BTU (British Thermal Unit)

What is a BTU?

BTU stands for British Thermal Unit. It is a unit of heat energy that indicates how much heat an air conditioner can remove (or a heater can add) in one hour. In HVAC, BTU/hr is used to express cooling and heating capacity.

BTU and Cooling Tons

• 1 ton of cooling = 12,000 BTU/hr
• Convert BTU to tons by dividing by 12,000 (e.g., 24,000 BTU ≈ 2 tons)

Why BTU Matters in AC Sizing

• Comfort: The right BTU rating prevents rooms from feeling hot, humid, or unevenly cooled.
• Efficiency: Oversized systems short-cycle and waste energy; undersized systems run constantly and still struggle.
• Humidity Control: Proper sizing helps maintain sensible and latent cooling for comfort.

Quick Rules of Thumb

• Start with ~20 BTU per square foot for typical residential spaces with 8-foot ceilings.
• Increase capacity for taller ceilings, high sun exposure, large windows, heavy occupancy, or kitchens.
• Decrease capacity for well-insulated, shaded, or tightly sealed spaces.

When to Go Beyond Rules of Thumb

• Use a Manual J load calculation for precise sizing, especially on new installs, additions, or high-performance homes.
• Consider insulation levels, infiltration rates, window area/orientation, shading, climate zone, and duct performance.

How to Use BTU Estimates

1. Calculate a baseline BTU/hr using square footage and ceiling height.
2. Adjust for sunlight, occupants, and internal heat gains (kitchens, electronics).
3. Convert to tons if needed and compare against equipment specs and efficiency ratings.
4. Verify with a detailed load calculation before final equipment selection.