Thermal Comfort (PMV-PPD) Simulator

HVAC design for office buildings: Office building HVAC is designed so that PMV falls in the comfort band (|PMV|<0.5, PPD<10%). Designers assume a metabolic rate of 1.2 met (office work) and clothing of 0.5 clo in summer / 1.0 clo in winter, and adjust not only air temperature but the window insulation (radiant temperature) and supply-air velocity (air movement). Perimeter zones near windows tend to feel cold due to radiation, so designers add a separate HVAC loop for the perimeter zone or use Low-E glass to limit the drop in radiant temperature.

The basis of Cool Biz and Warm Biz: Set-point guidelines such as "28°C in summer, 20°C in winter" are backed by the calculation that, with clothing adjusted to 0.5 clo in summer and 1.0 clo in winter, PMV stays inside the comfort band. Put the other way round, moving the set point without changing the dress code pushes PMV out of the comfort band and dissatisfaction jumps. PMV calculation is the common language for discussing HVAC and clothing together.

Comfort assessment for homes and shops: In homes, underfloor heating and radiant panels raise the radiant temperature directly, so comfort is achieved at a lower air temperature, which saves energy. Conversely, over-cooling a summer shop swings PMV negative for lightly dressed customers, who then complain of cold. In spaces where customers and staff have different clothing and metabolic rates, checking PMV for both conditions is the practical approach.

Linking with building simulation and BEMS: Feeding the indoor temperature, radiation and air-velocity distribution computed by load-calculation software or CFD lets you evaluate PMV and PPD at each point in the space. Building energy management systems (BEMS) increasingly embed PMV control, using PMV rather than temperature as the control target. This tool is useful for intuitively checking the single-point PMV calculation that underpins all of that.

The most common misconception is that "controlling air temperature alone makes a room comfortable". Of the six factors that set PMV, air temperature is just one. Near a window in winter the air may be 24°C, yet body heat radiates to the cold glass surface, the mean radiant temperature drops and you feel cold. In summer, radiation from sun-heated walls and ceiling can make a room feel hot even after the set point is lowered. Do not trust the thermostat number alone — look at the environment including radiation, air movement and humidity.

Next, the belief that "if PMV=0 everyone is satisfied". As the PPD equation shows, even at the optimum point where PMV is exactly 0, PPD is 5% — 1 in 20 people are dissatisfied. This comes from individual differences (basal metabolism, acclimatisation, preference) and cannot be designed away. So the HVAC target is not "everyone satisfied" but "keep dissatisfaction below 10% (|PMV|<0.5)". Understand that "complaints never reach zero" is not a design fault but a fundamental limit of the thermal environment.

Finally, that "PMV is a universal index usable in any space" is not true. PMV-PPD was built for a uniform, air-conditioned interior in steady state, such as an office. In a naturally ventilated space where occupants open windows and change clothing, people adapt actively, so PMV overestimates dissatisfaction compared with reality (an adaptive model is used in that case). Local discomfort such as only the feet being cold or only the head receiving strong radiation does not show up in PMV. In practice PMV is used alongside local comfort indices such as draught rate (DR) and vertical temperature difference, and treated as a "whole-body average" guide.