California climate zones and HVAC design temperatures

Design temperatures for major cities

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: 111°F (Imperial County); coldest winter design: 6°F (Mono County).

Typical equipment sizes in Los Angeles

One-story house with average insulation (R-13 walls, R-30 attic, double-pane windows), at Los Angeles's design temperatures. Your house can need more or less; use the calculators for yours.

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 2B: hot, dry (1 county, 0% of people)

Zone 3B: warm, dry (25 counties, 76% of people)

Zone 3C: warm, marine (14 counties, 22% of people)

Zone 4B: mixed, dry (8 counties, 1% of people)

Zone 4C: mixed, marine (2 counties, 0% of people)

Zone 5B: cool, dry (6 counties, 1% of people)

Zone 6B: cold, dry (2 counties, 0% of people)

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.