What size AC do I need?

Size should come from an ACCA Manual J load calculation. Central ACs are sized in tons (1 ton = 12,000 Btu/h), and the right size depends on climate, insulation, windows, air leakage and ducts. DOE says one ton per 400 to 500 square feet is a poor rule; a very efficient house may need one ton per 800 to 1,000.

Tons and Btu

Air conditioner capacity is measured in Btu per hour or in tons. One ton is 12,000 Btu/h, so a 36,000 Btu/h unit is a 3-ton unit. Home central systems are usually sold in half-ton steps. Model numbers often include a capacity code, such as 24, 36 or 48, for thousands of Btu/h.

The capacity on the label is measured at fixed test conditions: 80°F indoors and 95°F outdoors. Your house will see different conditions, so a proper sizing uses the manufacturer’s performance data at your local design temperatures, not just the nominal tonnage.

For a heat pump, the heating load matters as much as the cooling load. In a cold climate it can be the larger of the two, and the size chosen decides how often the backup heat runs.

Why square-foot rules fail

DOE’s consumer guide is blunt: the common estimate of one ton for every 400 to 500 square feet “fails to take into account local climate and the efficiency of a house.” It notes one ton can condition 800 to 1,000 square feet of a very efficient house. Two houses of the same size can need very different systems because of:

• Climate: local design temperatures and humidity.
• Insulation levels in the attic and walls.
• Windows: area, direction, shading and glass type.
• Air leakage through the building shell.
• Ducts: whether they run through a hot attic and how tight they are.
• People and appliances inside the house.

“Same size as the old one” has the same problem. The old unit may have been oversized, and the house may have changed since, with new windows or more insulation.

Manual J, S and D

ACCA, the contractors’ association, publishes the three standard methods. The International Residential Code requires equipment to be sized with Manual S based on loads from Manual J or another approved method, and ducts to be designed with Manual D. How strictly this is checked depends on your jurisdiction.

Training material from the International Code Council lists the usual mistakes: picking equipment by its AHRI rated capacity instead of the manufacturer’s data at design conditions, rounding up to the next size, and pushing past the Manual S limits. Each one pushes toward a bigger unit.

Climate changes the target

In hot, humid areas, an air conditioner has two jobs: lowering temperature and removing moisture. Equipment that cools the air quickly but runs briefly leaves moisture behind. In hot, dry areas, the main job is sensible cooling on peak afternoons, which is why the Southwest also has an EER2 minimum. For heat pumps, Manual S uses different sizing limits depending on whether moisture control or heating performance matters most in your climate.

Signs of an oversized system

• Short cycling: the system runs only a few minutes, shuts off, and starts again soon after.
• The house feels cool but clammy.
• Uneven temperatures between rooms.
• Extra wear on the compressor and controls from frequent starts.

DOE warns that oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality. Undersized systems have the opposite problem: they run constantly and can’t hold the set temperature on the hottest days.

A hot room doesn’t always mean too little capacity. Leaky, crushed or undersized ducts can starve a room of air while the rest of the house is fine. ENERGY STAR says sealing and insulating ducts can improve a system’s efficiency by as much as 20%, and sometimes more, so it is worth checking the ducts before paying for a bigger unit.

What to ask the contractor

• Will you do a Manual J load calculation, and can I have a copy?
• What design temperatures, insulation levels and air leakage did you assume?
• What is the calculated cooling load, and what is this unit’s capacity at our design conditions?
• Why this size and not a half-ton smaller?
• Can the existing ducts carry the airflow this unit needs?
• Does our building department want the load calculation with the permit?

Questions

How many square feet does a 3-ton AC cool? There is no fixed answer. DOE says rules like one ton per 400 to 500 square feet ignore climate and efficiency, and an efficient house may need one ton per 800 to 1,000 square feet. A Manual J calculation gives the real number.

Is a bigger air conditioner better? No. An oversized unit short-cycles, removes less moisture, and can leave the house cool but clammy. DOE warns it can also lead to mold and poor indoor air quality.

Should I replace my AC with the same size? Not automatically. The old unit may have been oversized, and insulation or windows may have changed. A new load calculation settles it.

Is a load calculation required by code? The International Residential Code requires equipment sized with ACCA Manual S from Manual J loads or another approved method. Whether it is checked at permit time depends on your jurisdiction.

Sources

• DOE: Consumer Guide to Home Heating and Cooling (sizing) — https://www.energy.gov/sites/default/files/2021-08/ES-HomeHeatingandCooling_081221.pdf
• DOE: Energy Saver 101, home cooling — https://www.energy.gov/sites/prod/files/HomeCooling101-final.pdf
• ACCA: Manual J — https://www.acca.org/standards/technical-manuals/manual-j
• ACCA: Manual S — https://www.acca.org/standards/technical-manuals/manual-s
• ACCA: Manual D — https://www.acca.org/standards/technical-manuals/manual-d
• ICC: Residential HVAC design process (IRC M1401.3, M1601.1) — https://iccsafe.org/wp-content/uploads/Residential-HVAC-Design-Process.pdf
• DOE: 2023 central air conditioner standards guidance (EER2) — https://www1.eere.energy.gov/buildings/appliance_standards/pdfs/2023_CAC_Standards_FAQ_10-5-2022_Final.pdf
• ENERGY STAR: Heat and cool efficiently (duct sealing) — https://www.energystar.gov/saveathome/heating-cooling