Calculating AHU Tonnage for Commercial Fit outs -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

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Jun 20, 2024

Calculating AHU Tonnage for Commercial Fit outs

Accurately calculating the Air Handling Unit (AHU) tonnage is essential for maintaining a comfortable indoor environment in commercial spaces. This guide will walk you through the detailed process of calculating AHU tonnage, including important considerations and a helpful thumb rule table for quick reference.

Calculating AHU Tonnage for Commercial Fit outs

Key Factors in AHU Tonnage Calculation

  1. Space Area (Square Feet)
    • The total area of the space that needs to be conditioned.
  2. Heat Load Factors
    • Occupancy Load: Heat generated by occupants.
    • Equipment Load: Heat generated by office equipment and machinery.
    • Lighting Load: Heat generated by lighting fixtures.
    • Solar Heat Gain: Heat gained through windows and walls exposed to sunlight.
    • Ventilation Load: Heat gain or loss due to fresh air ventilation requirements.
  3. Cooling Load Calculation (BTU/hr)
    • The total cooling load in BTU/hr is the sum of all heat load factors.
  4. Conversion to Tonnage
    • 1 ton of cooling is equivalent to 12,000 BTU/hr.

Detailed Steps for AHU Tonnage Calculation

  1. Calculate Space Area
    • Determine the total square footage (sq ft) of the area to be conditioned.
  2. Estimate Heat Loads
    • Occupancy Load: Generally, each person adds approximately 400 BTU/hr. Occupancy Load = Number of Occupants×400 BTU/hr
    • Equipment Load: List all equipment and their respective heat loads. Equipment Load=∑Equipment Power (W)×3.412 BTU/hr
    • Lighting Load: Sum the wattage of all lighting fixtures and convert to BTU/hr. Lighting Load=∑Lighting Power (W)×3.412 BTU/hr
    • Solar Heat Gain: Estimate based on window area and orientation. Use appropriate multipliers for direct sunlight exposure. Solar Heat Gain=Window Area (sq ft)×Solar Factor (BTU/hr/sq ft)
    • Ventilation Load: Account for the heat load due to fresh air intake. This includes sensible and latent heat components.
  3. Sum of Heat Loads
    • Add up all the calculated heat loads to get the total cooling load in BTU/hr. Total Cooling Load=Occupancy Load+Equipment Load+Lighting Load+Solar Heat Gain+Ventilation Load
  4. Convert Total Cooling Load to Tonnage
    • Divide the total cooling load in BTU/hr by 12,000 to convert to tons. Required Tonnage=Total Cooling Load (BTU/hr)/12,000

Practical Example

Let’s calculate the AHU tonnage for a 10,000 sq ft office space with the following details:

  • Occupants: 50 people
  • Equipment Load: 10 kW (10,000 W)
  • Lighting Load: 5 kW (5,000 W)
  • Window Area: 500 sq ft with a solar factor of 50 BTU/hr/sq ft
  • Ventilation Load: 20,000 BTU/hr
  1. Occupancy Load

Occupancy Load=50×400=20,000 BTU/hr

  1. Equipment Load

Equipment Load=10,000×3.412=34,120 BTU/hr

  1. Lighting Load

Lighting Load=5,000×3.412=17,060 BTU/hr

  1. Solar Heat Gain

Solar Heat Gain=500×50=25,000 BTU/hr

  1. Total Cooling Load

Total Cooling Load=20,000+34,120+17,060+25,000+20,000=116,180 BTU/hr

  1. Required Tonnage

Required Tonnage=116,18012,000≈9.68 tons

Thumb Rule Table for Quick Reference

For quick estimations, the following thumb rule table can be used. Note that these are general guidelines and actual requirements may vary based on specific conditions.

Type of Space

Area per Ton (sq ft/ton)

Office Space

200 - 300

Retail Space

250 - 350

Server Rooms

100 - 150


150 - 250

Conference Rooms

100 - 200

Industrial Spaces

300 - 400


Accurate AHU tonnage calculation ensures efficient and effective HVAC system performance in commercial fitouts. By considering all heat load factors and using detailed calculations, you can design a system that meets the cooling demands of your space. For quick estimates, refer to the thumb rule table, but always verify with detailed calculations for precise requirements.


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