Colorado climate zones and HVAC design temperatures
Colorado spans 4 IECC climate zones. 94% of people live in zone 5B (cool, dry). In Denver, heating and cooling equipment is sized for 92°F in summer and 7°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Denver | 5B | 92°F | 7°F | 53°F |
| Colorado Springs | 5B | 90°F | 9°F | 54°F |
| Aurora | 5B | 91°F | 8°F | 54°F |
| Fort Collins | 5B | 93°F | 5°F | 52°F |
| Grand Junction | 5B | 97°F | 15°F | 56°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: 99°F (Kiowa County); coldest winter design: -11°F (Gunnison County).
Typical equipment sizes in Denver
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Denver'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 4B: mixed, dry (5 counties, 1% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Baca County | 95°F | 9°F |
| Bent County | 98°F | 9°F |
| Las Animas County | 93°F | 9°F |
| Otero County | 98°F | 9°F |
| Prowers County | 99°F | 7°F |
Zone 5B: cool, dry (34 counties, 94% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adams County | 93°F | 4°F |
| Arapahoe County | 91°F | 8°F |
| Boulder County | 91°F | 10°F |
| Broomfield County | 93°F | 4°F |
| Cheyenne County | 95°F | 6°F |
| Crowley County | 98°F | 9°F |
| Custer County | 82°F | 5°F |
| Delta County | 98°F | 16°F |
| Denver County | 92°F | 7°F |
| Douglas County | 91°F | 8°F |
| El Paso County | 90°F | 9°F |
| Elbert County | 91°F | 8°F |
| Fremont County | 91°F | 8°F |
| Garfield County | 94°F | 8°F |
| Gilpin County | 91°F | 10°F |
| Huerfano County | 93°F | 9°F |
| Jefferson County | 91°F | 8°F |
| Kiowa County | 99°F | 7°F |
| Kit Carson County | 95°F | 6°F |
| La Plata County | 89°F | 6°F |
| Larimer County | 93°F | 5°F |
| Lincoln County | 92°F | 1°F |
| Logan County | 93°F | 4°F |
| Mesa County | 97°F | 15°F |
| Montezuma County | 93°F | 8°F |
| Montrose County | 92°F | 11°F |
| Morgan County | 94°F | 3°F |
| Phillips County | 97°F | 5°F |
| Pueblo County | 97°F | 8°F |
| Sedgwick County | 97°F | 5°F |
| Teller County | 88°F | 4°F |
| Washington County | 93°F | 4°F |
| Weld County | 94°F | 0°F |
| Yuma County | 97°F | 5°F |
Zone 6B: cold, dry (12 counties, 3% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Alamosa County | 85°F | -5°F |
| Archuleta County | 85°F | 8°F |
| Chaffee County | 83°F | 4°F |
| Conejos County | 85°F | -5°F |
| Costilla County | 85°F | -5°F |
| Dolores County | 93°F | 8°F |
| Eagle County | 88°F | 1°F |
| Moffat County | 91°F | -9°F |
| Ouray County | 77°F | 9°F |
| Rio Blanco County | 89°F | -2°F |
| Saguache County | 82°F | 5°F |
| San Miguel County | 77°F | 9°F |
Zone 7: very cold (13 counties, 3% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Clear Creek County | 91°F | 10°F |
| Grand County | 91°F | 10°F |
| Gunnison County | 82°F | -11°F |
| Hinsdale County | 77°F | 9°F |
| Jackson County | 85°F | -1°F |
| Lake County | 74°F | -2°F |
| Mineral County | 85°F | 8°F |
| Park County | 70°F | 3°F |
| Pitkin County | 83°F | 3°F |
| Rio Grande County | 85°F | -5°F |
| Routt County | 85°F | -1°F |
| San Juan County | 77°F | 9°F |
| Summit County | 68°F | 1°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.