Running constantly is a symptom, not a personality
A healthy AC cycles: it cools to the set point, rests, and repeats. One that runs nonstop without hitting the target is telling you it's losing a race — it removes heat slower than heat comes in. Either its capacity dropped, or your house is letting heat in faster than it should.
Causes on the AC side
- Low refrigerant. The system still cools, just weakly. It'll run forever and never win. Needs a leak check, not just a top-off.
- Dirty coils — indoor or outdoor. Heat can't transfer through a blanket of grime. This is the most common finding on systems that "just got weaker over the years."
- A choked filter or crushed/leaky ducts. If cold air can't reach the rooms, the rooms never cool. Duct leaks in a hot Miami attic are brutal — you're paying to cool the attic.
- An aging compressor. Output falls off quietly, years before it fails outright.
Causes on the house side
Attic insulation that's flattened out, a west-facing wall of unshaded windows, or leaky door seals all raise the heat load. If your AC is also 10+ years old, the combination of a slightly weaker system and a slightly leakier house is exactly the "runs all day, never gets there" pattern.
The afternoon test
Note the indoor temperature at 9 AM and again at 4 PM with the thermostat set constant. If the system holds fine in the morning and loses ground only in the afternoon heat, it's a capacity problem (AC-side or house-side). If it can't reach the set point even at night, look harder at refrigerant, coils, and ducts.