Why square footage alone fails
Two 1,800 square foot Jacksonville homes can have genuinely different cooling loads. One is shaded by oaks with new double-pane windows and R-38 attic insulation. The other faces west with single-pane glass and R-11 insulation from 1978. Same square footage, substantially different load.
A rule of thumb cannot see any of that, so it defaults to guessing high. Guessing high is how most Florida systems end up oversized.
What a Manual J load calculation measures
Manual J is the industry-standard method for calculating what a specific house actually needs. A real one accounts for:
- Square footage, ceiling heights, and the floor plan's volume.
- Window area, glazing type, and which direction each faces.
- Insulation levels in the attic, walls, and floor.
- Measured or estimated duct leakage, and whether ducts run through unconditioned attic space.
- Air infiltration — how leaky the building envelope is.
- Occupancy and internal heat gains from appliances and lighting.
- Local design conditions for Northeast Florida, not a national average.
The symptoms of an oversized system
You can usually diagnose oversizing from the couch:
- Cycles that last only a few minutes, repeatedly, through the hottest part of the day.
- The house hits setpoint quickly but feels damp.
- Big temperature swings between rooms.
- Higher bills than neighbors with similar homes — short cycling is hard on compressors and inefficient.
What to ask for in a quote
Ask whether a Manual J was performed on your house, and ask to see the output. A contractor who sized your system by walking the house and counting rooms is guessing, and you are the one who lives with the guess.
A quote worth comparing lists the model numbers, the tonnage, the SEER2 rating, and what is and is not included. Hold two of those side by side and the differences become obvious. Hold two vague quotes side by side and you are just comparing prices on unknown equipment.
Want it measured instead of guessed?
The readings that settle this — pressures, superheat and subcooling, amp draw, static pressure — take a visit. You get the price in writing before anything is replaced.
(904) 467-1100