Moderate Thermal Environments

The main standards for comfortable thermal environment are ISO EN 7730 and ASHRAE 55-92.2 The research that forms the basis for these two standards is mainly performed under environmental conditions similar to those for commer­cial and residential buildings, with activity levels of 1 to 2 met, norma! indoor clothing (0.5 to 1.0 clo), and a limited range of environmental parameters, General Thermal Comfort

ISO EN 7730 standardizes the PMV-PPD index as the method for evalua­tion of moderate thermal environments. To quantify the degree of comfort, the PMV (predicted mean vote) index gives a value on a 7-point thermal sen­sation scale: +3 hot, +2 warm, +1 slightly warm, 0 neutral, -1 slightly cool, -2 cool, -3 cold. An equation in the standard calculates the PJV1V index based on the six factors (clothing, activity, air and mean radiant temperatures, air speed, and humidity).

The PMV index can be determined when the activity (metabolic rate) and the clothing (thermal resistance) are estimated and the following environmen­tal parameters are measured: air temperature, mean radiant temperature, rela­tive air velocity, and partial water vapor pressure (see ISO EN 7726).

The PMV index is derived for steady-state conditions but can be applied with good approximation with minor fluctuations of one or more of the vari­ables, provided that time-weighted averages of the variables during the previ­ous 1 h period are applied. Because the PMV index assumes that all evaporation from the skin is transported through the clothing to the environ­ment, this method is not applicable to hot environments. It can be used for a range of PMV index of -2 to +2, i. e., thermal environments where sweating is minimal.

Use of the PMV index is only recommended when the six main parame­ters are within the following intervals:

M = 46-232 W/m2 (0.8 to 4 met)

Jdo = 0-0.310 m2 °C/W (0 to 2 clo)

*/=10-30-0 tr = 10-40 °C va — 0-1 m/s pa = 0-2700 Pa

The metabolic rate can be estimated by ISO EN 9886, and the thermal re­sistance of clothing can be estimated by ISO EN 9920, taking into account the type of work and the time of year. For varying metabolic rates, it is recom­mended to estimate a time-weighted average during the previous 1 h period. For sedentary people, the insulation of a chair must also be taken into account.

The PMV index can be used to check whether a given thermal environ­ment complies with specified comfort criteria and to establish requirements for different levels of acceptability. By setting PMV = 0, an equation is estab­lished that predicts combinations of activity, clothing, and environmental pa­rameters that will provide a thermally neutral sensation. Figure 6.1 shows the optimal operative temperature as a function of activity and clothing for differ­ent levels of acceptability.


Moderate Thermal Environments


Moderate Thermal Environments

M °C/w

. W/m


Moderate Thermal Environments


C: (ppd < 1 s%)

. W/m2


0 0.5 1.0 1.5 clo


FIGURE 6.1 The optimal operative temperature as a function of clotting and activity for the three categories of thermal environment The three diagrams also show the range around the optimal temper­ature for each of the three categories. The air velocity in the space Is assumed to be < 0.1 m/s. The rel­ative air velocity, *a, caused by body movement is estimated to be zero for a metabolic rate M less than I met and v0 = 0.3(W — I) for M > I met The diagrams are determined for a relative humidity of 50%, but the humidity has only a slight influence on the optimal and permissible temperature ranges.

The PM V index predicts the mean value of the thermal preferences of a large group of people exposed to the same environment. But individual votes are scattered around this mean value, and it is useful to predict the number of people likely to feel uncomfortably warm or cool. The PPD (predicted per­centage of dissatisfied) index establishes a quantitative prediction of the num­ber of thermally dissatisfied people. The PPD predicts the percentage of a large group of people likely to feel too warm or cool, i. e., voting hot (+3), warm (+2), cool {-2), or cold (-3) on the 7-point thermal sensation scale.

Once the PMV value has been determined, the PPD can be found from Fig. 6.2 or from the equation

PPD = 100 — 95^_a03353PMv4’0’2i79PMV‘). (6.2) Local Thermal Discomfort

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable if local influences on the body from asym­metric temperature radiation, draft, vertical air temperature differences, or con­tact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced. The data for local thermal discomfort is mainly based on studies of people un­der low activity levels (1.2 met). For higher activities it can be expected that peo­ple are less sensitive to local thermal discomfort. The relations between dissatisfaction and local discomfort parameters are found in CR 1752.

Draft: Local Air Velocities

One of the most critical factors is draft. Many people at low activity levels (seated/standing) are very sensitive to air velocities, and therefore draft is a very common cause for occupant complaints in ventilated and air-conditioned spaces. Fluctuations of the air velocity have a significant influence on a person’s sensation of draft. The fluctuations may be ex-

Moderate Thermal Environments

Predicted mean vote (PMV)

FIGURE 6.2 Predicted percentage of dissatisfied (PPD) as a function of predicted mean vote (PMV).

Pressed either by the standard deviation of the air velocity or by the turbu­lence intensity Tu, which is equal to standard deviation divided by the mean air velocity, va. The percentage ot people feeling draft (draft rating, DR) may be estimated from the equation

DR = (34 — ta)(va — 0,05)°’62(3.14 + 0.37 — SDi'(?) , (6.3 ;


Va = mean air velocity (3 min), m s_l

SO?/., = standard deviation of air velocity (3 min), m s “1

Tu =- air temperature, °C

For va < 0.05 m s"1, use va = 0.05 m s“1. For DR > 100%, use DR = 100%.

The model applies to people at light, mainly sedentary activity with a thermal sensation for the whole body close to neutral. The sensation of draft is lower at activities higher than sedentary and for people feeling warmer than neutral.

