Why Air Conditioners Freeze Up in South Texas

Inside your indoor coil, refrigerant boils at a low temperature and absorbs heat from the air blowing across it. The coil surface normally sits in the low 40s — cold enough to pull moisture out of the air, warm enough that the condensate drips into the pan and drains away.

Two things can drive that surface below 32 degrees: not enough warm air moving across the coil, or not enough refrigerant to absorb the available heat. Once it drops below freezing, condensate turns to frost instead of draining. Frost blocks airflow, which makes the coil even colder, and the system ices over completely.

Humidity is the accelerator. In the Valley, indoor coils pull large volumes of water out of the air, so there is often plenty of moisture available to freeze the moment coil temperature drops.

Long run hours compound it. A system running twelve hours a day never gets an off cycle long enough to thaw a partially frosted coil, so a small problem becomes a solid block of ice by the next morning.

• A clogged filter — the most common cause by a wide margin
• A dirty evaporator coil that air cannot pass through
• A blower wheel coated with dust, moving far less air than rated
• A failing blower motor or ECM module running at reduced speed
• Closed, blocked, or covered supply registers
• Undersized or crushed return ducts, common in older retrofits
• Collapsed or disconnected flexible duct in the attic

• A leak at the evaporator coil, flare fittings, or service valves
• Undercharge left behind by a previous top-off without a leak repair
• A restricted metering device (TXV or piston) starving the coil
• A failing TXV sensing bulb that never opens far enough

• Set the thermostat to Off for cooling and the fan to On
• Allow two to four hours for a fully iced coil to melt
• Place towels near the air handler — melting ice produces a lot of water
• Never chip ice off the coil; fins and tubing damage easily
• Do not use a heat gun or hair dryer on the coil
• Replace the filter before restarting the system

A system that freezes twice has a cause that has not been corrected. Continuing to thaw and restart it risks liquid refrigerant returning to the compressor, which damages internal components and can end the equipment's life.

A technician measures static pressure and airflow first, then refrigerant pressures with superheat and subcooling. Those numbers separate an airflow problem from a charge problem in a few minutes.