Alabama climate zones and HVAC design temperatures
Alabama spans 2 IECC climate zones. 81% of people live in zone 3A (warm, humid). In Huntsville, heating and cooling equipment is sized for 91°F in summer and 23°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Huntsville | 3A | 91°F | 23°F | 63°F |
| Birmingham | 3A | 93°F | 27°F | 67°F |
| Montgomery | 3A | 94°F | 30°F | 68°F |
| Mobile | 2A | 93°F | 32°F | 70°F |
| Tuscaloosa | 3A | 94°F | 28°F | 67°F |
Summer: exceeded in 1% of hours. Winter: colder in only 1% of hours. Water in: estimated groundwater temperature for tankless water heater sizing. Computed for each county from NOAA hourly weather data, 2015-2024; see where our numbers come from. Hottest summer design in the state: 97°F (Covington County); coldest winter design: 21°F (Jackson County).
Typical equipment sizes in Huntsville
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Huntsville's design temperatures. Your house can need more or less; use the calculators for yours.
| House | Central AC | Gas furnace (95% AFUE) | Heat pump |
|---|---|---|---|
| 1,500 sq ft | 2 tons | 40,000 BTU | 2 tons + 5 kW backup |
| 2,000 sq ft | 3 tons | 40,000 BTU | 3 tons + 5 kW backup |
| 2,500 sq ft | 3.5 tons | 40,000 BTU | 3.5 tons + 5 kW backup |
AC size calculator Furnace size calculator Heat pump size calculator
Climate zone by county
From the 2021 International Energy Conservation Code (IECC), as published by the U.S. Department of Energy. The zone sets the insulation and window requirements in the energy code; the number runs from 1 (hottest) to 8 (coldest), and the letter is A for moist, B for dry and C for marine.
Zone 2A: hot, humid (9 counties, 19% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Baldwin County | 90°F | 32°F |
| Coffee County | 93°F | 32°F |
| Covington County | 97°F | 33°F |
| Dale County | 93°F | 30°F |
| Escambia County | 94°F | 30°F |
| Geneva County | 94°F | 31°F |
| Henry County | 94°F | 33°F |
| Houston County | 94°F | 33°F |
| Mobile County | 93°F | 32°F |
Zone 3A: warm, humid (58 counties, 81% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Autauga County | 94°F | 30°F |
| Barbour County | 95°F | 30°F |
| Bibb County | 92°F | 29°F |
| Blount County | 91°F | 23°F |
| Bullock County | 94°F | 29°F |
| Butler County | 94°F | 29°F |
| Calhoun County | 92°F | 27°F |
| Chambers County | 93°F | 27°F |
| Cherokee County | 90°F | 23°F |
| Chilton County | 92°F | 29°F |
| Choctaw County | 95°F | 28°F |
| Clarke County | 94°F | 30°F |
| Clay County | 92°F | 27°F |
| Cleburne County | 89°F | 26°F |
| Colbert County | 94°F | 25°F |
| Conecuh County | 94°F | 30°F |
| Coosa County | 92°F | 28°F |
| Crenshaw County | 94°F | 29°F |
| Cullman County | 91°F | 23°F |
| Dallas County | 95°F | 30°F |
| DeKalb County | 90°F | 23°F |
| Elmore County | 94°F | 30°F |
| Etowah County | 92°F | 23°F |
| Fayette County | 97°F | 23°F |
| Franklin County | 91°F | 22°F |
| Greene County | 94°F | 28°F |
| Hale County | 94°F | 28°F |
| Jackson County | 93°F | 21°F |
| Jefferson County | 93°F | 27°F |
| Lamar County | 94°F | 26°F |
| Lauderdale County | 94°F | 25°F |
| Lawrence County | 93°F | 23°F |
| Lee County | 91°F | 29°F |
| Limestone County | 93°F | 23°F |
| Lowndes County | 95°F | 30°F |
| Macon County | 91°F | 29°F |
| Madison County | 91°F | 23°F |
| Marengo County | 94°F | 28°F |
| Marion County | 91°F | 22°F |
| Marshall County | 90°F | 27°F |
| Monroe County | 94°F | 30°F |
| Montgomery County | 94°F | 30°F |
| Morgan County | 93°F | 23°F |
| Perry County | 94°F | 28°F |
| Pickens County | 94°F | 28°F |
| Pike County | 94°F | 29°F |
| Randolph County | 93°F | 27°F |
| Russell County | 94°F | 27°F |
| Shelby County | 92°F | 29°F |
| St. Clair County | 93°F | 27°F |
| Sumter County | 94°F | 28°F |
| Talladega County | 92°F | 27°F |
| Tallapoosa County | 92°F | 28°F |
| Tuscaloosa County | 94°F | 28°F |
| Walker County | 97°F | 23°F |
| Washington County | 93°F | 32°F |
| Wilcox County | 94°F | 30°F |
| Winston County | 91°F | 22°F |
Climate data as of 2026-10. Design temperatures may differ by a few degrees from the ACCA Manual J tables your contractor uses; winter values run a little warmer because recent winters have been milder.