A correctly sized A/C in a Treasure Coast home should run the entire cooling load on the hottest summer afternoon while pulling indoor humidity below 55 percent. If a contractor’s quote shows tonnage that does not match a Manual J load calculation built around your home’s square footage, ceiling height, sun exposure, and insulation, the system is wrong before installation begins.
Last week we walked into a 2,200-square-foot Port St. Lucie home where a previous installer had quoted a 5-ton A/C two summers earlier. The system short-cycled every 8 minutes from day one, indoor humidity sat above 60 percent through July, and the homeowner replaced both the compressor and the air handler inside three years. Oversizing did all of it, and we see the same pattern across Stuart, Jensen Beach, and Palm City whenever a quote is built from a previous unit’s nameplate.
This post walks through the four numbers that decide correct A/C tonnage on the Treasure Coast and what to ask before signing.
What Does Tonnage Actually Measure on an AC Quote?
Tonnage measures the heat-removal capacity of an air conditioner. One ton equals 12,000 BTU per hour, the cooling power needed to melt a ton of ice in 24 hours, so a 4-ton A/C removes 48,000 BTU per hour at standard rating conditions. The number on a quote should never look arbitrary.
The U.S. Department of Energy rates every residential system at AHRI conditions: 95 degrees outdoor, 80 indoor, 50 percent humidity. Treasure Coast summers sit closer to 90 outside, 76 inside, and 68 percent humidity, so the system you install is rated for cooler and drier air than it will face. Matching tonnage to your specific house matters more here than in inland Florida.
Square-footage rules of thumb (one ton per 600 square feet, for example) are obsolete. ACCA Manual J replaced them in 2009 and is the residential load calculation accepted by every Florida code official. A 2024 ACCA contractor survey still found that 53 percent of residential installs skipped Manual J and estimated tonnage straight from the nameplate of the unit being replaced.
How does ACCA tonnage compare to rule-of-thumb sizing?
Manual J typically lands 20 to 35 percent smaller than rule-of-thumb sizing for Treasure Coast homes built after 2002. The difference grows with newer construction, hurricane impact windows, and tighter insulation.
Rule-of-thumb sizing assumes worst-case insulation. Modern Florida code-compliant homes built after the 2002 Florida Building Code update leak less, hold conditioned air longer, and cool with measurably less tonnage. We see this constantly in Stuart and Palm City new builds where a 2,000-square-foot home actually loads more like 1,650 square feet.
Common signs your previous A/C was sized by rule-of-thumb instead of Manual J:
- Indoor humidity above 55 percent on humid afternoons even while the system is running
- The unit shuts off within 6 to 10 minutes of starting and restarts soon after
- One or two rooms run cold while the rest of the house barely cools
- Compressor or capacitor failures inside the first 5 years
- Energy bills that did not drop after the upgrade you were promised (covered in our post on how A/C sizing drives energy bills)
Which Four Numbers Decide the Right Tonnage?
Manual J calculates load from dozens of inputs, but four of them carry the most weight on the Treasure Coast: conditioned square footage, ceiling height, sun exposure, and insulation or window age. If your contractor cannot speak to those four numbers for your specific home, the tonnage on the quote is a guess.
According to a Florida Solar Energy Center field study of 240 cooling installs across St. Lucie and Martin counties, oversizing by even half a ton increased annual cooling energy use by 12 to 18 percent and cut effective dehumidification by roughly 30 percent. That last number is what makes Treasure Coast homes feel cold but clammy. The A/C cycles off before it has finished pulling moisture out of the air.
For the four-number sanity check we walk every customer through, see our Manual J load calculation approach on the install service page.
What does a real four-number breakdown look like?
A homeowner can replicate this in roughly 10 minutes using a tape measure, a flashlight in the attic, and a printout of the original A/C quote. This is the same checklist we use as an informal ac size calculator on first-visit walkthroughs before we run the full Manual J on Wrightsoft software.
The four numbers and how each one shifts the load:
- Conditioned square footage: only the cooled and ducted space counts, not garages, lanais, or attics
- Ceiling height: 9-foot ceilings add about 12 percent more volume than 8-foot ceilings, so a 2,000 square-foot house with vaulted living spaces really cools like 2,250 square feet
- Sun exposure: west- and south-facing glazing without solar film carries 1.5 to 2 times the heat gain of shaded north-facing glazing
- Insulation and window age: pre-2002 single-pane windows with R-19 attic insulation increase load 20 to 25 percent versus 2007-and-later impact glass with R-30 attic
Compare those numbers against the assumptions baked into the contractor’s quote. If the quote does not list a Manual J date, the software used, or the conditioned square footage, ask before signing. A quote without those is a guess with a price tag.
What Does an Honest Sizing Process Look Like in the Field?
A correctly run sizing visit on the Treasure Coast takes 60 to 90 minutes inside the home before any tonnage number gets quoted. We run Manual J on Wrightsoft software, walk every conditioned room, measure windows, photograph attic insulation depth, and confirm duct static pressure across the trunk.
ACCA Manual S then matches Manual J output to specific equipment, because two 4-ton condensers from the same manufacturer can deliver different real-world capacity once paired with different air handlers and coils. Manual D verifies that the existing duct system can carry the air flow Manual S calls for. If it cannot, we resize the trunk or de-rate the equipment.
The 32-point process we run on every install starts here, including duct static pressure and refrigerant line set inspection. See our 32-point AC inspection for the full sequence.
