Warm summers with low humidity. Cool winters requiring moderate heating. Climate zone 3B covers parts of California, Texas, Oklahoma, New Mexico, including cities like Fresno, Austin, San Antonio, Lubbock, Amarillo. Heating drives equipment selection and runtime in zone 3B. Cooling capacity is a secondary concern.
Climate zone 3B covers central and west Texas, Oklahoma, and parts of southern California's interior: Austin, San Antonio, Dallas-Fort Worth, Lubbock, and Fresno. Summers regularly push past 100°F with low humidity. Winters are mostly mild, but rare freeze events have rewritten how Texans size and back up their HVAC since 2021.
Triple-Digit Texas Summers and What That Means for Sizing
Austin averages 30+ days per year above 100°F, with some recent years exceeding 70 such days. San Antonio runs similar. The cooling season effectively runs from late April through October, which is six and a half months of continuous AC operation.
Unlike zone 1B (Phoenix), the Texas hill country gets enough humidity in late spring and fall that pure sensible-capacity sizing doesn't work. The AC needs to dehumidify during shoulder seasons and deliver brute force cooling capacity at 100 to 105°F peaks. Variable-speed equipment is the right answer, but it's also the most expensive ($9,000 to $13,500 installed). A two-stage AC at $7,500 to $10,000 hits 80 percent of the comfort improvement at a meaningful cost discount.
Post-Uri Backup Heating: How Texas HVAC Specifications Changed
Winter Storm Uri (February 2021) killed at least 246 Texans, knocked out power to 4.5 million homes, and cost the state an estimated $195 billion. Most Texas residential HVAC at the time was heat pumps with electric resistance backup, which failed exactly when needed most because the grid itself collapsed.
Senate Bill 3 (2021) mandated weatherization of Texas power generators, and ERCOT now reports 321 of 324 inspected generators as compliant with the new standards. Battery storage grew from near zero in 2021 to 17 GW today, and solar from 5 GW to 35 GW. Despite that, most central Texas HVAC contractors now recommend either dual-fuel installs (heat pump + gas furnace) or whole-home generator/battery backup for any all-electric installation. Adding the gas furnace runs $2,500 to $4,500 extra. A 20kW standby generator runs $8,000 to $14,000 installed.
Sizing Pitfalls Specific to Texas Hill Country
Two recurring sizing errors in zone 3B: builders specifying AC based on summer-only loads (ignoring the 1 percent of hours below 25°F), and contractors using a flat 500 sq ft per ton rule that ignores the masonry construction common in this region. South Texas homes built with brick veneer and concrete slabs have meaningful thermal mass that flattens cooling load peaks. A house that calculates to 4.0 tons under standard Manual J inputs often performs comfortably with 3.0 to 3.5 tons because the mass smooths peak demand.
Manual J does account for thermal mass when you enter the actual wall construction. Skipping that input is one of the most common reasons Austin and San Antonio homes get oversized.
Design Temperatures for Zone 3B
Design temperatures are the outdoor conditions your HVAC system needs to handle. Winter design temp is the temperature your house must stay warm at. Summer design temp is the temperature your house must stay cool at. Use these as Manual J inputs.
| Condition | Range | What it means for sizing |
|---|---|---|
| Winter design temp | 15°F to 30°F | Heating equipment must hold setpoint at this outdoor temp. |
| Summer design temp | 85°F to 95°F | Cooling equipment must hold setpoint at this outdoor temp. |
| Heating degree days | 2,000 to 3,500 | Higher numbers mean longer, colder winters and more heating runtime. |
| Cooling degree days | 1,000 to 2,500 | Higher numbers mean longer, hotter summers and more AC runtime. |
| Load priority | Heating dominated | Heating drives equipment selection and runtime in zone 3B. Cooling capacity is a secondary concern. |
Cities in Climate Zone 3B
These US cities are typically classified as climate zone 3B. Zone boundaries follow county lines, so suburbs of these cities sometimes fall into adjacent zones.
