Texas 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: 104°F (Dimmit County); coldest winter design: 13°F (Moore County).

Typical equipment sizes in Houston

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

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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 1A: very hot, humid (3 counties, 5% of people)

Zone 2A: hot, humid (81 counties, 69% of people)

Zone 2B: hot, dry (14 counties, 2% of people)

Zone 3A: warm, humid (58 counties, 15% of people)

Zone 3B: warm, dry (70 counties, 8% of people)

Zone 4B: mixed, dry (28 counties, 2% 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.