An AC freezes up when the evaporator coil runs below 32 degrees while humid air passes over it. Four things push it there: too little airflow, a low refrigerant charge, a restriction in the refrigerant circuit, or an outdoor temperature below the unit's cooling minimum. The ice is the symptom. The readings name the cause.

Ice on an air conditioner looks like an emergency and is usually a simple story. It is also one of the more informative faults in residential HVAC, because the four conditions that produce it leave different fingerprints on the gauges. The ice tells you something is wrong. The numbers tell you what.

Why a Coil Makes Ice

In cooling, the evaporator coil is supposed to run below the dew point of the air passing through it and above freezing. That is how a system pulls water out of the air and sends it down a drain instead of collecting it on the fins. Coil temperature is not guessed at. It is read off suction pressure and converted to a saturation temperature. On R-410A, 32 degrees corresponds to about 101.6 psig at sea level, so a suction pressure parked under that number is a coil growing frost.

Once frost starts, it feeds itself. Ice closes up the fin spacing, airflow drops further, the coil gets colder still, and the ice spreads outward along the suction line toward the compressor. A system found iced at the outdoor unit froze at the evaporator first. It is also why ice shows up after the system has been running a while rather than in the first ten minutes.

Cause One: Not Enough Air Across the Coil

This is the common one. A coil is sized to absorb heat from a specific volume of air. Starve it and less heat arrives, the refrigerant does not fully boil off inside the coil, and coil temperature falls.

California's energy code sets the floor for verified airflow at 350 cfm per ton of nominal cooling capacity. The Department of Energy's review of residential HVAC installation field studies found that this is exactly where most systems fail. Low airflow turned up in more than half of systems in every region studied, with individual studies putting 50 to 93 percent of tested systems below the recommended range. In the New Jersey baseline study that review summarizes, 53 percent of measured airflows came in under 350 cfm per ton.

The restrictions are mundane and they stack: a loaded filter, a dirty blower wheel, a coil nobody has washed, closed or furniture-blocked registers, crushed flex duct, an undersized return. The same DOE review notes that filters and coils alone account for more than half of total system static pressure, which is why a filter that only looks a little grey still matters. It also found that filters rated MERV 11 and below carry a modest energy penalty, under 5 percent, and that four inch filters outperform one and two inch cartridges. Depth buys filtration without buying restriction.

Ducts belong in this list too. The DOE review found 67 to 100 percent of systems tested leaking 100 CFM25 or more to outside, and the New Jersey homes averaged 329 CFM25. A return pulling attic air is both an air quality problem and a capacity problem, and in a Los Angeles attic in September it is a large one.

Static pressure is the reading that separates a dirty filter from a duct problem, and it is one of the measurements we take on every cooling service visit rather than a number we reach for only after something breaks.

Cause Two: Low Charge or a Restriction

Refrigerant is not consumed. A system low on charge has a leak, which is why adding refrigerant without finding the leak is a bill you pay again next season.

Undercharge is common. The DOE review found 29 to 78 percent of systems undercharged depending on the study, and in the New Jersey baseline 68 percent of systems were not charged correctly: 47 percent under and 21 percent over. One study in that review, by Kim and Braun, put an undercharge of 12 to 19 percent at an average 12.87 percent loss of cooling capacity and 7.6 percent loss of efficiency. That is a system that still blows cold air and takes an hour longer than it should to reach setpoint.

A low charge ices a coil differently than low airflow does. There is not enough refrigerant to fill the circuit, so frost forms in the first passes and spreads as it blocks air, which tends to produce a partially iced coil rather than one solid block. A restriction, a plugged filter drier or a failing metering device, arrives at the same place from the other direction.

California requires third party HERS verification of refrigerant charge on new ducted air conditioners and heat pumps in Title 24 climate zones 2 and 8 through 15, which covers most of the inland Los Angeles area. A changeout that skipped it is worth asking about, because a system that was undercharged on day one has been losing capacity quietly ever since.

Cause Three: It Got Cool Outside

This is the cause that surprises people, and late October in the Valley is when we see it. Cooling equipment has a minimum outdoor operating temperature. Carrier's residential service manual states it plainly: the minimum operating temperature in cooling mode is 55 degrees outdoor ambient, and running below that calls for low ambient accessories, one of which is an evaporator freeze thermostat added for exactly this reason.

The mechanism is straightforward. A lower outdoor temperature means a lower condensing temperature and lower head pressure. Suction pressure follows head pressure down, and the coil follows suction pressure. A house that sat at 88 degrees at four in the afternoon and 56 degrees by ten at night, with the thermostat still calling for cooling, is a coil heading for 32 degrees with nothing wrong with the equipment at all.

Cause Four: The Blower Quit

A blower that stops while the compressor keeps running will freeze a coil in minutes. The usual culprits are a failed motor or run capacitor, a control board or relay that drops out once it warms up, or a thermostat wiring fault. The signature is intermittent: the system ices, thaws overnight, works for a day, ices again.

Intermittent faults are found with data rather than with one visit, which is why a blower suspect gets an amp draw log and a supply air temperature log across a full run cycle instead of a single reading taken while the technician is standing there.

What the Readings Separate

Three of these four causes look identical from the hallway and are distinct on the gauges.

Superheat alone does not settle it, and a correct superheat reading does not prove the coil is staying above freezing. Airflow, static pressure, superheat and subcooling have to agree before anyone connects a hose to a charging port. A coil also has to be fully thawed before any of those readings mean anything, which is the part that makes a freeze-up a two-visit problem more often than homeowners expect.

What To Do Before Anyone Arrives

Turn cooling off at the thermostat and set the fan to ON. Room air moving across the coil thaws it faster than shutting the system off entirely, usually in one to three hours, longer if the coil has become a solid block.

Do not chip at the ice. Evaporator fins bend and the tubing behind them punctures, which converts a service call into a coil replacement.

Put towels under the air handler. A coil frozen for a day holds a surprising amount of water, and it releases faster than a condensate pan sized for normal operation can drain. Water on a ceiling is the expensive part of a frozen coil, not the coil.

Pull the filter and look at it, and check that no supply registers have been closed off. A loaded filter is a strong hint, not a diagnosis. Then leave the cooling off. Running a system iced up to see whether it clears is how liquid refrigerant gets back to a compressor.

What This Looks Like on a Heat Pump

In heating, frost on the outdoor coil is normal. The outdoor coil runs well below outdoor air temperature in heating mode, so it collects frost, and the system clears it on a defrost cycle. A heat pump steaming in a Woodland Hills driveway on a 45 degree morning is working correctly. A unit whose outdoor coil never clears, or that defrosts every few minutes, has a defrost control or charge problem and is worth a call. Indoor coil ice in heating mode is a different matter and always means something is wrong.