AC Tonnage and Sizing: What 3, 4 and 5 Tons Mean in Florida

What a 3, 4 or 5 ton unit is, and is not

A 3 ton unit is rated at 36,000 Btu per hour, a 4 ton unit at 48,000 and a 5 ton unit at 60,000. Those are capacity ratings, and they say nothing about how many square feet of floor a unit serves. A Building America guide hosted by the Department of Energy says equipment sizes typically come in half or one ton steps.

Why square feet per ton gives different answers

The Florida Solar Energy Center (FSEC) surveyed Florida air conditioning contractors about how they size systems for new homes. FSEC reported the results in a paper presented in 1996, so it describes older practice. It is the only Florida survey of contractor sizing methods that we found. Of the 489 contractors who answered, about a third used Manual J, about a third used software, and about a quarter used square footage. The paper warns that its sample probably leans toward contractors with more interest in sizing.

A square-foot rule gives one ton of capacity for each stated number of square feet of floor. The contractors who used one picked anything from 350 to 700 square feet per ton, with a weighted average of 502. By our arithmetic, an 1,800 square foot house comes to about 2.6 tons at 700 square feet per ton and about 5.1 tons at 350. The two ends of the range differ by a factor of two.

On their most recent job, the contractors often chose half-ton sizes, and many chose three tons regardless of the size of the house. The paper says that pattern suggests some contractors may not have been sizing at all. Some 38.5 percent of the contractors said they had at times oversized units on purpose, and the most popular reason was a customer request. The paper also cites a 1997 Florida study of over 400 homes, which found that more than half had cooling equipment sized above 120 percent of Manual J.

What the load calculation gives you, and what comes after it

A load calculation estimates how much heat one particular house gains on a hot day, in Btu per hour. ACCA, the Air Conditioning Contractors of America, calls Manual J the national standard for producing equipment sizing loads for single-family homes, townhouses, condominiums and manufactured homes. A University of Florida extension publication says the load must be calculated, not just estimated from square feet. The Department of Energy's Building Science Education site says rules of thumb are too often used to size comfort systems, which results in excessively oversized systems. The same page says loads in energy-efficient homes are often so small that they fall outside manufacturers' range of capacities. Our reading: for such a home, even the smallest unit offered may be larger than the load.

Manual J produces a load, and it does not pick a unit. Manual S is the companion for choosing equipment, and ACCA describes it as a guide for selecting and sizing residential heating, cooling, dehumidification and humidification equipment. Manual D, the third companion, gives duct sizing principles and calculations.

A load calculation assumes a design day, which is a hot outdoor temperature and an indoor temperature the system is meant to hold. Florida's energy code says the indoor design temperature for cooling must be at least 75°F. A Building America guide hosted by the Department of Energy says a properly sized system may only reach 76°F or 77°F, not 75°F, during the few hours hotter than the design temperature. Its example is a town in Virginia, where the design temperature is exceeded about 35 hours a year.

What Florida's codes say about sizing cooling equipment

Two parts of the Florida Building Code touch on sizing, and this page read the 8th Edition (2023) text of both. Section M1401.3 of the Florida Building Code, Residential, says equipment shall be sized by Manual S or another approved method, from loads calculated by Manual J or another approved method. One exception allows the next larger standard size when the manufacturer's published capacities cannot meet both the total and the sensible heat gains. Sensible heat is what a thermometer shows, and the latent load, also called the humidity load, is the moisture in the air.

The Florida Building Code, Energy Conservation, section R403.7.1, also calls for Manual S sizing from Manual J loads. It adds that the code does not allow designer safety factors or provisions for future expansion. Section R403.7.1.1 says cooling-only equipment must have a total capacity at least equal to the calculated total load and no more than 1.15 times that load, or be the closest available size. In plain terms, that is at most 15 percent above the calculated load, or the nearest size the manufacturer makes. It also says the equipment's latent capacity, its moisture removal, must not be less than the calculated latent load. Section R403.7.1.1 also has two exceptions for attached single- and multiple-family units. Equipment capacity may be less than the calculated total sensible load, but not less than 80 percent of that load. When signed and sealed by a Florida-registered engineer, equipment in those units may be sized in accordance with good design practice. For heat pumps, section R403.7.1.2.1 puts the same 1.15 limit on total cooling capacity.

