Climate Zone 4C - Mixed, Marine
Mild temperatures with marine influence. Moderate heating needs, minimal cooling. This climate zone encompasses parts of Washington, Oregon and includes major cities like Seattle, Portland, Olympia, Salem.
Common Questions About Zone 4C
Zone 4C definition
This page explains what IECC climate zone 4C means in HVAC and building-code terms.
Cities in zone 4C
Use the city examples below to answer common lookup searches for this zone.
Heating and cooling profile
See whether zone 4C is heating-dominant, cooling-dominant, or balanced.
HVAC requirements
Compare design temperatures, degree days, code requirements, and equipment guidance for this zone.
What IECC Climate Zone 4C Means
Temperature class
Mixed (Zone 4)
Moisture class
Marine (C)
Load priority
Heating-dominant
Quick definition
Mild temperatures with marine influence. Moderate heating needs, minimal cooling.
Common City Lookups for Zone 4C
| City | IECC zone answer | Why it matters |
|---|---|---|
| Seattle | Seattle is commonly treated as IECC climate zone 4C in this guide. | Heating priority |
| Portland | Portland is commonly treated as IECC climate zone 4C in this guide. | Heating priority |
| Olympia | Olympia is commonly treated as IECC climate zone 4C in this guide. | Heating priority |
| Salem | Salem is commonly treated as IECC climate zone 4C in this guide. | Heating priority |
Design Temperatures
Winter Design Temperatures
Used for heating load calculations and equipment sizing
Summer Design Temperatures
Used for cooling load calculations and equipment sizing
Heating & Cooling Degree Days
Heating Degree Days (HDD)
Base 65°F. Higher numbers indicate more heating requirements and longer heating seasons.
Cooling Degree Days (CDD)
Base 65°F. Higher numbers indicate more cooling requirements and longer cooling seasons.
HVAC System Considerations
Key Considerations
- •Heating priority
- •Minimal cooling needs
- •High humidity management
- •Corrosion resistance
- •Mold prevention
Common Equipment
- ✓Heat pumps
- ✓Gas furnaces
- ✓Ductless mini-splits
- ✓Radiant heating systems
Energy Code Requirements
Equipment Efficiency
- ⚠SEER 13+ minimum
- ⚠HSPF 8.2+ for heat pumps
- ⚠AFUE 80+ for furnaces
- ⚠Ventilation requirements
Insulation Requirements
States with Zone 4C Areas
Quick Reference
Calculate Load
Use our load calculators with the design temperatures and requirements specific to Zone 4C.