For people at higher activity levels or at ambient temperatures above the comfort range, an increased air velocity may improve the general thermal comfort. This influence is taken into account by using the PMV equation. Also, high local velocities (spot cooling) may decrease discomfort from high activity or high ambient temperatures.

Vertical Air Temperature Difference

A high vertical air temperature difference between one’s head and ankles may cause discomfort. Figure 6.3 shows the percentage of dissatisfied as a func­tion of the vertical air temperature difference between head and ankles (1.1 and 0.1 m above the floor, respectively). The figure applies when the temperature in­creases with height. People are less sensitive to decreasing temperature.




20 —




Moderate Thermal Environments

0 2 4 6 8 10

Air temperature difference between head and feet (K)

FIGURE 6.3 Local discomfort caused by vertical air temperature difference (applies when the temperature increases with height).

If the floor is too warm or too cool, occupants may feel uncomfortable due to warm or cool feet. For people wearing light indoor shoes, it is the tem­perature of the floor rather than the material of the floor covering that is im­portant to comfort. Figure 6.4 shows the percentage of dissatisfied for seated or standing people as a function of floor temperature.

Radiant Temperature Asymmetry

Radiant asymmetry may also cause discomfort. People are most sensitive to radiant asymmetry caused by warm ceilings or cool walls (windows). Fig­ure 6.5 shows the percentage of dissatisfied as a function of radiant tem­perature asymmetry caused by a warm ceiling, a cool wall, a cool ceiling, or a warm wall. These data apply for sedentary people and low ceiling height. A study involving a high ceiling (9 m) and asymmetry from ceiling-mounted gas — fired infrared heaters showed a higher acceptable temperature asymmetry.3 For seated and standing people, a temperature asymmetry of 10-14 K resulted in less than 5% dissatisfied. Target Values for Acceptable Thermal Environments for Comfort

Thermal comfort is defined as the condition of mind that expresses sat­isfaction with the thermal environment. Dissatisfaction may be caused by thermal discomfort of the body as a whole as expressed with the PMV and PPI) indices, or it may be caused by unwanted cooling (or heating) of a particular part of the body. Due to individual differences, it is impossible to specify a thermal environment that will satisfy everybody. There will al­ways be a percentage of dissatisfied occupants, but it is possible to specify an environment predicted to be acceptable by a certain percentage of the occupants.

Moderate Thermal Environments

Floor temperature (°C)

FIGURE 6.4 Local thermal discomfort caused by warm or cold floors.

Moderate Thermal Environments

Radiant temperature asymmetry (°C)

■■ FIGURE 6.5 Local thermal discomfort caused by radiant temperature asymmetry.

Due to local or national priorities, technical developments, and climatic regions, in some cases a higher thermal quality (fewer dissatisfied) or a lower quality (more dissatisfied) may be sufficient. In both cases the PMV and PPD indices, the model of draft, and the relation between local thermal discomfort parameters and the expected percentage of dissatisfied people may be used to determine different ranges of parameters for the evaluation and design of the thermal environment.

While some existing standards specify only one level of comfort (e. g., ISO EN 7730, ASHRAE 55-92), a CEN report (CR 1752) recommend three categories, as shown in Table 6,3. Each category prescribes a maximum percentage of dissatis­fied for the body as a whole (PPD) and for each of the four types of local discom­fort. Some requirements are hard to meet in practice while others are quite easily met. The different percentages express a balance between the aim of providing few dissatisfied and what is practically obtainable using existing technology.

The three categories in Table 6.3 apply to spaces where persons are ex­posed to the same thermal environment. It is advantageous if some kind of in­dividual control over the thermal environment can be established for each person in a space. Individual control of the local air temperature, mean radi­ant temperature, or air velocity may contribute to reducing the rather large differences between individual requirements and therefore provide fewer dis­satisfied.

Modification of the clothing may also contribute to balancing individual differences. The effect of adding (or removing) different garments on the opti­mal operative temperature is given in Table 6.15.

Operative Temperature Range

For a given space there is an optimal operative temperature corresponding to PMV = 0, depending on the activity and the clothing of the occupants. Figure

6,1 shows the optimal operative temperature and the permissible temperature

Thermal state of the body as a whole

Local discomfort







Predicted mean vote (PMV)



Dissatisfied due to draft (DR)



Dissatisfied due to vertical air temperature difference



Dissatisfied due to warm or cool floor




Due to radiant



<’ 6

~0.2<PMV< +0.2



< 10

< 5



-0.5 < PMV < +0.5




< 5



-0.7 <PMV < +0.7




< 10

Range as a function of clothing and activity for each of the three categories. The optimal operative temperature is the same for the three categories, while the per­missible range around the optimum operative temperature varies.

The operative temperature at all locations within the occupied zone of a space should at all times be within the permissible range. This means that the permissible range should cover both spatial and temporary variations, includ­ing fluctuations caused by the control system.

Figure 6.1 applies for a relative humidity of 50%; however, in moderate environments the air humidity has only a modest impact on the thermal sensa­tion. Typically, a 10% increase in relative humidity is experienced as equally warm as a 0.3 °C increase in operative temperature.

The numbers of dissatisfied persons in Table 6.3 are not additive. Some of the people experiencing general thermal comfort (PMV-PPD) may be the same as the people experiencing local thermal discomfort. In practice, a higher or lower number of dissatisfied persons may be found using subjective question­naires in field investigations (ISO 10551).

Local Thermal Discomfort

Figure 6.6 and Tables 6.4-6.6 give ranges for local thermal discomfort pa­rameters for the three categories listed in Table 6.3. The acceptable mean air velocity is a function of local air temperature and turbulence intensity. The turbulence intensity may vary between 30% and 60% in spaces with mixed flow air distribution. In spaces with displacement ventilation or without me­chanical ventilation, the turbulence intensity may be lower.