Understanding where heat enters your room is crucial for properly sizing your AC. Different heat sources require different approaches to cooling.
Types of Heat Gain
External (Outdoor) Heat
- Solar radiation through windows (biggest source in summer)
- Heat conducted through walls and roof
- Warm air infiltration from doors/cracks
- Outdoor humidity increasing latent load
Internal Heat Sources
- People (600 BTU/person/hour)
- Lighting (all heat from incandescent bulbs)
- Appliances (stoves, refrigerators, electronics)
- Computers and servers
- Moisture from showers and cooking
Solar Heat Gain (Biggest Factor)
Solar radiation accounts for 30-50% of summer cooling load:
- West windows: Intense afternoon sun
- South windows: All-day sun exposure
- East windows: Morning sun
- North windows: Minimal direct sun
Reducing Solar Heat
- Install awnings or overhangs
- Use reflective window film
- Keep curtains/blinds closed
- Plant shade trees
Conductive Heat Through Walls and Roof
Heat conducted from outside depends on:
- Insulation quality: R-value rating
- Wall color: Dark colors absorb more heat
- Roofing material: Light colors reflect, dark absorbs
- Wall materials: Concrete conducts more than wood
A well-insulated wall might conduct 10 BTU/sq ft/day; a poor one conducts 50+ BTU/sq ft/day.
Internal Heat Sources
People
- 600 BTU/hour per person (sensible + latent)
- Sitting generates 300-400 BTU
- Light activity generates 600-800 BTU
- Heavy activity generates 1000+ BTU
Lighting
- Incandescent bulbs: 90% energy becomes heat
- LED bulbs: Only 10% becomes heat
- Fluorescent: 40-50% becomes heat
- Upgrading to LED significantly reduces cooling load
Kitchen Equipment
- Cooking appliances: 4,000-12,000 BTU/hour
- Open-flame stove: Adds significant latent heat
- Refrigerator: 1,000-2,000 BTU/hour continuous
- Dishwasher: 2,000-3,000 BTU/hour when running
Electronics
- Computer: 400-800 BTU/hour
- TV: 100-400 BTU/hour
- Multiple devices can significantly impact load
Infiltration and Ventilation
Air Leaks
- Poorly sealed doors and windows
- Can account for 10-30% of cooling load
- Sealing leaks is cost-effective
Required Ventilation
- Building codes require fresh outside air
- Brings in warm, humid air
- Increases cooling load proportionally
Sensible vs Latent Heat
Sensible Heat (Temperature Change)
- Heat that changes air temperature
- Measured by thermometer
- 30-70% of total cooling load
Latent Heat (Moisture Removal)
- Heat from water vapor (humidity)
- Not shown on thermometer
- 30-70% of total cooling load
- Higher in humid climates
Calculating Total Heat Gain
- Solar gain through windows
- Conductive gain through walls/roof
- Internal heat from people, lights, appliances
- Infiltration from door/window leaks
- Latent heat from humidity
Conclusion
Multiple heat sources contribute to your cooling load. Understanding these sources helps you minimize unnecessary load (reduce solar gain, improve insulation, upgrade lighting) and properly size your AC for actual needs.