Radiation is heat transfer through electromagnetic waves that requires no medium and occurs across empty space. Unlike conduction and convection, radiative heat transfer depends on surface emissivity and absolute temperature, following the Stefan-Boltzmann law. In HVAC systems, radiation accounts for 5 to 15 percent of total heat transfer but becomes significant in high-temperature applications and when designing thermal barriers.
Building Load Components
Solar radiation through windows generates 30 to 50 percent of summer cooling loads, reaching 200 BTU/hour per square foot on south-facing glass at peak solar conditions. Radiative cooling from building surfaces at night can reduce nighttime cooling loads by 10 to 20 percent. HVAC load calculations include solar gain multipliers and shading coefficients to account for radiative effects.
System Design Considerations
Technicians use low-emissivity coatings and reflective insulation to minimize radiative heat gain in equipment enclosures and ductwork. In high-temperature process applications reaching 400°F to 500°F, radiative heat transfer becomes dominant and requires specialized calculations. Proper surface treatments reduce equipment operating temperatures by 5 to 15 degrees Fahrenheit.