Illinois climate zones and HVAC design temperatures
Illinois spans 2 IECC climate zones. 89% of people live in zone 5A (cool, humid). In Chicago, heating and cooling equipment is sized for 89°F in summer and 6°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Chicago | 5A | 89°F | 6°F | 56°F |
| Aurora | 5A | 88°F | 2°F | 53°F |
| Rockford | 5A | 88°F | 0°F | 53°F |
| Peoria | 5A | 90°F | 4°F | 56°F |
| Springfield | 5A | 90°F | 7°F | 57°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: 93°F (Calhoun County); coldest winter design: -2°F (Jo Daviess County).
Typical equipment sizes in Chicago
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Chicago'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 + 8 kW backup |
| 2,000 sq ft | 2.5 tons | 60,000 BTU | 3 tons + 8 kW backup |
| 2,500 sq ft | 3 tons | 60,000 BTU | 3.5 tons + 8 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 4A: mixed, humid (44 counties, 11% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Alexander County | 90°F | 17°F |
| Bond County | 89°F | 7°F |
| Calhoun County | 93°F | 13°F |
| Christian County | 89°F | 8°F |
| Clark County | 89°F | 8°F |
| Clay County | 91°F | 12°F |
| Clinton County | 92°F | 12°F |
| Coles County | 88°F | 8°F |
| Crawford County | 89°F | 8°F |
| Cumberland County | 88°F | 8°F |
| Edwards County | 90°F | 12°F |
| Effingham County | 91°F | 12°F |
| Fayette County | 91°F | 12°F |
| Franklin County | 91°F | 14°F |
| Gallatin County | 90°F | 14°F |
| Greene County | 90°F | 8°F |
| Hamilton County | 90°F | 12°F |
| Hardin County | 91°F | 15°F |
| Jackson County | 91°F | 14°F |
| Jasper County | 90°F | 11°F |
| Jefferson County | 91°F | 11°F |
| Jersey County | 93°F | 13°F |
| Johnson County | 91°F | 15°F |
| Lawrence County | 90°F | 12°F |
| Macoupin County | 89°F | 7°F |
| Madison County | 93°F | 13°F |
| Marion County | 91°F | 12°F |
| Massac County | 91°F | 15°F |
| Monroe County | 93°F | 13°F |
| Montgomery County | 89°F | 7°F |
| Perry County | 91°F | 14°F |
| Pope County | 91°F | 15°F |
| Pulaski County | 90°F | 17°F |
| Randolph County | 90°F | 13°F |
| Richland County | 90°F | 11°F |
| Saline County | 91°F | 14°F |
| Shelby County | 88°F | 8°F |
| St. Clair County | 92°F | 12°F |
| Union County | 91°F | 14°F |
| Wabash County | 90°F | 12°F |
| Washington County | 91°F | 11°F |
| Wayne County | 90°F | 12°F |
| White County | 90°F | 12°F |
| Williamson County | 91°F | 14°F |
Zone 5A: cool, humid (58 counties, 89% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adams County | 90°F | 5°F |
| Boone County | 88°F | 0°F |
| Brown County | 90°F | 8°F |
| Bureau County | 89°F | 2°F |
| Carroll County | 86°F | -1°F |
| Cass County | 89°F | 6°F |
| Champaign County | 89°F | 5°F |
| Cook County | 89°F | 6°F |
| De Witt County | 88°F | 4°F |
| DeKalb County | 87°F | 0°F |
| Douglas County | 89°F | 5°F |
| DuPage County | 88°F | 2°F |
| Edgar County | 89°F | 6°F |
| Ford County | 89°F | 5°F |
| Fulton County | 90°F | 4°F |
| Grundy County | 88°F | 1°F |
| Hancock County | 90°F | 3°F |
| Henderson County | 89°F | 2°F |
| Henry County | 90°F | 2°F |
| Iroquois County | 88°F | 3°F |
| Jo Daviess County | 86°F | -2°F |
| Kane County | 88°F | 2°F |
| Kankakee County | 88°F | 3°F |
| Kendall County | 88°F | 0°F |
| Knox County | 88°F | 1°F |
| Lake County | 86°F | 2°F |
| LaSalle County | 89°F | 2°F |
| Lee County | 88°F | 0°F |
| Livingston County | 89°F | 3°F |
| Logan County | 90°F | 8°F |
| Macon County | 89°F | 7°F |
| Marshall County | 88°F | 2°F |
| Mason County | 90°F | 4°F |
| McDonough County | 89°F | 2°F |
| McHenry County | 89°F | 4°F |
| McLean County | 88°F | 4°F |
| Menard County | 90°F | 7°F |
| Mercer County | 90°F | 2°F |
| Morgan County | 89°F | 6°F |
| Moultrie County | 89°F | 7°F |
| Ogle County | 87°F | -1°F |
| Peoria County | 90°F | 4°F |
| Piatt County | 89°F | 5°F |
| Pike County | 90°F | 8°F |
| Putnam County | 89°F | 2°F |
| Rock Island County | 90°F | 2°F |
| Sangamon County | 90°F | 7°F |
| Schuyler County | 89°F | 2°F |
| Scott County | 89°F | 6°F |
| Stark County | 88°F | 2°F |
| Stephenson County | 86°F | -1°F |
| Tazewell County | 90°F | 4°F |
| Vermilion County | 88°F | 5°F |
| Warren County | 88°F | 1°F |
| Whiteside County | 88°F | 0°F |
| Will County | 89°F | 4°F |
| Winnebago County | 88°F | 0°F |
| Woodford County | 88°F | 2°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.