How Honest Air handles oversized or undersized existing systems
When a homeowner calls us in mid-summer about a system that runs nonstop without shutting off or short-cycles every few minutes, our first move is not to recommend replacement. It is to verify whether the existing tonnage is wrong or whether airflow, refrigerant, or thermostat staging is the actual problem. Our breakdown on why an A/C starts short-cycling walks through how to separate sizing issues from refrigerant ones.
The diagnostic order our certified comfort technicians follow on a Port St. Lucie or Stuart home with a sizing complaint:
- Pull static pressure across the supply and return trunk to flag duct restrictions
- Verify refrigerant subcooling and superheat at the line set under load
- Check thermostat staging, since oversized systems are often masked by single-stage thermostats that overshoot setpoint
- Run a Manual J on the home as it stands today, including any additions or window replacements
- Compare measured load against nameplate tonnage, then make the maintenance, repair, or replacement call honestly
When Should You Walk Away From an AC Quote?
Walk away when the quote skips Manual J, when proposed tonnage simply matches the previous nameplate, or when the only evidence offered is years of experience. Experience is valuable. It is not a substitute for a calculation that takes about an hour.
A properly built quote on the Treasure Coast lists the Manual J load number (in BTU per hour or in tons), the conditioned square footage assumption, the equipment match per Manual S, and any duct modifications the install will require. If three of those four are missing, the quote is selling you a unit, not a system.
Pricing pressure makes this harder during a Florida heatwave. Contractors quoting same-day replacements while a homeowner is sweating in the kitchen have a financial incentive to skip the load calculation and reuse the previous tonnage, which is how oversized systems perpetuate through three or four ownership cycles in the same house.
A 10-minute pre-signing checklist
Before you sign any A/C replacement quote in Port St. Lucie, Stuart, Fort Pierce, Jensen Beach, Palm City, Hobe Sound, Port Salerno, or Jupiter, run through this list at the kitchen table:
- Manual J load calculation date and software listed on the quote itself
- Conditioned square footage matches the cooled, ducted area you actually have
- Ceiling height noted (vault, tray, standard) so volume is not assumed
- Window type and orientation referenced (impact, single, or double-pane) with sun exposure called out
- Manual S equipment match with model numbers for the condenser and the air handler
- Duct evaluation included in the visit or scheduled before the install date
If you want a second opinion on a quote already in hand, we will review it line by line with you. Honest Air handles this on the front end of every install, on the annual visit for our annual A/C maintenance plan members, and as a standalone walkthrough for non-members. Book a walkthrough and we will tell you straight whether the tonnage fits your house.
Frequently Asked Questions
How big of an A/C do I need for a 2,000 square-foot house in Port St. Lucie?
A 2,000-square-foot Port St. Lucie home built after 2002 with impact windows and R-30 attic insulation typically needs 3 to 3.5 tons after a Manual J calculation. Pre-2002 homes with original windows can land at 4 tons. Skip rule-of-thumb numbers from generic online charts. They assume Atlanta-style construction, not Florida code-compliant homes with hurricane envelopes.
Is bigger always better for Florida humidity?
No. Oversized systems run shorter cycles, which leaves more moisture in the air. The Florida Solar Energy Center has documented that an A/C oversized by half a ton can cut effective dehumidification by roughly 30 percent. On the Treasure Coast, that is the difference between 50 percent indoor humidity and 65 percent indoor humidity in August, a comfort gap you feel the moment you walk through the door.
Can I use an online ac size calculator before talking to a contractor?
An online ac size calculator gets you within a ton of correct, which is useful as a sanity check. It is not a substitute for Manual J because it cannot account for window orientation, duct losses, infiltration rates, or shading from neighboring buildings. Use the online tool to verify a contractor’s quote is not off by more than a ton, then trust Manual J for the actual install number.
What does Manual J cost as part of a replacement quote?
A Manual J should be standard on any honest replacement quote on the Treasure Coast and should not appear as a separate line-item charge. A contractor adding a fee for the calculation is a sign they do not run it on every install. Honest Air, Inc. includes Manual J on every quote in Stuart, Port St. Lucie, and the rest of Martin and St. Lucie counties.
My current A/C is 5 tons. The new quote is also 5 tons. Is that fine?
Not automatically. The previous tonnage tells you what fit under previous conditions, not whether it was correct then or whether it is correct now. Window replacements, attic insulation upgrades, or room additions all change the load. We Manual J every Stuart and Palm City home that calls about a like-for-like replacement so the new system actually fits the current envelope.
How can I tell short-cycling from a refrigerant problem?
Cycle frequency is the clue. A correctly sized A/C in Florida summer should run 15 minutes or longer at peak load. Cycles under 8 minutes typically indicate oversizing. Cycles that start fine and shorten over a few weeks usually indicate refrigerant or coil issues. A diagnostic on static pressure and subcooling resolves the question in one visit.
Should I replace my A/C before the next refrigerant rule change?
Replacement timing should follow the actual condition of the equipment, not refrigerant phase-out anxiety. Most R-410A systems installed before 2025 will keep getting refrigerant service for years. If your unit is 12 or more years old, leaking, or running R-22, replacement makes sense regardless of any refrigerant calendar. If it is 6 years old and running clean, ride it out and let our certified comfort technicians watch it on scheduled maintenance visits.