| City | Climate type | HVAC priority |
|---|---|---|
| Fresno | Warm, dry | Moderate cooling loads |
| Austin | Warm, dry | Moderate cooling loads |
| San Antonio | Warm, dry | Moderate cooling loads |
| Lubbock | Warm, dry | Moderate cooling loads |
| Amarillo | Warm, dry | Moderate cooling loads |
Insulation Requirements for Zone 3B
IECC code sets minimum insulation R-values by climate zone. These are the numbers your local building inspector checks during framing. Better envelope insulation lowers your HVAC load and lets you install smaller equipment.
| Assembly | Required R-value | Where it goes |
|---|---|---|
| Walls (above grade) | R-13 to R-15 | Between studs, often with continuous foam outside the sheathing |
| Ceiling / attic | R-30 to R-38 | Loose-fill or batts on the attic floor, or spray foam on the roof deck |
| Floor / crawlspace | R-19 to R-25 | Between floor joists, or on crawlspace walls if conditioned |
For a state-by-state breakdown of insulation requirements, see our insulation R-values guide.
HVAC Equipment Requirements for Zone 3B
These are the system types that fit zone 3B conditions. Final selection depends on your fuel cost, your building envelope, and the actual Manual J load for your specific house.
| Equipment | Why it fits zone 3B |
|---|---|
| Heat pumps | Moderate cooling loads |
| Gas furnaces with AC | Heating needs in winter |
| Packaged units | Low humidity year-round |
| Evaporative cooling (supplemental) | Temperature swings |
Key HVAC Design Considerations in Zone 3B
These are the design issues that come up most in climate zone 3B:
- Moderate cooling loads
- Heating needs in winter
- Low humidity year-round
- Temperature swings
- Dust considerations
Energy Code Rules for Zone 3B
Most states in zone 3B have adopted the IECC for residential construction. Equipment efficiency rules typically include:
- SEER 13+ minimum
- HSPF 7.7+ for heat pumps
- AFUE 78+ for furnaces
- Window U-factor limits
Check your state's adopted code edition (some states are still on IECC 2018, others have moved to IECC 2021 or 2024). For the full state breakdown, see our HVAC building code requirements guide.
States with Zone 3B Areas
These states have counties classified as climate zone 3B. Not every county in these states is zone 3B, so check the IECC map for your specific county.
Frequently Asked Questions
What does IECC climate zone 3B mean?
Climate zone 3B is warm and dry. Warm summers with low humidity. Cool winters requiring moderate heating.
What are the design temperatures for climate zone 3B?
In climate zone 3B, the summer design temperature runs 85°F to 95°F and the winter design temperature runs 15°F to 30°F. These are the outdoor temperatures your HVAC system needs to handle without falling behind on the hottest summer day and coldest winter day.
Is climate zone 3B heating or cooling dominated?
Climate zone 3B is generally heating dominated. Heating degree days run 2,000 to 3,500. Cooling degree days run 1,000 to 2,500. Heating drives equipment selection and runtime in zone 3B. Cooling capacity is a secondary concern.
Which cities are in climate zone 3B?
Major US cities in IECC climate zone 3B include Fresno, Austin, San Antonio, Lubbock, Amarillo. Zone boundaries follow county lines, so check the IECC map for your exact county if you're outside these cities.
What insulation does climate zone 3B require?
IECC code-required insulation for climate zone 3B: walls R-13 to R-15, ceiling R-30 to R-38, floor R-19 to R-25. These are minimum R-values for new construction and major renovations. Higher R-values lower your HVAC load and let you use smaller equipment.
What HVAC equipment works best in climate zone 3B?
Common equipment choices for climate zone 3B include Heat pumps, Gas furnaces with AC, Packaged units, Evaporative cooling (supplemental). Final selection depends on your building envelope, fuel availability, electric rates, and the actual Manual J load calculation for your home.
How do I size HVAC for climate zone 3B?
Use the climate zone 3B design temperatures (winter 15°F to 30°F, summer 85°F to 95°F) as the outdoor design conditions in a Manual J load calculation. Our free residential load calculator plugs these in automatically when you select your city.
What energy code applies in climate zone 3B?
Most states in climate zone 3B have adopted some version of the IECC (International Energy Conservation Code) for residential construction. Equipment efficiency rules apply: SEER 13+ minimum. HSPF 7.7+ for heat pumps. AFUE 78+ for furnaces. Window U-factor limits.