Hot summers and cold winters with high humidity. Significant heating and cooling loads. Climate zone 4A covers parts of Virginia, Kentucky, Missouri, Kansas, Maryland, Delaware, New Jersey, including cities like Richmond, Louisville, Kansas City, St. Louis, Baltimore. Heating drives equipment selection and runtime in zone 4A. Cooling capacity is a secondary concern.
Climate zone 4A is the mid-Atlantic and southern Midwest: Kansas City, St. Louis, Baltimore, Richmond, Louisville, and the I-70 corridor. This zone has the most balanced heating and cooling loads of any IECC zone. Summers run 85 to 95°F with meaningful humidity, winters drop to 0 to 20°F. Heating and cooling each account for roughly half of annual HVAC energy use, which makes the dual-fuel decision (heat pump vs gas furnace as primary heat) the central question for most homeowners.
The Dual-Fuel Decision: Where the Math Tips
Dual-fuel systems pair a heat pump with a gas furnace, and the thermostat switches between them based on outdoor temperature and the relative cost of electricity vs natural gas. A typical changeover setpoint in zone 4A is 30 to 35°F: above that, the heat pump runs because it's cheaper at 200 to 400 percent efficiency; below that, the gas furnace takes over because heat pump efficiency drops faster than gas furnace efficiency.
The right changeover temperature is a function of your local electricity rate (cents/kWh) and your natural gas rate (dollars/therm). Baltimore homeowners on BGE pay roughly 14 cents/kWh and $1.30/therm, which makes the heat pump cheaper down to about 30°F. St. Louis homes on Ameren and Spire Energy pay closer to 12 cents/kWh and $1.10/therm, which pushes the crossover down to 25°F. Kansas City homes on Evergy and Spire fall in between.
Why Humidity Control Matters in 4A Summers
Zone 4A summer humidity is the sneaky problem. Outdoor temperatures peak at 90 to 95°F, but average summer dew points sit at 65 to 70°F, which is muggier than most homeowners think. A standard single-stage AC sized for sensible cooling load satisfies the thermostat in 8 to 12 minutes and shuts off, leaving indoor humidity at 60+ percent.
Variable-speed heat pump compressors solve this. Running at 40 to 60 percent capacity for hours pulls moisture continuously. Where the budget doesn't stretch that far, a separately controlled whole-house dehumidifier ($1,200 to $2,000) in the return duct gives you humidity control independent of cooling demand.
Zone 4A Equipment Cost Comparison
Typical replacement costs for a 2,000 square foot home in metro Kansas City, St. Louis, or Baltimore:
| System type | Install cost | Best for |
|---|---|---|
| Single-stage AC + 95% gas furnace | $7,500 to $10,500 | Lowest upfront cost, simple controls |
| Two-stage AC + 95% gas furnace | $8,500 to $12,000 | Better humidity control, modest premium |
| Dual-fuel heat pump + 95% gas furnace | $10,500 to $14,500 | Lowest annual operating cost in 4A |
| Variable-speed heat pump (all-electric) | $11,000 to $15,500 | Best efficiency, requires backup strategy for cold snaps |
Design Temperatures for Zone 4A
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 | 0°F to 20°F | Heating equipment must hold setpoint at this outdoor temp. |
| Summer design temp | 80°F to 90°F | Cooling equipment must hold setpoint at this outdoor temp. |
| Heating degree days | 3,000 to 5,500 | Higher numbers mean longer, colder winters and more heating runtime. |
| Cooling degree days | 1,000 to 2,000 | Higher numbers mean longer, hotter summers and more AC runtime. |
| Load priority | Heating dominated | Heating drives equipment selection and runtime in zone 4A. Cooling capacity is a secondary concern. |
Cities in Climate Zone 4A
These US cities are typically classified as climate zone 4A. Zone boundaries follow county lines, so suburbs of these cities sometimes fall into adjacent zones.
| City | Climate type | HVAC priority |
|---|---|---|
| Richmond | Mixed, moist | High heating and cooling loads |
| Louisville | Mixed, moist | High heating and cooling loads |
| Kansas City | Mixed, moist | High heating and cooling loads |
| St. Louis | Mixed, moist | High heating and cooling loads |
| Baltimore | Mixed, moist | High heating and cooling loads |
Insulation Requirements for Zone 4A
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-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-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 4A
These are the system types that fit zone 4A 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 4A |
|---|---|
| Heat pumps with backup heat | High heating and cooling loads |
| Gas furnaces with AC | Humidity control year-round |
| Dual-fuel systems | Seasonal system changeover |
| Central dehumidification | Freeze protection |
Key HVAC Design Considerations in Zone 4A
These are the design issues that come up most in climate zone 4A:
- High heating and cooling loads
- Humidity control year-round
- Seasonal system changeover
- Freeze protection
- Mold and moisture management
Energy Code Rules for Zone 4A
Most states in zone 4A have adopted the IECC for residential construction. Equipment efficiency rules typically include:
- SEER 13+ minimum
- HSPF 7.7+ for heat pumps
- AFUE 80+ for furnaces
- Duct testing 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 4A Areas
These states have counties classified as climate zone 4A. Not every county in these states is zone 4A, so check the IECC map for your specific county.
Frequently Asked Questions
What does IECC climate zone 4A mean?
Climate zone 4A is mixed and moist. Hot summers and cold winters with high humidity. Significant heating and cooling loads.
What are the design temperatures for climate zone 4A?
In climate zone 4A, the summer design temperature runs 80°F to 90°F and the winter design temperature runs 0°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 4A heating or cooling dominated?
Climate zone 4A is generally heating dominated. Heating degree days run 3,000 to 5,500. Cooling degree days run 1,000 to 2,000. Heating drives equipment selection and runtime in zone 4A. Cooling capacity is a secondary concern.
Which cities are in climate zone 4A?
Major US cities in IECC climate zone 4A include Richmond, Louisville, Kansas City, St. Louis, Baltimore. 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 4A require?
IECC code-required insulation for climate zone 4A: walls R-13 to R-19, ceiling R-38 to R-49, 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 4A?
Common equipment choices for climate zone 4A include Heat pumps with backup heat, Gas furnaces with AC, Dual-fuel systems, Central dehumidification. 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 4A?
Use the climate zone 4A design temperatures (winter 0°F to 20°F, summer 80°F to 90°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 4A?
Most states in climate zone 4A 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 80+ for furnaces. Duct testing requirements.