The right AC size for a Florida home is determined by a Manual J load calculation that accounts for square footage, ceiling height, insulation, window orientation, infiltration, and latent (humidity) load — which most rules-of-thumb ignore. An oversized AC in Florida cools the air faster than it can dehumidify it, leaving the home cold-and-clammy at 76°F with 65% relative humidity. The right answer is a properly sized variable-speed system, not a bigger fixed-stage one. As a rough sanity check, Florida homes typically run 400–600 sq ft per ton, depending on construction and orientation.
Why "bigger AC = more comfortable" is wrong in Florida
In dry climates, oversized AC is wasteful but not uncomfortable. In Florida, oversized AC is actively uncomfortable because of how humidity removal works:
- An air conditioner removes moisture only while running.
- An oversized unit reaches the thermostat setpoint quickly and shuts off.
- Short, frequent on-cycles dehumidify less than longer cycles.
- The result: cold air at a low temperature with a sticky, clammy 60–70% relative humidity — and potential for mold.
A correctly sized system runs longer cycles at lower output, removes more latent load (humidity), and holds the home at 75–76°F with 50–55% RH — the "shoulders relax" comfort range.
The Manual J calculation: what's actually in it
Manual J is the residential load calculation methodology published by ACCA. Key inputs:
- Sensible load (heat to remove from air): conduction through walls/roof, solar gain through windows, internal gains (people, lights, appliances).
- Latent load (moisture to remove): infiltration, occupancy, building construction.
- Orientation: a west-facing wall in a Florida home contributes 2–3× the load of a north-facing wall.
- Insulation R-values: actual installed R-values, not rated.
- Window U-values and SHGC: solar heat gain coefficient determines how much solar load enters.
- Infiltration rate: how leaky the building envelope is.
Most rule-of-thumb sizing (e.g., "1 ton per 500 sq ft") ignores all of this and routinely produces equipment that's 30–50% oversized for the actual load.
What variable-speed (inverter-driven) does differently
Modern variable-speed compressors can ramp from 25% to 110% of rated capacity, matching output to actual load. In Florida specifically, this matters because:
- Cycling frequency drops dramatically (longer runtime = better dehumidification)
- Part-load efficiency is dramatically better (most operating hours are at part load)
- Indoor humidity holds at 50–55% RH instead of bouncing 55–70%
For a typical 2,500 sq ft Florida home, a properly sized variable-speed system delivers measurably better comfort and lower power bills than an oversized fixed-stage system, even if the rated tonnage is smaller.
Common Florida sizing mistakes
- Adding a half-ton "for safety." A 4-ton system on a 3.5-ton load runs short cycles and dehumidifies poorly.
- Ignoring west-facing window load. A west-facing great room can account for 30%+ of total cooling load on a summer afternoon.
- Sizing to peak load only. Equipment runs at part-load 90% of the time. Optimizing for peak alone underweights the variable-speed advantage.
- Skipping the duct calculation (Manual D). A correctly sized unit attached to undersized ducts still under-performs and creates static pressure problems.
- Reusing existing tonnage on retrofit. If the original installer oversized it, replacing like-for-like perpetuates the problem.