Thermal comfort principle: A human body feels thermally comfortable when the metabolic heat generation is equal to the __________.

Introduction / Context:
Human thermal comfort is a balance problem. The body produces metabolic heat and must reject an equivalent amount to maintain a stable core temperature. HVAC design aims to support this balance through control of air temperature, humidity, air speed, and mean radiant temperature.

Given Data / Assumptions:

• Steady metabolic rate appropriate to activity level.
• Clothing insulation is steady during the period considered.
• Environmental parameters are uniform around the occupant.

Concept / Approach:
The comfort condition is met when metabolic heat generation equals total heat dissipation from the body by convection, radiation, evaporation (sweating and respiration), and small conduction terms. If generation exceeds dissipation, heat is stored in the body and the person feels too warm; if dissipation exceeds generation, the person feels too cool.

Step-by-Step Solution:

Verification / Alternative check:
Thermal comfort models (e.g., PMV/PPD by Fanger) mathematically enforce this energy balance, incorporating clothing insulation and air movement. Neutral sensation corresponds to negligible net storage.

Why Other Options Are Wrong:

• Heat stored in the human body: Storage indicates imbalance and discomfort if sustained.
• Sum of (a) and (b): Physically meaningless; you cannot add “dissipated” and “stored” to set a balance.
• Difference of (a) and (b): Also not the criterion; the correct condition is zero storage.
• Net radiant heat received: Only one component of loss/gain; not the full balance.

Common Pitfalls:
Ignoring evaporation’s importance in warm/humid climates; neglecting air speed effects on convective loss.

Final Answer:
heat dissipated to the surroundings