It is common for occupants to complain while the thermostat in an office or a cabin reads 22 °C, and the cause is usually not the temperature.
Six variables
ISO 7730 defines comfort as a function of six variables: air temperature, mean radiant temperature, air velocity, relative humidity, metabolic rate and clothing insulation [1]. From these, PMV (predicted mean vote) and PPD (predicted percentage dissatisfied) are computed.
The critical point is this: a thermostat measures only one of them. Unless the other five are known, nothing has been said about comfort.
Local discomfort is assessed separately
Even with an acceptable mean PMV, local effects generate complaints:
- Draught. Air velocity and turbulence intensity at neck and ankle height. At low temperature even a low velocity is uncomfortable.
- Vertical temperature difference. If the difference between ankle and head exceeds a threshold, dissatisfaction rises.
- Radiant asymmetry. A cold glazed surface or a warm ceiling causes discomfort even when the air temperature is right.
- Floor temperature.
These four are questions about the distribution, not the mean, and cannot be assessed without a field solution. See the note on when CFD is necessary.
Modelling notes
Radiation must be included. The mean radiant temperature carries as much weight in the comfort equation as the air temperature. PMV cannot be computed without solving the surface temperatures. See the radiation note.
Buoyancy governs. Occupant, lighting and equipment heat loads set the flow pattern in a low-velocity space. See the mixed convection note.
The target is a map, not a single number. The output should be the distribution of PMV and draught risk in the occupied zone — not a single room average.
References
- ISO 7730:2005. Ergonomics of the thermal environment — Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. ISO, Geneva.
- Churchill, S. W. & Chu, H. H. S. (1975). Correlating equations for laminar and turbulent free convection from a vertical plate. International Journal of Heat and Mass Transfer, 18(11), 1323–1329. DOI: 10.1016/0017-9310(75)90243-4