Hot summers and cold winters with low humidity. Large temperature variations. Climate zone 4B covers parts of Colorado, Kansas, Oklahoma, New Mexico, Utah, including cities like Denver, Colorado Springs, Albuquerque, Oklahoma City, Wichita. Heating drives equipment selection and runtime in zone 4B. Cooling capacity is a secondary concern.
Climate zone 4B is the high plains and Rocky Mountain front range below 7,000 feet: Denver, Colorado Springs, Pueblo, Oklahoma City, and parts of New Mexico and Utah. Cold dry winters drop to -5 to 10°F design temps, summers stay relatively cool because of altitude. The defining HVAC challenge is twofold: altitude derating that affects every piece of combustion equipment, and the bone-dry winter air that makes whole-house humidification practically mandatory.
Combustion Equipment Derating Above 4,000 Feet
Denver sits at 5,280 feet. At that altitude, atmospheric pressure is roughly 83 percent of sea-level pressure, which means there's less oxygen available for combustion. Standard gas furnaces and water heaters rated for sea-level operation must be derated when installed above 2,000 feet, typically reducing input BTU capacity by 4 percent per 1,000 feet of elevation.
A nominal 80,000 BTU furnace at sea level delivers roughly 64,000 BTU in Denver. Manufacturers publish altitude-specific orifice sizes (smaller jets to match the thinner air), and a contractor who doesn't change the orifices during install ends up with rich combustion, soot deposits, and a system that fails CO testing. This is one of the most common installation errors in front-range Colorado.
Why Winter Humidification Is Standard, Not Optional, in Zone 4B
Winter relative humidity in Denver and Cheyenne regularly drops below 15 percent, which is drier than the Sahara Desert. At those levels you get cracked wood floors, static electricity that damages electronics, persistent dry skin and respiratory complaints, and HVAC ductwork that delivers air so dry the home feels colder than the thermostat reading.
Whole-house humidifiers plumbed into the furnace return air run $400 to $900 installed and bring indoor RH up to the recommended 30 to 50 percent range. The bypass type is cheapest and works fine for most homes. Fan-powered or steam humidifiers ($800 to $2,500) handle larger homes and tighter target humidity better. Skipping the humidifier in zone 4B isn't a comfort preference, it's an indoor air quality and home preservation issue.
Xcel Energy Cold Climate Heat Pump Rebates
Colorado's natural gas furnace rebate program ended January 1, 2026 (the Xcel Energy gas furnace rebate for 95+ AFUE units no longer applies to new 2026 installs). Heat pumps are now the dominant rebate path. Under Xcel's Clean Heat Plan, qualifying cold-climate heat pumps earn $2,250 per heating ton sized at the 5°F load. Most Denver-area homes need 3 to 4 tons, putting the rebate at $6,750 to $9,000.
The math has shifted hard: a dual-fuel install (heat pump + 95% AFUE furnace) used to be the default in zone 4B. With gas rebates gone and heat pump rebates climbing, full electrification with a cold-climate heat pump and small electric resistance backup is now competitive on first cost and clearly cheaper to operate.
Design Temperatures for Zone 4B
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 | 5°F to 20°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 | 3,500 to 6,000 | Higher numbers mean longer, colder winters and more heating runtime. |
| Cooling degree days | 500 to 1,500 | Higher numbers mean longer, hotter summers and more AC runtime. |
| Load priority | Heating dominated | Heating drives equipment selection and runtime in zone 4B. Cooling capacity is a secondary concern. |
Cities in Climate Zone 4B
These US cities are typically classified as climate zone 4B. Zone boundaries follow county lines, so suburbs of these cities sometimes fall into adjacent zones.
| City | Climate type | HVAC priority |
|---|---|---|
| Denver | Mixed, dry | High heating loads |
| Colorado Springs | Mixed, dry | High heating loads |
| Albuquerque | Mixed, dry | High heating loads |
| Oklahoma City | Mixed, dry | High heating loads |
| Wichita | Mixed, dry | High heating loads |
Insulation Requirements for Zone 4B
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-15 to R-19 | Between studs, often with continuous foam outside the sheathing |
| Ceiling / attic | R-38 to R-49 | Loose-fill or batts on the attic floor, or spray foam on the roof deck |
| Floor / crawlspace | R-25 to R-30 | 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 4B
These are the system types that fit zone 4B 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 4B |
|---|---|
| Gas furnaces with AC | High heating loads |
| Heat pumps | Moderate cooling loads |
| Boilers with cooling | Large temperature swings |
| Evaporative cooling (supplemental) | Low humidity year-round |
Key HVAC Design Considerations in Zone 4B
These are the design issues that come up most in climate zone 4B:
- High heating loads
- Moderate cooling loads
- Large temperature swings
- Low humidity year-round
- Altitude considerations
Energy Code Rules for Zone 4B
Most states in zone 4B 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
- Enhanced insulation requirements
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 4B Areas
These states have counties classified as climate zone 4B. Not every county in these states is zone 4B, so check the IECC map for your specific county.
Frequently Asked Questions
What does IECC climate zone 4B mean?
Climate zone 4B is mixed and dry. Hot summers and cold winters with low humidity. Large temperature variations.
What are the design temperatures for climate zone 4B?
In climate zone 4B, the summer design temperature runs 85°F to 95°F and the winter design temperature runs 5°F to 20°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 4B heating or cooling dominated?
Climate zone 4B is generally heating dominated. Heating degree days run 3,500 to 6,000. Cooling degree days run 500 to 1,500. Heating drives equipment selection and runtime in zone 4B. Cooling capacity is a secondary concern.
Which cities are in climate zone 4B?
Major US cities in IECC climate zone 4B include Denver, Colorado Springs, Albuquerque, Oklahoma City, Wichita. 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 4B require?
IECC code-required insulation for climate zone 4B: walls R-15 to R-19, ceiling R-38 to R-49, floor R-25 to R-30. 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 4B?
Common equipment choices for climate zone 4B include Gas furnaces with AC, Heat pumps, Boilers with cooling, 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 4B?
Use the climate zone 4B design temperatures (winter 5°F to 20°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 4B?
Most states in climate zone 4B 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. Enhanced insulation requirements.