Morgan Hill's Climate Zone 4 Designation and What It Means for HVAC

What the City's designation actually says

The City of Morgan Hill's Adopted Codes & Design Criteria page lists the 2025 California Building, Residential, Plumbing, Mechanical, Electrical, Fire, Energy, and CalGreen codes, and identifies Morgan Hill's Design Criteria Climate Zone as Climate Zone 4. That's the official local source for this classification, and it's the basis for the design conditions used in code compliance and energy calculations for the area.

What a climate zone is used for

A climate zone designation feeds into the outdoor design temperatures and energy code requirements used when a Manual J load calculation is prepared for a specific home. It is not, by itself, a statement about how hot or humid conditions typically are, and it does not translate into a specific equipment type, tonnage, or efficiency tier. Two homes in the same climate zone can have very different calculated loads depending on their own envelope, windows, orientation, shading, and infiltration.

What actually determines equipment type and size

A home-specific Manual J load calculation — covered in detail in our HVAC sizing page — accounts for the building envelope, window area and orientation, insulation levels, air infiltration, internal heat gains, and the outdoor design temperatures applicable to the project. That calculation, not the climate zone alone, is what determines the heating and cooling capacity a system needs to meet.

Equipment type is a separate decision that also depends on factors like available electrical capacity, fuel source, ductwork condition, and homeowner priorities. See our heat pump vs. furnace-and-AC comparison for how those factors are weighed.

Ductwork, orientation, and shading

Do not assume duct location, insulation, glazing, shading, or construction details from the neighborhood or apparent age of a home — check the actual house. Where ductwork runs through an attic, it can be exposed to higher temperatures, and leakage or thin insulation can reduce delivered capacity. Window area, orientation, and shading are also inputs a load calculation accounts for. Each of these is a reason a load calculation for your specific home — rather than a climate-zone or square-footage shortcut — is the right basis for equipment selection, as covered in our HVAC sizing page and our ductwork page.