Metabolic rate is the rate at which the human body produces heat through internal chemical processes, expressed in watts per square meter of body surface area (W/m²). It varies with activity level and directly determines how much cooling or heating an HVAC system must provide to maintain thermal comfort. Resting metabolic rate is approximately 58 W/m², while strenuous activity can exceed 290 W/m².
Technical Details
ASHRAE Standard 55 defines metabolic rates using the met unit, where 1 met equals 58.2 W/m². Seated office work generates 1.2 met; light standing work produces 1.6 met; and walking at 1.4 m/s equals 2.8 met. Thermal comfort models like PMV and PPD incorporate metabolic rate to predict occupant satisfaction. Metabolic rates increase by 5 percent per degree Celsius of body core temperature above normal.
Applications in HVAC Design
HVAC comfort zones adjust operative temperature based on metabolic rate and clothing insulation. Occupancy classification guides system setpoints for offices, gymnasiums, and industrial spaces with different activity levels.
Practical Significance
Accounting for metabolic rate prevents over-conditioning spaces and enables energy-efficient, activity-appropriate climate control.