Home › AC sizing
AC sizing and Manual J in Casselberry, Florida
Casselberry's median home has 1,442 sq ft of living area, which a real Manual J load calculation usually sizes at 2.5 to 3 tons - not the 4 tons a rule of thumb gives. In a climate driven by latent load, an oversized system short-cycles and leaves the house cold and humid.
Why bigger is the expensive mistake in Casselberry
The rule of thumb people use in their heads is one ton per 500 square feet. Applied to Casselberry’s median improved residential parcel — 1,442 sq ft of living area — that gives just under 3 tons. Applied generously it gives 4. A real Manual J load calculation on the same house usually lands on 2.5 or 3 tons, and the difference between 3 and 4 is the difference between a house that is comfortable and a house that is cold and clammy.
Here is why, and it is specific to this climate. Central Florida summer air carries sustained dewpoints in the seventies. A large share of the work an air conditioner does here is latent — condensing water out of the air — rather than sensible, which is dropping the temperature. Dehumidifying takes run time. An oversized system satisfies the thermostat quickly, shuts off, and never runs long enough to pull the water out. You get 74°F at 60% relative humidity, which feels worse than 76°F at 48% and grows things.
What a Manual J actually measures
| Input | Why it moves the answer in Casselberry |
|---|---|
| Conditioned floor area and ceiling height | Median 1,442 sq ft citywide, but the top-20 subdivisions range from 781 sq ft at Marbeya Club to 2,630 sq ft at Legacy Park. One city, a 3.4× spread. |
| Window area, orientation and glazing | Single-pane aluminium in the 1970s stock, code-compliant glazing in the post-2001 stock. Often the largest single line in the calculation. |
| Insulation levels | 1970s attic insulation that has settled is not what the original build specified. This is also what Duke will pay up to $800 to improve. |
| Duct location and leakage | Attic ducts in a vented attic reaching 130°F. In the 57.1% of the city built 1970–89 the trunk is often original. |
| Infiltration | A 1959 Heftler Homes house and a 2023 Grayson Square house are not the same envelope by any measure. |
| Internal gains and occupancy | Real people and real appliances, not a table default. |
Seminole County Property Appraiser CAMA extract (Parcels.csv, dated 2026-09-05). Improved residential parcels — single-family, townhome and condominium — in city tax district C1, City of Casselberry. n = 8,563.
Four questions that separate the quotes
What to ask for, and what a good answer looks like:
- “Can I see the load calculation?” A real one is a document with your address on it and the inputs listed. “It is a three ton, same as what is there” is not a load calculation, it is a transcription.
- “What is the existing external static pressure?” A number, in inches of water column, measured. If the ducts cannot pass the airflow, the tonnage on the sticker is fiction.
- “What is the sensible heat ratio of the proposed equipment?” In a latent-load climate this matters as much as the tonnage. A variable-capacity system that can run long at part load is often the better answer in a small Casselberry house than a larger single-stage one.
- “Is the existing system oversized?” If the old system was wrong, matching it repeats the mistake with a newer warranty on it.
Being sold a bigger system than the one you have?
Ask for the load calculation before you ask for the price. In this climate the smaller correct answer is usually the more comfortable one.
Phone: (689) 308-3675 · Monday-Friday, 8:00 AM - 5:00 PM
Where the facts on this page come from
Every figure above is either a primary-source document or a named third-party cost guide. All of it was checked in September 2026. Fee schedules, rebate amounts and market prices change without notice — if you are about to spend money on one of these numbers, open the source and check it.