Proper AC sizing is crucial for comfort, efficiency, and equipment longevity. Both undersized and oversized units cause problems.
Why Size Matters
Undersized AC - Runs constantly without reaching set temperature - Can't keep up on hot days - Higher energy bills from continuous operation - Excessive wear from overwork
Oversized AC - Short cycles (turns on/off frequently) - Doesn't run long enough to dehumidify - Hot and cold spots - Increased wear from frequent starts - Higher upfront cost
Sizing Basics: BTUs and Tons
Air conditioners are rated in BTUs (British Thermal Units) per hour or tons of cooling capacity.
- 1 ton = 12,000 BTUs/hour
- Residential systems typically range from 1.5 to 5 tons
Rules of Thumb (Use Cautiously)
A rough estimate is 20 BTUs per square foot: - 1,000 sq ft = 20,000 BTUs (1.5-2 tons) - 1,500 sq ft = 30,000 BTUs (2.5 tons) - 2,000 sq ft = 40,000 BTUs (3.5 tons)
Important: These are rough guidelines only. Many factors affect actual requirements.
Factors That Affect Sizing
1. Climate Bay Area homes need less cooling than Phoenix or Houston homes of the same size.
2. Insulation Well-insulated homes require smaller systems. Older homes with poor insulation need more capacity.
3. Windows - Number, size, and orientation - Single vs. double-pane - Window treatments
South and west-facing windows add significant heat load.
4. Ceiling Height Standard calculations assume 8-foot ceilings. Taller ceilings increase volume and capacity needs.
5. Sun Exposure Shaded homes require less cooling than homes with full sun exposure.
6. Occupancy More people = more heat. Each person adds about 600 BTUs/hour.
7. Heat-Generating Appliances Kitchens, laundry rooms, and home offices with equipment add heat load.
8. Ductwork Leaky or poorly insulated ducts require larger systems to compensate for losses.
Manual J: The Professional Method
HVAC professionals use Manual J calculations—a comprehensive methodology considering all factors above plus: - Wall construction - Roof type and color - Foundation type - Infiltration rates - Internal heat gains
Manual J produces accurate sizing specific to your home.
Why DIY Sizing Falls Short
Online calculators and rules of thumb often oversize. Contractors sometimes oversize "to be safe." Neither approach is optimal.
A professional load calculation costs $100-$200 but ensures proper sizing for your specific home.
Getting the Right Size
- Request Manual J calculations from contractors
- Get multiple quotes and compare recommendations
- Be wary of significantly different sizing suggestions
- Don't accept "bigger is better" arguments
Signs Your Current System Is Wrong Size
Undersized: - Home never reaches set temperature on hot days - System runs constantly - Higher than expected bills
Oversized: - Home feels clammy/humid - System cycles every few minutes - Uneven temperatures
Proper sizing is worth the effort. It ensures comfort, efficiency, and equipment longevity for the 15-20 year life of your system.