Introduction #

HVAC capacity calculation theory is essential, but practical examples make the concepts concrete. This guide provides detailed, step-by-step calculations for various facility types, from small offices to large manufacturing plants. Each example includes actual building data, load components, diversity factors, and final equipment selection.

Example 1: Small Office Space #

Facility Overview #

  • Type: Office space
  • Size: 200 m² (20m × 10m)
  • Ceiling Height: 3 meters
  • Occupancy: 15 people
  • Location: Moderate climate
  • Windows: 30 m² (south-facing)

Step 1: Calculate Space Volume #

Volume = Length × Width × Height
Volume = 20 × 10 × 3 = 600 m³

Step 2: Building Envelope Load #

Wall Area:

Walls: 2 × (20 + 10) × 3 = 180 m²
U-value: 0.3 W/m²·K (insulated)
Temperature difference: 10°C (indoor 22°C, outdoor 32°C)

Wall load = 0.3 × 180 × 10 = 540 W = 0.54 kW

Roof Load:

Roof area: 20 × 10 = 200 m²
U-value: 0.2 W/m²·K (well-insulated)
Temperature difference: 15°C

Roof load = 0.2 × 200 × 15 = 600 W = 0.60 kW

Window Load:

Window area: 30 m²
U-value: 2.8 W/m²·K (double-pane)
Temperature difference: 10°C

Window load = 2.8 × 30 × 10 = 840 W = 0.84 kW

Total Envelope Load:

0.54 + 0.60 + 0.84 = 1.98 kW

Step 3: Solar Heat Gain #

Window area: 30 m² (south-facing)
SHGC: 0.5
Shading: 0.7 (30% reduction)
Solar factor: 800 W/m² (south, peak)

Solar gain = 30 × 0.5 × 0.7 × 800 = 8,400 W = 8.4 kW

Step 4: Internal Heat Gains #

People Load:

Occupancy: 15 people (seated, office work)
Sensible: 15 × 70W = 1,050 W = 1.05 kW
Latent: 15 × 60W = 900 W = 0.90 kW

Lighting Load:

Total lighting: 3 kW
Usage factor: 0.9
Heat gain: 3 × 0.9 × 1.0 = 2.7 kW

Equipment Load:

Computers, printers: 4 kW
Usage factor: 0.8
Heat gain: 4 × 0.8 × 0.9 = 2.88 kW

Total Internal Gains:

Sensible: 1.05 + 2.7 + 2.88 = 6.63 kW
Latent: 0.90 kW
Total: 7.53 kW

Step 5: Ventilation Load #

Occupancy: 15 people
Ventilation rate: 0.01 m³/s per person
Total ventilation: 15 × 0.01 = 0.15 m³/s

Sensible: 1.2 × 0.15 × 1.006 × 10 = 1.81 kW
Latent: 1.2 × 0.15 × 2,500 × 0.008 = 3.6 kW

Step 6: Sum All Loads #

Load Component Sensible (kW) Latent (kW) Total (kW)
Envelope 1.98 0 1.98
Solar 8.40 0 8.40
People 1.05 0.90 1.95
Lighting 2.70 0 2.70
Equipment 2.88 0 2.88
Ventilation 1.81 3.60 5.41
Total 18.82 4.50 23.32 kW

Step 7: Convert to Tons and Select Equipment #

Total cooling: 23.32 kW
Convert to tons: 23.32 ÷ 3.517 = 6.63 tons
Select: 7.5 tons (next standard size)

Example 2: Manufacturing Workshop #

Facility Overview #

  • Type: Manufacturing workshop
  • Size: 1,000 m² (40m × 25m)
  • Ceiling Height: 5 meters
  • Occupancy: 20 people
  • Equipment: 150 kW motors
  • Process Heat: 20 kW

Step 1: Calculate Space Volume #

Volume = 40 × 25 × 5 = 5,000 m³

Step 2: Building Envelope Load #

Wall Load:

Walls: 2 × (40 + 25) × 5 = 650 m²
U-value: 0.4 W/m²·K (moderate insulation)
Temperature difference: 15°C

Wall load = 0.4 × 650 × 15 = 3,900 W = 3.9 kW

Roof Load:

Roof: 40 × 25 = 1,000 m²
U-value: 0.3 W/m²·K
Temperature difference: 20°C

Roof load = 0.3 × 1,000 × 20 = 6,000 W = 6.0 kW

Total Envelope:

3.9 + 6.0 = 9.9 kW

Step 3: Internal Heat Gains #

People:

20 people (light work)
Sensible: 20 × 100W = 2.0 kW
Latent: 20 × 120W = 2.4 kW

Equipment:

Motors: 150 kW
Usage factor: 0.7
Heat gain factor: 0.85
Equipment load = 150 × 0.7 × 0.85 = 89.25 kW

Process Heat:

Process: 20 kW
Usage factor: 0.6
Process load = 20 × 0.6 = 12.0 kW

Lighting:

