigerant does not get consumed. A system low on charge has a leak, full stop. Low charge drops the pressure in the evaporator, which drops its boiling temperature below freezing, and the coil ices up.
This is why simply adding refrigerant is not a repair. It is a temporary measure that puts the same leak back on the clock, and on R-22 systems it is an expensive one. Finding and fixing the leak — or deciding the system is not worth it — is the actual decision point.
How to thaw it safely
Switch the system to OFF at the thermostat, then set the fan to ON. Running the blower without cooling pushes room-temperature air across the coil and melts the ice far faster than leaving everything off. Expect one to three hours for a fully iced coil.
Put towels down near the air handler. All of that ice becomes water, and it can easily exceed what the drain pan was built to handle at once.
Two things not to do
Do not chip, scrape, or pry at the ice. The evaporator coil is thin aluminum and copper, and puncturing it converts a service call into a coil replacement.
Do not keep running the system in cooling while it is frozen. Liquid refrigerant that fails to boil off in an iced coil can return to the compressor, and compressors do not compress liquid. That is how a cheap repair becomes a system replacement.
After it thaws
Put in a clean filter, open the vents, and restart cooling. If the system runs normally and holds temperature, the cause was airflow and you have likely solved it.
If it freezes again within a day or two, stop running it and have the charge checked properly. A refreeze after a filter change points at refrigerant or the blower, and both need measurement rather than guesswork.
Want it measured instead of guessed?
The readings that settle this — pressures, superheat and subcooling, amp draw, static pressure — take a visit. You get the price in writing before anything is replaced.
(904) 467-1100