What this symptom may mean
The indoor coil is meant to run cold, but just above freezing. When airflow drops or refrigerant runs low, the coil dips below freezing and moisture from humid air turns to frost, then solid ice.
Once ice forms it blocks even more air, so the problem feeds itself. You may notice frost on the larger copper line outside, water near the indoor unit, or vents that slowly lose airflow.
Possible causes
Clogged air filter
A filter loaded with dust and pollen is the most common reason a healthy system freezes.
Low refrigerant charge
A leak lowers coil pressure and temperature below freezing, and the leak has to be found and fixed.
Blower problems
A weak motor or dirty blower wheel reduces airflow across the coil.
Blocked returns or closed vents
Shutting too many registers, or blocking a return grille, starves the system of air.
Dirty indoor coil
Grime on the coil acts like insulation and encourages frost to build.
Safe checks you can make
- Set the thermostat to OFF. Running the fan alone on ON can speed thawing if water is draining normally.
- Allow several hours for the ice to melt completely.
- Replace the air filter while the system is off.
- Open every supply register and clear anything blocking return grilles.
- Keep towels near the indoor unit, and look at the condensate line outside to confirm meltwater is draining.
What not to do
- Don't chip, scrape or pry ice off the coil or copper lines. A punctured tube becomes a leak.
- Don't keep running cooling mode on a frozen system, which can send liquid refrigerant back to the compressor.
- Don't aim a hair dryer or heat gun at the coil or wiring.
- Don't add refrigerant yourself, even if a leak seems obvious.
When professional evaluation may be appropriate
If the coil freezes again within a day or two after a new filter and fully open vents, simple airflow is not the cause. Low refrigerant and blower faults need professional testing, and a leak must be located before any charge is added.
Call to be connected with an independent provider for refrigerant leak repair or general AC repair in the Montgomery, AL area.
- Ice returns after a full thaw and a new filter
- Oily residue around copper joints
- The blower sounds weak or whines
- Meltwater overflowing the drain pan
What an HVAC provider may inspect
- Refrigerant pressures, superheat and subcooling
- Leak detection at the coil, line set and outdoor unit
- Blower motor speed and wheel condition
Keeping it from happening again
Freezing is mostly an airflow problem until proven otherwise. Change filters on schedule, more often during heavy spring pollen, and avoid closing off unused rooms. A yearly check of refrigerant pressures and the blower helps catch a slow leak before it ices the coil on a humid July day.
