AC Blows Cold, Then Warm, Then Cold Again

The easy explanation first

Check the fan setting. If your thermostat fan is set to ON rather than AUTO, the blower runs continuously — including after the compressor has satisfied the setpoint and shut off. During those stretches you get room-temperature air out of the vents, which feels exactly like the system “stopped cooling.”

Switch it to AUTO. In South Florida that is the right setting anyway: continuous fan re-evaporates condensed moisture off the coil back into the house, undoing the dehumidification you just paid for.

If it is on AUTO, the pattern matters

Note roughly how long each phase lasts. It genuinely narrows the cause:

• Cools well for 10–30 minutes, then warm for a long stretch, then cold again — classic evaporator coil freezing and thawing. The coil ices over, blocks airflow, the system stops cooling; the ice melts, and it works again.
• Cools briefly, warm for 5–15 minutes, repeating all afternoon — often the compressor overheating and tripping its internal overload, then restarting once it cools.
• Short bursts of cold, a few minutes apart, all day — short cycling, usually oversizing or a control problem.
• Gets worse as the afternoon gets hotter — points at a system running near its limit: dirty condenser, low charge, or capacity that no longer matches the load.

The freezing cycle, since it is the most common

A coil freezes for two reasons: not enough air across it, or not enough refrigerant in it. Restricted airflow — a loaded filter, a dirty blower wheel, closed vents — makes the coil colder than it should be until condensation on the fins turns to ice. Low refrigerant does the same thing by dropping the coil pressure.

You can break the cycle yourself: system OFF, fan ON for two to three hours to thaw, replace the filter, restart. If it freezes again within a day, stop treating the symptom. Refrigerant does not get used up — if it is low, it leaked, and topping it up without finding the leak wastes money and vents refrigerant.

The one that should not wait

If the warm phases come with the outdoor unit shutting down hard, a burning or hot-electrical smell, or a breaker that trips during the cycle, switch the system off at the breaker and call. Those combinations point at an electrical fault or a compressor in trouble, and running it through more cycles makes the outcome worse.

What to tell us when you call

Four things get us close before anyone arrives: how long the cold phase lasts, how long the warm phase lasts, whether there is visible ice on the copper lines or the indoor coil, and whether the outdoor fan is spinning during the warm phase. That last one alone separates several causes.