Our reading, with a limit: section R503.1.2 lists the energy code sections that apply to new heating, cooling and duct systems in an alteration to an existing building. It names R403.1, R403.2, R403.3 and R403.6, and R403.7 is not on that list. Two other passages bear on that reading. Section R503.1 says alterations to any building shall comply with the requirements of the code for new construction. Section R403.7.1 uses the words "new or replacement" equipment, but in its sentence on efficiency ratings, not sizing. So the 1.15 limit may or may not bind a like-for-like replacement, while the Residential code's M1401.3 applies to equipment in general. This is not legal advice, and we did not check how any Tampa Bay building department applies either section to a replacement. Hillsborough County says that from December 31, 2026, permit submissions must meet the 9th Edition (2026) Florida Building Code, so section numbers and wording may change.

The size on the label is a rating

Section R403.7.1.1 calls the published AHRI total capacity a nominal, rating-test value that shall not be used for equipment sizing. AHRI is the industry group whose directory publishes certification documents for matched indoor and outdoor units. The code says the manufacturer's expanded performance data must be used instead. Expanded performance data are the maker's tables of what a unit delivers at different outdoor and indoor conditions. The code ties that selection to the outdoor design temperature from the load calculation and to the indoor design values. A split system is an outdoor unit plus an indoor unit. For one, the code says the maker and model number of both units must be submitted with their sensible and total cooling capacities at the design conditions. Our reading: a unit labeled 3 ton and what it delivers at your design conditions can differ, which is why the code asks for the tables.

A bigger ton needs more air

A Department of Energy guideline says air conditioners are typically designed for 350 to 400 cfm of air across the indoor coil for each ton. Cfm means cubic feet per minute, a measure of airflow. For humid climates, the same guideline recommends 350 cfm per ton. By our arithmetic, that is about 1,050 to 1,200 cfm for a 3 ton unit, 1,400 to 1,600 for a 4 ton and 1,750 to 2,000 for a 5 ton. Our reading: ducts built for a 3 ton unit may not carry the air a 4 ton unit is designed to move. As an illustration, not a measurement of any house, ducts that carry only 1,200 cfm would give a 4 ton unit 300 cfm per ton, below that range.

The same University of Florida extension publication says an oversized unit can overcome, from a room temperature standpoint, some issues. It lists leaking ductwork, poor AC maintenance, poorly insulated ducts, missing insulation and excessive infiltration, which is uncontrolled outdoor air coming into the home. Our reading: a bigger unit can hide a defect without fixing it.

Return air vent too small covers one common airflow problem. AC installation done right covers the airflow readings an installer can show you. Changing ducts, airflow settings or refrigerant charge is work for a licensed contractor.

What Tampa Bay permit offices publish

The Pinellas County Building Department's March 2021 Permitting Guide lists energy forms and HVAC sizing calculations among items that might apply to specific projects. That office serves unincorporated Pinellas County, Belleair Beach, Belleair Bluffs, Belleair Shore and Indian Rocks Beach, so check which office covers your address. The City of Largo says equal changeouts, which replace equipment like for like or size for size, are typically done over the counter with no plan review. It adds that any new installation has requirements, and plans or supporting documents may be required. Our reading: moving from a 3 ton to a 4 ton unit is not size for size, so ask the office what paperwork it wants. The City of Tampa's Construction Services FAQ lists an A/C change-out among many projects that do not require plan review. HVAC permits in Florida covers what a permit is and why it matters.

What to ask for in writing

These questions work for any contractor's proposal.

1. The load calculation report for this house, not only a tonnage, with the total, sensible and latent loads in Btu per hour.
2. The outdoor and indoor design temperatures the calculation used.
3. The model numbers of the indoor and outdoor units, and their sensible and total capacity at those design conditions from the manufacturer's data.
4. Why this size and not the next one down, and whether it is larger than the system it replaces.
5. If the unit is larger than the old one, what changes to the ducts and returns, and the airflow in cfm the system is set to deliver.
6. Whether the permit office wants a sizing calculation or energy forms for this job, and who submits them.
7. If the proposal adds capacity for safety or future expansion, the reason in writing.

Get each answer in the written proposal, because a spoken answer is hard to check later. Changing equipment involves refrigerant and live electrical parts, so leave that work to a licensed contractor.