Does humid-subtropical climate change my HVAC sizing?

Atlanta's Koppen Cfa classification — humid subtropical — drives HVAC sizing in ways a Manual J calculation cannot ignore. Summer design conditions per ASHRAE 169 sit near 92 to 95 degrees dry bulb at 75 to 77 degrees coincident wet bulb, which means your equipment is dehumidifying nearly as hard as it is cooling for roughly four months a year. Oversized single-stage equipment short-cycles, hits sensible setpoint in eight minutes, shuts off, and never pulls the latent load out of the air. The result is the classic 72-degree-and-clammy Atlanta house with mold blooming behind the baseboards. The fix is enforced at the code level. Georgia adopted the 2020 International Energy Conservation Code with state amendments, and IECC Section R403.7 requires HVAC sizing per ACCA Manual J for the load, Manual S for equipment selection, and Manual D for duct design. Most Atlanta jurisdictions, including the City of Atlanta and Fulton County, demand a stamped Manual J as part of permit submittal for any new equipment install or substantial alteration. Right-sizing typically shows that the 5-ton system the previous contractor installed should be a 3-ton variable-speed heat pump with a properly matched coil. Variable-speed compressors run longer at lower output, which is exactly what humid Atlanta wants. Best-practice remodels add a dedicated dehumidifier sized to 70 to 90 pints per day for any conditioned basement, and an ERV — energy recovery ventilator — to satisfy ASHRAE 62.2 ventilation rates without dumping outdoor moisture into the envelope. ENERGY STAR Home Performance with ENERGY STAR specifies these whole-house controls for the Southeast climate. Done right, an Atlanta system runs 60-percent duty cycle in July, holds 50 to 55 percent indoor relative humidity, and uses 30 to 40 percent less kWh than the oversized predecessor.