Learning Center
Why Is My AC Freezing Up?
There are exactly two root causes of AC coil icing — and one of them you can diagnose yourself in 60 seconds.
If your AC is frozen right now: turn the thermostat to FAN ONLY and let it thaw before restarting. Do NOT run it in COOL mode with ice present — see the steps below.
Root Causes
The Two Causes of a Frozen AC
Cause 1: Restricted Airflow
The evaporator coil must have warm air flowing across it constantly to maintain the proper operating temperature. When airflow drops — from a clogged filter, closed supply vents, blocked return vents, or a failing blower motor — the coil gets too cold and the moisture in the air freezes onto its surface instead of draining away.
Specific causes include:
- • Dirty or severely clogged air filter (the #1 cause overall)
- • One or more supply or return vents blocked by furniture, rugs, or closed
- • Dirty evaporator coil (insulating the coil surface)
- • Dirty blower wheel reducing fan capacity
- • Collapsed or kinked ductwork restricting airflow to certain sections
- • Blower motor running below normal speed (capacitor, motor issue)
What to do:
Check and replace the filter first — always. If the filter is clean, check that all vents in the home are fully open and unobstructed. If those check out, call for service. Do NOT run the system in COOL mode while it's frozen.
Cause 2: Low Refrigerant
Refrigerant is the heat-transfer medium that flows through the evaporator coil. When the refrigerant level is low (due to a leak — refrigerant doesn't deplete without a leak), the pressure inside the evaporator coil drops below designed range. Lower pressure means lower temperature — cold enough to freeze moisture from the air onto the coil surface.
Specific causes include:
- • Refrigerant leak anywhere in the refrigerant circuit (most common location: evaporator coil, service valves, or line set connections)
- • Refrigerant was never properly charged at installation
- • Previous repair left the system undercharged
What to do:
Low refrigerant cannot be self-diagnosed at home — it requires pressure gauges and a licensed technician. If the filter is clean and the freeze-up recurs, low refrigerant is the likely cause. EPA regulations require finding and repairing the leak before recharging.
What to Do Right Now — Step by Step
Turn the thermostat OFF (or to FAN ONLY mode)
Do NOT turn it to HEAT. FAN ONLY allows the blower to continue running, which speeds thawing. Switching to HEAT mode on a heat pump when there's a refrigerant issue can cause additional problems.
Leave it for 2–4 hours to fully thaw
A severely frozen coil can take several hours to thaw completely. Attempting to restart before full thaw risks liquid slugging the compressor (see below).
Check and replace the air filter
Even if the filter doesn't look completely clogged — replace it. A partially clogged filter can cause icing, especially during peak demand days with restricted airflow.
Check all vents in the home
Make sure every supply and return vent is fully open and unobstructed. Closing vents in unused rooms is a common practice that actually causes airflow problems.
Restart carefully
Once fully thawed and with a fresh filter, restart the system in COOL mode. Monitor for 30–60 minutes. If it freezes again, or if it froze with a clean filter, call for service.
If it freezes again — call for service
Recurring freeze-up after a clean filter and clear vents almost always means low refrigerant. Do not continue running a system that freezes repeatedly — compressor damage is the likely outcome.
Frequently Asked Questions
System Freezing Repeatedly?
If your system froze with a clean filter and clear vents, call us. Low refrigerant requires a licensed technician — and EPA regulations require finding and fixing the leak before recharging. NC License #27258.