PPD (Predicted Percentage Dissatisfied) quantifies the percentage of occupants expected to be thermally dissatisfied in a given environment, ranging from 0 to 100 percent. It complements PMV by translating thermal sensation predictions into occupant satisfaction rates. ASHRAE and ISO standards use PPD targets to define acceptable thermal comfort zones. A PPD of 10 percent is typically considered acceptable for general comfort; 5 percent represents high-quality design.
Technical Details
PPD is calculated from PMV using the relationship: PPD = 100 – 95 × exp(-0.03353 × PMV⁴ – 0.2179 × PMV²). Even at PMV = 0 (perfect thermal neutrality), PPD remains approximately 5 percent due to individual physiological and psychological variations. ISO 7730 defines PPD acceptance criteria: 10 percent PPD maximum for Category II comfort (normal expectation); 5 percent for Category I (high expectation). PPD directly influences HVAC performance verification and occupant satisfaction surveys.
Applications in HVAC Systems
Commissioning protocols use PPD targets to validate system performance. Complaint resolution often references PPD calculations to assess whether conditions meet standards.
Practical Significance
PPD translates technical comfort metrics into business-relevant satisfaction indicators for facility managers and building operators.