How to Calculate Cooling Load: Formula & Example
Cooling Load Formula
The ASHRAE cooling load temperature difference (CLTD) method calculates the total heat that must be removed from a space. The cooling load has three main components:
Qtotal = Qenvelope + Qsolar + Qinternal
- Qenvelope = conduction through walls, roof, floor, and windows
- Qsolar = solar heat gain through windows (SHGC × Solar Heat Gain)
- Qinternal = heat from people (~600 BTU/h each), equipment, and lighting
Step-by-Step Example
Scenario: 250 sq ft office in Atlanta (Zone 3), 2 people, 2 computers, south-facing window.
Step 1: Envelope load = 250 sq ft × 20 BTU/sq ft = 5,000 BTU/h
Step 2: Solar gain = 30 sq ft window × 200 BTU/sq ft (south, clear) = 6,000 BTU/h
Step 3: People = 2 × 600 = 1,200 BTU/h
Step 4: Equipment = 2 × 400 = 800 BTU/h
Total: 5,000 + 6,000 + 1,200 + 800 = 13,000 BTU/h
Standards Reference
- ASHRAE Fundamentals — Chapter 18 (Nonresidential Cooling and Heating Loads)
- ASHRAE Fundamentals — Chapter 19 (Residential Cooling and Heating Loads)
- GB 50736-2012 — Design Code for HVAC of Civil Buildings (China)
- SHASE-S 101 — Load Calculation for Air Conditioning (Japan)
Frequently Asked Questions
How do you calculate cooling load?
Cooling load = Base index × Area × Orientation factor × Insulation factor × Window factor × Climate factor. The ASHRAE CLTD method accounts for solar heat gain, internal loads, and thermal mass.
What is the formula for cooling load?
Q = A × q × C_orientation × C_insulation × C_window × C_climate. Where A = area (m²), q = base cooling index (W/m²), and C = correction factors. This is the simplified unit index method from ASHRAE.
What is the difference between cooling load and heating load?
Cooling load includes solar heat gain and internal heat (people, equipment), which reduce the cooling needed from HVAC. Heating load ignores internal gains (they help heat the building) but must handle larger temperature differences in winter.