AC Freezing Up — Ice on the Lines or the Indoor Coil in a Phoenix Summer
Ice on the copper line or the indoor coil at 110° outside means airflow is blocked or refrigerant is low. What to do right now, the causes in order, and what each repair costs.
Ice on an air conditioner in Tempe in August sounds wrong, and it is. The indoor coil runs below freezing when air isn't moving across it fast enough or there isn't enough refrigerant in the system. Moisture freezes on the coil, the ice blocks more air, and it builds until the house stops cooling.
What to do right now
- Thermostat to Off. Fan to On. The blower moves warm house air across the coil and melts it in an hour or two without running the compressor.
- Put towels or a pan under the indoor unit. That ice is going to be water. See AC leaking water.
- Check the filter. If it's gray and bowed, that may be the whole problem.
- Don't chip the ice. The coil fins and copper are thin.
Once it's thawed and the filter is clean, you can try cooling again. If it freezes again within a day, it isn't the filter.
Causes, most to least common
- Dirty filter. The cheapest fix in HVAC. A clogged filter cuts airflow across the coil. Filter guide for Phoenix dust.
- Low refrigerant from a leak. Low charge drops coil pressure and temperature. A system doesn't "use up" refrigerant; if it's low, it's leaking. We find the leak. We don't just top it off.
- Blower motor or capacitor failing. A blower running slow or not at all freezes the coil fast.
- Dirty indoor coil. Years of dust past a loose filter rack coat the coil and act like a second clogged filter.
- Closed or blocked vents and returns. Closing too many registers in unused rooms starves the coil.
- Thermostat set very low on a humid night. Running at 65° all night in monsoon humidity can ice a marginal system.
What the fixes cost
| New filter | $5–$40 | You can do this one |
| Blower capacitor | $150–$400 | Same-visit fix |
| Blower motor | $450–$1,200 | ECM variable-speed motors are the high end |
| Indoor coil cleaning | $250–$600 | Depends on access; attic units cost more |
| Refrigerant leak search, repair and recharge | $500–$1,500 | R-410A. R-22 systems: different math |
| Evaporator coil replacement | $1,200–$2,500 | Usually a repair-or-replace conversation |
Why running it frozen is expensive
When the coil is a block of ice, liquid refrigerant can make it back to the compressor. Compressors pump gas, not liquid. Repeat that for a few days and the $300 problem becomes a $1,500 to $3,000 compressor. Shut it off when you see ice.
Heat pumps in winter are different
Light frost on the outdoor coil of a heat pump on a cold morning is normal; the defrost cycle clears it. A solid block of ice outside that's still there at noon is a defrost problem. See heat pump repair.
Questions people ask
Why is my AC freezing up when it's 110° outside?+
The indoor coil gets too cold because air isn't moving across it (dirty filter, failing blower, dirty coil, closed vents) or the system is low on refrigerant from a leak. Outdoor temperature doesn't prevent it.
How long does it take a frozen AC to thaw?+
With the thermostat off and the fan on, usually one to three hours. Overnight if the ice is heavy.
Can I add refrigerant myself?+
No. Handling refrigerant requires EPA 608 certification, and adding it without fixing the leak means it leaks out again.
Will a frozen AC fix itself?+
It will thaw. It won't fix itself. If a clean filter doesn't stop it, something else is wrong.