Tennessee climate zones and HVAC design temperatures
Tennessee spans 2 IECC climate zones. 52% of people live in zone 3A (warm, humid). In Nashville, heating and cooling equipment is sized for 93°F in summer and 21°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Nashville | 3A | 93°F | 21°F | 64°F |
| Memphis | 3A | 94°F | 24°F | 66°F |
| Knoxville | 4A | 91°F | 22°F | 63°F |
| Chattanooga | 3A | 93°F | 26°F | 65°F |
| Clarksville | 4A | 91°F | 18°F | 62°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: 94°F (Lawrence County); coldest winter design: 15°F (Pickett County).
Typical equipment sizes in Nashville
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Nashville'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 | 2.5 tons | 40,000 BTU | 2.5 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 3A: warm, humid (34 counties, 52% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Bedford County | 90°F | 19°F |
| Chester County | 93°F | 20°F |
| Coffee County | 90°F | 19°F |
| Crockett County | 93°F | 20°F |
| Davidson County | 93°F | 21°F |
| Decatur County | 93°F | 20°F |
| Dyer County | 93°F | 22°F |
| Fayette County | 93°F | 21°F |
| Franklin County | 90°F | 21°F |
| Gibson County | 93°F | 20°F |
| Giles County | 91°F | 23°F |
| Grundy County | 90°F | 21°F |
| Hamilton County | 93°F | 26°F |
| Hardeman County | 93°F | 20°F |
| Hardin County | 91°F | 23°F |
| Haywood County | 93°F | 20°F |
| Henderson County | 93°F | 20°F |
| Hickman County | 93°F | 21°F |
| Lauderdale County | 93°F | 22°F |
| Lawrence County | 94°F | 25°F |
| Lewis County | 94°F | 25°F |
| Lincoln County | 91°F | 23°F |
| Madison County | 93°F | 20°F |
| Marion County | 93°F | 26°F |
| Marshall County | 90°F | 19°F |
| Maury County | 93°F | 21°F |
| McNairy County | 91°F | 23°F |
| Moore County | 90°F | 19°F |
| Perry County | 90°F | 18°F |
| Rutherford County | 93°F | 19°F |
| Shelby County | 94°F | 24°F |
| Tipton County | 93°F | 21°F |
| Wayne County | 94°F | 25°F |
| Williamson County | 93°F | 21°F |
Zone 4A: mixed, humid (61 counties, 48% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Anderson County | 91°F | 22°F |
| Benton County | 90°F | 18°F |
| Bledsoe County | 85°F | 18°F |
| Blount County | 91°F | 22°F |
| Bradley County | 93°F | 26°F |
| Campbell County | 91°F | 22°F |
| Cannon County | 93°F | 19°F |
| Carroll County | 90°F | 18°F |
| Carter County | 89°F | 19°F |
| Cheatham County | 93°F | 21°F |
| Claiborne County | 89°F | 19°F |
| Clay County | 91°F | 18°F |
| Cocke County | 91°F | 22°F |
| Cumberland County | 85°F | 18°F |
| DeKalb County | 91°F | 18°F |
| Dickson County | 91°F | 18°F |
| Fentress County | 85°F | 18°F |
| Grainger County | 89°F | 19°F |
| Greene County | 89°F | 19°F |
| Hamblen County | 89°F | 19°F |
| Hancock County | 88°F | 19°F |
| Hawkins County | 89°F | 19°F |
| Henry County | 90°F | 18°F |
| Houston County | 91°F | 18°F |
| Humphreys County | 91°F | 18°F |
| Jackson County | 91°F | 18°F |
| Jefferson County | 91°F | 22°F |
| Johnson County | 87°F | 19°F |
| Knox County | 91°F | 22°F |
| Lake County | 91°F | 19°F |
| Loudon County | 91°F | 22°F |
| Macon County | 90°F | 16°F |
| McMinn County | 93°F | 26°F |
| Meigs County | 85°F | 18°F |
| Monroe County | 91°F | 22°F |
| Montgomery County | 91°F | 18°F |
| Morgan County | 91°F | 22°F |
| Obion County | 91°F | 19°F |
| Overton County | 91°F | 18°F |
| Pickett County | 89°F | 15°F |
| Polk County | 91°F | 25°F |
| Putnam County | 91°F | 18°F |
| Rhea County | 85°F | 18°F |
| Roane County | 91°F | 22°F |
| Robertson County | 93°F | 21°F |
| Scott County | 91°F | 22°F |
| Sequatchie County | 93°F | 26°F |
| Sevier County | 91°F | 22°F |
| Smith County | 91°F | 18°F |
| Stewart County | 91°F | 18°F |
| Sullivan County | 89°F | 19°F |
| Sumner County | 93°F | 21°F |
| Trousdale County | 93°F | 19°F |
| Unicoi County | 89°F | 19°F |
| Union County | 89°F | 19°F |
| Van Buren County | 91°F | 18°F |
| Warren County | 91°F | 18°F |
| Washington County | 89°F | 19°F |
| Weakley County | 91°F | 19°F |
| White County | 91°F | 18°F |
| Wilson County | 93°F | 19°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.