Lighting: 15 kW
Usage: 0.9
Lighting load = 15 × 0.9 = 13.5 kW

Total Internal:

Sensible: 2.0 + 89.25 + 12.0 + 13.5 = 116.75 kW
Latent: 2.4 kW
Total: 119.15 kW

Step 4: Ventilation Load #

Ventilation: 20 × 0.015 = 0.3 m³/s
Sensible: 1.2 × 0.3 × 1.006 × 15 = 5.43 kW
Latent: 1.2 × 0.3 × 2,500 × 0.010 = 9.0 kW

Step 5: Sum All Loads #

Load Component Sensible (kW) Latent (kW) Total (kW)
Envelope 9.9 0 9.9
People 2.0 2.4 4.4
Equipment 89.25 0 89.25
Process 12.0 0 12.0
Lighting 13.5 0 13.5
Ventilation 5.43 9.0 14.43
Total 132.08 11.4 143.48 kW

Step 6: Convert to Tons and Select #

Total cooling: 143.48 kW
Convert to tons: 143.48 ÷ 3.517 = 40.8 tons
Select: 45 tons (with 10% margin)

Example 3: Warehouse with Office #

Facility Overview #

  • Type: Warehouse with attached office
  • Warehouse: 2,000 m² (50m × 40m), 6m height
  • Office: 200 m² (20m × 10m), 3m height
  • Occupancy: 5 people (warehouse), 10 people (office)
  • Minimal equipment in warehouse

Warehouse Calculation #

Volume:

50 × 40 × 6 = 12,000 m³

Envelope Load:

Walls: 2 × (50 + 40) × 6 = 1,080 m²
U-value: 0.5 W/m²·K (minimal insulation)
Temperature difference: 20°C

Wall load = 0.5 × 1,080 × 20 = 10,800 W = 10.8 kW

Roof: 50 × 40 = 2,000 m²
Roof load = 0.5 × 2,000 × 25 = 25,000 W = 25.0 kW

Total envelope: 35.8 kW

Internal Gains:

People: 5 × 150W = 0.75 kW
Lighting: 10 kW × 0.9 = 9.0 kW
Total: 9.75 kW

Ventilation:

5 × 0.005 = 0.025 m³/s
Sensible: 1.2 × 0.025 × 1.006 × 20 = 0.60 kW

Warehouse Total:

35.8 + 9.75 + 0.60 = 46.15 kW = 13.1 tons

Office Calculation #

Volume:

20 × 10 × 3 = 600 m³

Envelope Load:

Similar to Example 1: 1.98 kW

Internal Gains:

People: 10 × 70W = 0.70 kW
Lighting: 2 kW × 0.9 = 1.8 kW
Equipment: 3 kW × 0.8 = 2.4 kW
Total: 4.9 kW

Ventilation:

10 × 0.01 = 0.1 m³/s
Sensible: 1.2 × 0.1 × 1.006 × 10 = 1.21 kW
Latent: 1.2 × 0.1 × 2,500 × 0.008 = 2.4 kW

Office Total:

1.98 + 4.9 + 1.21 + 2.4 = 10.49 kW = 3.0 tons

Combined System #

Warehouse: 13.1 tons
Office: 3.0 tons
Total: 16.1 tons
Select: 20 tons (with margin)

Frequently Asked Questions #

Q1: What's the typical cooling load per square meter? #

A: Varies by facility type:

  • Office: 80-120 W/m²
  • Manufacturing: 150-300 W/m²
  • Warehouse: 30-60 W/m²

Always verify with detailed calculations.

Q2: How do I account for process heat? #

A: Add process heat as internal gain:

  • Use actual process heat output
  • Apply usage factor (0.6-0.8 typical)
  • Add to sensible load

Q3: What's the difference between sensible and latent load? #

A:

  • Sensible: Changes temperature (measured in kW)
  • Latent: Removes moisture (measured in kW)

Total cooling = Sensible + Latent

Q4: How do I convert kW to tons? #

A:

  • 1 ton = 3.517 kW
  • kW ÷ 3.517 = tons

Q5: Should I size for peak conditions? #

A: Yes, size for design peak conditions (typically 1-2% of time). This ensures system can handle worst-case scenarios.

Q6: How much safety margin should I add? #

A: Typical margins:

  • 10-15%: Standard applications
  • 15-20%: Variable loads or growth plans
  • 20-25%: Critical applications

Conclusion #

These examples demonstrate practical HVAC capacity calculations for various facility types. Key components:

  • Building envelope: Walls, roof, windows
  • Solar gains: Through windows and roof
  • Internal gains: People, lighting, equipment, process
  • Ventilation: Outdoor air requirements
  • Diversity factors: Not all loads simultaneous

Use the HVAC Capacity Calculator to quickly estimate capacity, but always verify with detailed calculations for final equipment selection.