Washington climate zones and HVAC design temperatures
Washington spans 4 IECC climate zones. 72% of people live in zone 4C (mixed, marine). In Seattle, heating and cooling equipment is sized for 85°F in summer and 30°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Seattle | 4C | 85°F | 30°F | 56°F |
| Spokane | 5B | 94°F | 15°F | 53°F |
| Tacoma | 4C | 85°F | 25°F | 54°F |
| Vancouver | 4C | 90°F | 28°F | 57°F |
| Yakima | 5B | 97°F | 15°F | 55°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 (Benton County); coldest winter design: 9°F (Pend Oreille County).
Typical equipment sizes in Seattle
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Seattle'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 | 2.5 tons | 40,000 BTU | 2.5 tons + 5 kW backup |
| 2,500 sq ft | 2.5 tons | 40,000 BTU | 2.5 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 4C: mixed, marine (14 counties, 72% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Clark County | 90°F | 28°F |
| Cowlitz County | 86°F | 27°F |
| Grays Harbor County | 75°F | 31°F |
| Jefferson County | 71°F | 29°F |
| King County | 85°F | 30°F |
| Lewis County | 86°F | 28°F |
| Mason County | 86°F | 26°F |
| Pacific County | 75°F | 31°F |
| Pierce County | 85°F | 25°F |
| Skagit County | 81°F | 27°F |
| Snohomish County | 80°F | 29°F |
| Thurston County | 86°F | 26°F |
| Wahkiakum County | 75°F | 31°F |
| Whatcom County | 79°F | 24°F |
Zone 5B: cool, dry (18 counties, 21% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Adams County | 96°F | 12°F |
| Asotin County | 97°F | 19°F |
| Benton County | 99°F | 18°F |
| Chelan County | 94°F | 14°F |
| Columbia County | 96°F | 16°F |
| Douglas County | 94°F | 14°F |
| Franklin County | 98°F | 17°F |
| Garfield County | 97°F | 19°F |
| Grant County | 96°F | 12°F |
| Kittitas County | 94°F | 12°F |
| Klickitat County | 97°F | 22°F |
| Lincoln County | 91°F | 11°F |
| Okanogan County | 97°F | 11°F |
| Skamania County | 91°F | 30°F |
| Spokane County | 94°F | 15°F |
| Walla Walla County | 96°F | 16°F |
| Whitman County | 90°F | 14°F |
| Yakima County | 97°F | 15°F |
Zone 5C: cool, marine (4 counties, 6% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Clallam County | 68°F | 32°F |
| Island County | 71°F | 29°F |
| Kitsap County | 85°F | 26°F |
| San Juan County | 78°F | 28°F |
Zone 6B: cold, dry (3 counties, 1% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| Ferry County | 97°F | 11°F |
| Pend Oreille County | 92°F | 9°F |
| Stevens County | 92°F | 9°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.