AC That Never Shuts Off in Port St. Lucie, FL
During a Port St. Lucie August, long run times are normal. Continuous run times that never satisfy the thermostat are not. The first step in diagnosis is establishing which of the two is happening.
Normal versus abnormal
On a 93-degree afternoon with high humidity, a correctly sized system may run most of the hour. That is by design; oversized equipment short-cycles and leaves the house humid. The warning sign is a system that runs continuously and still drifts above setpoint, or one that runs continuously overnight when the load is much lower.
Common causes
- Lost capacity from low charge, a dirty condenser coil, or a restricted evaporator
- A thermostat in a hot spot, in direct sun, or reading inaccurately
- Thermostat fan left in the ON position, which makes the system seem never to stop
- Significant duct leakage into the attic
- A contactor welded closed, keeping the compressor energized
- Heat gain from missing insulation, aging window seals or added west-facing glass
What it costs
A system that runs continuously without reaching setpoint pays full power for partial comfort and accumulates compressor hours it should not. It is also a common precursor to a complete no-cool failure later in the season.
Frequently asked questions
- Is it cheaper to leave the AC on all day?
- In humid Florida, a modest setback while away is usually fine, but deep setbacks force long recovery runs and can leave humidity high. A few degrees is the practical middle ground.
- A new thermostat made it worse. Why?
- Incorrect wiring, the wrong equipment-type configuration, or an aggressive recovery setting can all cause continuous running after a thermostat change.
- Should the outdoor unit ever stop?
- Yes, once the setpoint is met. If it never stops, even overnight, it should be checked.