Mild temperatures with marine influence. Moderate heating needs, minimal cooling. Climate zone 4C covers parts of Washington, Oregon, including cities like Seattle, Portland, Olympia, Salem. Heating drives equipment selection and runtime in zone 4C. Cooling capacity is a secondary concern.
Climate zone 4C is the Pacific Northwest marine: Seattle, Portland, Olympia, Salem, and the I-5 corridor west of the Cascades. Winters are cool but mild (20 to 35°F design temps), summers are short and historically pleasant, and humidity stays moderate year-round thanks to ocean influence. Two big shifts have reshaped HVAC here in the past five years: rapidly growing heat pump adoption driven by aggressive utility and state rebates, and the rise of summer AC retrofits forced by increasingly severe heat waves and wildfire smoke seasons.
The PNW Heat Pump Adoption Surge
Seattle and Portland sit at the leading edge of US heat pump adoption. The City of Seattle's Clean Heat Program, Portland's Cooling Portland program (which added $10.3 million in PCEF funding to serve 10,000 additional households through 2026), and Oregon's Heat Pump Purchase Program stack on top of federal rebates to make heat pump installs cost-competitive with gas furnace replacements. King County's Energize Program is funding another $6 million in technical assistance and retrofits through 2029.
The climate also favors heat pumps: zone 4C winter design temps sit well above the threshold where cold-climate heat pumps start to lose meaningful capacity. A standard heat pump with no backup heat at all covers virtually all annual heating need in coastal Washington and Oregon, which removes the dual-fuel complication that drives equipment cost up in colder zones.
Heat Waves and the AC Retrofit Reality
Until roughly 2015, most Pacific Northwest housing was built with no cooling at all. The 2021 heat dome that pushed Portland to 116°F and Seattle to 108°F killed more than 100 people in the region, mostly in homes with no AC. Cooling Portland and Seattle Clean Heat program data show that 60+ percent of new heat pump installs in the metro area are retrofits into homes that previously had gas or oil heat and no cooling at all.
Heat pump installs solve both problems at once: a single piece of equipment provides the new cooling capability and a major heating efficiency upgrade. Cost ranges $9,000 to $16,000 for a ducted system, $7,500 to $12,000 for a 2 to 3 zone ductless mini-split if the home doesn't have existing ductwork.
Wildfire Smoke and Filter Specifications
Wildfire smoke season (typically late August through October) now forces zone 4C homeowners to think about indoor air quality at a level Florida or Boston don't have to. Standard MERV 8 furnace filters don't capture wildfire smoke particulates (PM2.5). ASHRAE and EPA both recommend MERV 13 minimum as the practical bar for smoke filtration without overwhelming residential blower motors.
Most modern heat pump air handlers in PNW installs ship with 4-inch media filter cabinets that accept MERV 13 or higher. Adding a portable HEPA air cleaner ($150 to $500) for the bedroom during heavy smoke days is the practical layered approach. Sealing the home up during smoke events also requires running mechanical ventilation with carbon filtration to prevent CO2 buildup, which is a step beyond standard residential HVAC scope.
- Most ducted heat pump installs in Seattle/Portland: $9,000 to $16,000
- MERV 13 minimum filter rating for wildfire smoke (EPA and ASHRAE)
- Heat pumps cover ~100% of annual heating load in zone 4C with no backup needed
- PCEF Cooling Portland adds $10.3M to serve 10,000+ households through 2026
- Seattle Clean Heat and King County Energize Programs layer with NEEP and federal IRA rebates
Design Temperatures for Zone 4C
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 | 20°F to 35°F | Heating equipment must hold setpoint at this outdoor temp. |
| Summer design temp | 70°F to 80°F | Cooling equipment must hold setpoint at this outdoor temp. |
| Heating degree days | 3,000 to 5,000 | Higher numbers mean longer, colder winters and more heating runtime. |
| Cooling degree days | 0 to 500 | Higher numbers mean longer, hotter summers and more AC runtime. |
| Load priority | Heating dominated | Heating drives equipment selection and runtime in zone 4C. Cooling capacity is a secondary concern. |
Cities in Climate Zone 4C
These US cities are typically classified as climate zone 4C. Zone boundaries follow county lines, so suburbs of these cities sometimes fall into adjacent zones.
Insulation Requirements for Zone 4C
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-20 | 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 4C
These are the system types that fit zone 4C 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 4C |
|---|---|
| Heat pumps | Heating priority |
| Gas furnaces | Minimal cooling needs |
| Ductless mini-splits | High humidity management |
| Radiant heating systems | Corrosion resistance |
Key HVAC Design Considerations in Zone 4C
These are the design issues that come up most in climate zone 4C:
- Heating priority
- Minimal cooling needs
- High humidity management
- Corrosion resistance
- Mold prevention
Energy Code Rules for Zone 4C
Most states in zone 4C have adopted the IECC for residential construction. Equipment efficiency rules typically include:
- SEER 13+ minimum
- HSPF 8.2+ for heat pumps
- AFUE 80+ for furnaces
- Ventilation 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 4C Areas
These states have counties classified as climate zone 4C. Not every county in these states is zone 4C, so check the IECC map for your specific county.
Frequently Asked Questions
What does IECC climate zone 4C mean?
Climate zone 4C is mixed and marine. Mild temperatures with marine influence. Moderate heating needs, minimal cooling.
What are the design temperatures for climate zone 4C?
In climate zone 4C, the summer design temperature runs 70°F to 80°F and the winter design temperature runs 20°F to 35°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 4C heating or cooling dominated?
Climate zone 4C is generally heating dominated. Heating degree days run 3,000 to 5,000. Cooling degree days run 0 to 500. Heating drives equipment selection and runtime in zone 4C. Cooling capacity is a secondary concern.
Which cities are in climate zone 4C?
Major US cities in IECC climate zone 4C include Seattle, Portland, Olympia, Salem. 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 4C require?
IECC code-required insulation for climate zone 4C: walls R-15 to R-20, 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 4C?
Common equipment choices for climate zone 4C include Heat pumps, Gas furnaces, Ductless mini-splits, Radiant heating systems. 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 4C?
Use the climate zone 4C design temperatures (winter 20°F to 35°F, summer 70°F to 80°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 4C?
Most states in climate zone 4C have adopted some version of the IECC (International Energy Conservation Code) for residential construction. Equipment efficiency rules apply: SEER 13+ minimum. HSPF 8.2+ for heat pumps. AFUE 80+ for furnaces. Ventilation requirements.