Heat Load Calculator

Understanding Heat Load

Heat load calculation is essential for proper HVAC sizing. Underestimating leads to inadequate cooling, while overestimating wastes energy. A thorough heat load analysis considers all sources of heat entering or generated within a space.

Components of Heat Load

Total heat load consists of external and internal sources:

• Transmission Load: Heat transfer through walls, roof, floor, and windows
• Solar Load: Heat from sunlight entering through windows
• Occupant Load: Body heat from people (400-600 BTU per person)
• Equipment Load: Heat from appliances, computers, lighting
• Ventilation Load: Heat from fresh air intake
• Infiltration Load: Heat from air leaking through cracks

Transmission Heat Gain

Heat enters through the building envelope based on construction materials:

• Concrete walls: 0.8-1.2 BTU/sq ft/°F difference
• Insulated walls: 0.3-0.5 BTU/sq ft/°F
• Single-pane glass: 1.0-1.3 BTU/sq ft/°F
• Double-pane glass: 0.5-0.7 BTU/sq ft/°F
• Uninsulated roof: 0.8-1.0 BTU/sq ft/°F

Solar Heat Gain

Windows facing different directions have varying solar loads:

• North-facing: Minimal (10-20 BTU/sq ft)
• East-facing: Morning heat (40-60 BTU/sq ft)
• South-facing: All-day heat (60-80 BTU/sq ft)
• West-facing: Afternoon heat, worst case (80-100 BTU/sq ft)

Internal Heat Gains

Heat generated inside the space:

• People: 400 BTU (seated), 600 BTU (active) per person
• Lighting: 3.4 BTU per watt
• Computers: 500-800 BTU per desktop
• Appliances: Check wattage × 3.4 = BTU

Sample Heat Load Calculation

For a 200 sq ft office with 2 people and 2 computers:

• Envelope: 200 × 25 = 5,000 BTU
• Solar (West window): 20 sq ft × 80 = 1,600 BTU
• People: 2 × 500 = 1,000 BTU
• Computers: 2 × 600 = 1,200 BTU
• Lighting: 100W × 3.4 = 340 BTU
• Total: 9,140 BTU (0.76 ton)