Predicted Percentage Dissatisfied (PPD) is a metric that quantifies the expected percentage of occupants who will express dissatisfaction with the thermal environment, expressed as a percentage from 0 to 100. PPD translates abstract thermal sensation predictions into actionable comfort performance targets. ASHRAE and ISO standards specify PPD thresholds to define acceptable indoor environments and guide HVAC design decisions.
Technical Details
PPD is derived mathematically from PMV using the formula: PPD = 100 – 95 × exp(-0.03353 × PMV⁴ – 0.2179 × PMV²). The relationship is non-linear, meaning small PMV deviations near neutral create larger PPD changes. At PMV = 0, PPD equals approximately 5 percent, reflecting unavoidable individual differences. ISO 7730 establishes PPD limits: Category I (high expectation) requires PPD ≤ 5 percent; Category II (normal expectation) allows PPD ≤ 10 percent; Category III (moderate expectation) permits PPD ≤ 15 percent.
Applications in HVAC Commissioning
Commissioning agents use PPD targets to validate system performance and verify occupant satisfaction objectively. Building performance reports document achieved PPD versus design targets.
Practical Significance
PPD provides facility managers with quantifiable comfort performance metrics, supporting evidence-based decisions for system optimization and renovation investments.