Idaho climate zones and HVAC design temperatures
Idaho spans 2 IECC climate zones. 72% of people live in zone 5B (cool, dry). In Boise, heating and cooling equipment is sized for 97°F in summer and 16°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Boise | 5B | 97°F | 16°F | 56°F |
| Meridian | 5B | 97°F | 16°F | 56°F |
| Nampa | 5B | 96°F | 15°F | 55°F |
| Idaho Falls | 6B | 91°F | 1°F | 48°F |
| Pocatello | 6B | 93°F | 5°F | 50°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: 98°F (Elmore County); coldest winter design: -10°F (Blaine County).
Typical equipment sizes in Boise
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Boise'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 | 2.5 tons + 5 kW backup |
| 2,000 sq ft | 2.5 tons | 40,000 BTU | 3 tons + 5 kW backup |
| 2,500 sq ft | 3.5 tons | 60,000 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 5B: cool, dry (22 counties, 72% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Ada County | 97°F | 16°F |
| Benewah County | 91°F | 12°F |
| Canyon County | 96°F | 15°F |
| Cassia County | 91°F | 11°F |
| Clearwater County | 93°F | 19°F |
| Elmore County | 98°F | 13°F |
| Gem County | 96°F | 15°F |
| Gooding County | 93°F | 13°F |
| Idaho County | 88°F | 12°F |
| Jerome County | 93°F | 13°F |
| Kootenai County | 91°F | 12°F |
| Latah County | 90°F | 14°F |
| Lewis County | 88°F | 12°F |
| Lincoln County | 93°F | 13°F |
| Minidoka County | 91°F | 11°F |
| Nez Perce County | 97°F | 19°F |
| Owyhee County | 97°F | 16°F |
| Payette County | 98°F | 15°F |
| Power County | 93°F | 5°F |
| Shoshone County | 91°F | 12°F |
| Twin Falls County | 93°F | 11°F |
| Washington County | 98°F | 15°F |
Zone 6B: cold, dry (22 counties, 28% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adams County | 87°F | 2°F |
| Bannock County | 93°F | 5°F |
| Bear Lake County | 82°F | -8°F |
| Bingham County | 93°F | 5°F |
| Blaine County | 85°F | -10°F |
| Boise County | 97°F | 16°F |
| Bonner County | 88°F | 12°F |
| Bonneville County | 91°F | 1°F |
| Boundary County | 90°F | 9°F |
| Butte County | 91°F | 1°F |
| Camas County | 93°F | 13°F |
| Caribou County | 82°F | -8°F |
| Clark County | 89°F | 1°F |
| Custer County | 91°F | -2°F |
| Franklin County | 93°F | 5°F |
| Fremont County | 89°F | 1°F |
| Jefferson County | 89°F | 1°F |
| Lemhi County | 89°F | -2°F |
| Madison County | 89°F | 1°F |
| Oneida County | 93°F | 5°F |
| Teton County | 85°F | -4°F |
| Valley County | 87°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.