District of Columbia climate zones and HVAC design temperatures
All of District of Columbia is in IECC climate zone 4A (mixed, humid). In Washington, heating and cooling equipment is sized for 92°F in summer and 23°F in winter.
Design temperatures for major cities
| City | Zone | Summer 1% | Winter 99% | Water in |
|---|---|---|---|---|
| Washington | 4A | 92°F | 23°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: 92°F (District of Columbia); coldest winter design: 23°F (District of Columbia).
Typical equipment sizes in Washington
One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Washington'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 | 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 4A: mixed, humid (1 county, 100% of people)
| County | Summer 1% | Winter 99% |
|---|---|---|
| District of Columbia | 92°F | 23°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.