Why AC Sizing Matters
Installing the wrong size air conditioner is one of the costliest HVAC mistakes. An improperly sized system will:
- Cost more to operate
- Wear out faster
- Provide poor comfort
- Create humidity problems
- Void manufacturer warranties
Yet many Bay Area contractors still use outdated “rules of thumb” instead of proper load calculations. Here’s what you need to know.
The Myth of “Bigger is Better”
Many homeowners assume a larger AC cools better. This is false.
An oversized AC:
- Cycles on/off rapidly (short-cycling)
- Never runs long enough to remove humidity
- Creates temperature swings
- Wastes energy starting up repeatedly
- Wears out components faster
- Costs more upfront
An undersized AC:
- Runs constantly
- Can’t keep up on hot days
- Costs more to operate
- Wears out from overwork
The goal: Perfectly matched capacity to your home’s cooling needs.
Manual J Load Calculation: The Right Way
Professional HVAC contractors use Manual J load calculations — an engineering method that considers:
1. Square Footage
Base measurement, but only one factor among many.
2. Climate Zone
Bay Area microclimates vary dramatically:
- Coastal: Low cooling needs
- East Bay valleys: Substantial cooling needs
- Inland areas: Highest cooling requirements
3. Home Orientation
- South-facing homes absorb more heat
- East-facing bedrooms get morning sun
- West-facing rooms get intense afternoon heat
4. Insulation Levels
- Attic insulation (R-value)
- Wall insulation quality
- Floor/crawlspace insulation
- Air sealing effectiveness
5. Window Characteristics
- Total window square footage
- Window quality (single, double, triple pane)
- Low-E coatings
- Sun exposure per window
6. Home Construction
- Roof color (light vs. dark)
- Exterior wall materials
- Roof type and ventilation
- Cathedral ceilings or standard
7. Internal Heat Sources
- Number of occupants
- Appliances generating heat
- Home office equipment
- Lighting type (LED vs. incandescent)
8. Ductwork Condition
- Duct sizing
- Duct insulation
- Air leakage rate
- Location (attic vs. conditioned space)
Tonnage Explained
AC capacity is measured in “tons” — not weight, but cooling capacity.
One ton = 12,000 BTU/hour of cooling
Common residential sizes:
- 1.5 tons (18,000 BTU): Small homes, apartments
- 2 tons (24,000 BTU): 1,000-1,400 sq ft homes
- 2.5 tons (30,000 BTU): 1,400-1,800 sq ft homes
- 3 tons (36,000 BTU): 1,800-2,200 sq ft homes
- 3.5 tons (42,000 BTU): 2,200-2,600 sq ft homes
- 4 tons (48,000 BTU): 2,600-3,000 sq ft homes
- 5 tons (60,000 BTU): 3,000+ sq ft homes
Bay Area-Specific Considerations
Coastal Homes (SF, Daly City, Pacifica)
Typical needs: 300-500 sq ft per ton
- Mild summer temperatures
- Natural fog cooling
- Many homes don’t need AC at all
- When installed, often 1.5-2.5 tons total
Moderate Zones (Oakland, Berkeley, San Mateo)
Typical needs: 400-600 sq ft per ton
- Occasional hot days
- Moderate cooling needs
- 2-3 tons for average homes
Inland Valleys (Walnut Creek, Concord, Dublin, Livermore)
Typical needs: 500-700 sq ft per ton
- Regular 90-100°F+ summer days
- Higher cooling capacity needed
- 3-5 tons for larger homes
South Bay (San Jose, Sunnyvale, Santa Clara)
Typical needs: 450-650 sq ft per ton
- Warm summers
- Less marine influence than SF
- 2.5-4 tons typical
The Danger of “Rules of Thumb”
Some contractors still use outdated shortcuts:
“1 ton per 500 square feet"
"1 ton per 600 square feet"
"Same size as old system”
These ignore critical factors like:
- Your specific Bay Area microclimate
- Home improvements since last installation
- Actual insulation and window quality
- Ductwork condition
- Indoor vs. outdoor unit matching
Result: Wrong-sized system, unhappy homeowner.
Multi-Zone Homes
Some Bay Area homes have dramatically different cooling needs:
- East-facing bedrooms: Cool mornings, warm afternoons
- West-facing living rooms: Hot late afternoon
- Upstairs vs. downstairs: Heat rises
- Sun rooms or additions: Extra heat load
Solutions:
- Zoned system with dampers controlling airflow
- Multiple systems for extreme differences
- Ductless mini-splits for problem zones
Signs Your Current AC is Wrong Size
Too Large (Oversized):
- Runs for 5-10 minutes then shuts off
- House feels cold but humid/clammy
- Large temperature swings
- High electricity bills despite short run times
- Uneven cooling room-to-room
Too Small (Undersized):
- Runs constantly on hot days
- Never reaches set temperature
- Some rooms never cool properly
- High electricity bills from continuous operation
- System age faster than expected
What to Expect from Bay Area Climate Control
When we size your AC system, we:
1. Detailed Home Assessment
- Measure all conditioned spaces
- Evaluate insulation levels
- Assess window types and exposure
- Examine ductwork condition
- Note home orientation
2. Manual J Load Calculation
- Use industry-standard software
- Input all relevant variables
- Account for Bay Area microclimate
- Calculate room-by-room loads
3. Equipment Recommendation
- Match capacity to calculated load
- Recommend appropriate SEER rating
- Suggest zoning if beneficial
- Provide multiple options and pricing
4. Ductwork Evaluation
- Verify duct sizing adequate
- Check for leaks or damage
- Recommend modifications if needed
- Ensure proper airflow
Replacement Scenarios
Same Square Footage, Better Insulation
If you’ve added insulation or upgraded windows since your last AC installation, you likely need a smaller system now.
Home Addition
Added square footage? You might need:
- Larger central system, or
- Separate mini-split for addition, or
- Zoned system with multiple units
Changed Usage
- Converted garage to living space
- Added home office
- Enclosed patio
- Finished attic/basement
All affect cooling load and sizing requirements.
The Right Investment
Proper AC sizing requires:
- Time (comprehensive home evaluation)
- Expertise (Manual J calculations)
- Honesty (not upselling bigger equipment)
Cost: The right-sized system costs the same as the wrong-sized one.
Value: Comfort, efficiency, longevity — priceless.
Get a Proper Assessment
Bay Area Climate Control performs Manual J load calculations on every installation. We’ll never guess, estimate, or use rules of thumb with your comfort and investment.
Want to estimate your system size? Try our HVAC Sizing Calculator to get a preliminary estimate based on your home’s characteristics. Then contact us for a professional Manual J calculation.
Free in-home sizing assessment
Transparent calculations
Right-sized equipment recommendations
Call (510) 391-5597 or request your free assessment today.