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4.1 THERMAL COMFORTHuman thermal comfort is defined as the conditions in which a person would prefer neither warmer nor coolersurroundings. It is a rather complex concept, since it depends on various influencing parameters and it is thecombination of these parameters that creates the end result of comfort..

4.1.1 Influencing Parameters

Building occupants are always in search of thermal comfort, which in turn influences a person's performance(intellectual, manual and perceptual). Depending on the available means, occupants will attempt several actions tochange or control environmental conditions. In order to be most successful in these actions, one must have athorough quantitative, as well as qualitative, knowledge of the conditions establishing the parameters that influencethermal comfort. This will also enable building designers, to provide alternative means to the occupants forcontrolling their thermal comfort conditions, instead of lowering the thermostat during summer or increasing itduring winter.

The human body is like a complex internal combustion engine. To achieve thermal comfort, the body must balanceits heat gains and losses by properly adjusting its functions (i.e. perspiration), while also responding to theprevailing environmental conditions (i.e. temperature and humidity). Under good conditions the human body canfunction at optimum levels.

There are times, however, that comfort can not be achieved by the functions of the body itself, due to the severityof the prevailing conditions. Under such circumstances it is necessary to provide some assistance, either by natural,hybrid or mechanical means. It is important though, for rational use of available energy resources, to first exhaustall means of achieving comfort by natural or hybrid techniques and reducing heating and cooling loads,before having to resort to energy consuming mechanical systems.

Depending on the function of the building and its various spaces, indoor environment conditions will varysignificantly, since occupant needs are different. Clearly, there are significant variations of indoor conditionsdepending on the use of the building (i.e. offices, factories, shops, hospitals, schools, theatres, restaurants, hotels,athletic halls, museums, computer rooms, etc).

The most important parameters that influence thermal comfort are the: dry bulb temperature, relative humidity, airvelocity, barometric pressure, clothing, and activity. Thermal comfort can be achieved by many differentcombinations of these variables. In all cases, it is the end result that we are interested in achieving, which meansthat, it is the combined effect of these parameters on the human body that is important. The positive or negativeaffect of one parameter on comfort may be enhanced or counterbalanced by the change of another parameter.

The body's thermal equilibrium is a dynamic balance between heat production (as a result of human metabolic rate)and body heat transfer by convection, conduction, radiation and evaporation to or from the environment, as shownin Figure 21.

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Skin temperature (32.2oC)

Sensible & latent heat lossesfrom perspiration

Body temperature (37oC)

Radiative heat losses

Metabolism - heat production

Heat transfer to and from the environment

Vapour evaporation (perspiration)Low humidity enhances evaporation

Radiative heat gains

➚ ➚➚

Air movement enhances heat losses

Figure 21. Interactions of the human body with the environment.

Sweating and the resulting evaporative cooling sensation, is the main mechanism of thermal adjustment for thehuman body, under hot environmental conditions or high level of activity. Clothing will directly influence theamount of heat and mass (moisture) exchange from the body to its environment.

The control of environmental conditions in order to achieve thermal comfort can be performed by:• Passive controls (on the environment, clothing, metabolic rate), and• Active or hybrid controls (on the building).

Thermal comfort is directly dependent on air ventilation systems (natural, hybrid or mechanical) that supply thenecessary amounts of fresh air, which is controlled in terms of quantity, velocity, quality and thermal conditions.Indoor thermal conditions are primarily influenced by indoor temperature and relative humidity. The indoortemperature is defined in terms of air temperature and internal wall surface temperature in a given space (radianttemperature). The relative humidity is the ratio of the mole fraction of water vapour in a given moist air sample, tothe mole fraction in an air sample, saturated at the same temperature and pressure. Most air-conditioning systemsare in fact used to primarily remove the excess water vapor from the air.

4.1.2 The Comfort - PMV Theory

Human comfort is not just a simple heat balance, but it needs to take into account complex psychologicalprocesses. The thermal sensation is processed through several mental processes before it leads to an expression ofpreference or judgment. Of course the primary parameters are physical, like environmental conditions, activity,clothing, but there are also other influences, like the state of acclimatization of individual, personal expectationsand attitudes, or behavioral adjustments.

Although thermal comfort is not always perceived the same by all humans, several attempts have been made inorder to develop empirical correlations for relating comfort perceptions to specific physiological responses. Amongthe various models for the quantitative estimation of thermal comfort, the most widely used is the one suggested by

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(Fanger 1970). This work has grown to be the most popular way of quantitatively expressing thermal comfort andthermal sensation, known as the Predicted Mean Vote (PMV) Theory and the associated index of PredictedPercent of Dissatisfied (PPD) people.

The PMV and PPD indices have been introduced and empirically derived, by Fanger during the 1970's. The PMVindex is calculated through a complex mathematical function of human activity, clothing and four environmentalparameters. This equation has been empirically developed following an extensive study and monitoring of humanbeings under varying conditions, and a comprehensive statistical analysis of their responses. PMV relates theimbalance between the actual heat flow from the human body in a given environment and the heat flow required foroptimum comfort at the specified activity. For PMV calculations one can also refer to readily available tools on theinternet, for example, the human heat balance

(http://atmos.es.mq.edu.au/~rdedear/pmv/).The method has become since 1984, the basis of the International and the European Standard EN ISO 7730(Moderate thermal environments - Determination of the PMV and PPD indices and specification of theconditions for thermal comfort) for assessing thermal comfort in spaces with average temperatures. Other relatedstandards and regulations on thermal comfort include the International Standard ISO 9920 (Ergonomics of thethermal environment - Estimation of the thermal insulation and evaporative resistance of a clothing ensemble),ISO 8996 (Ergonomics - Determination of metabolic heat production). This method is also included in the newstandard that is being prepared by the European Commission on energy calculations and building labeling (CR1752 - Ventilation for buildings, Design criteria for the indoor environment). This new standard is currentlyunder assessment by national committees, and has not yet been accepted as a standard.

The PMV index quantifies the degree of discomfort, giving the predicted mean vote of a large group of subjectsaccording to the psychological scale shown in Figure 22. The PMV values range between -3 and +3. Negativevalues indicate an uncomfortable feeling due to a cold sensation, while positive values indicate an uncomfortablefeeling due to a hot sensation. Zero is the neutral point, representing comfort.

-3 -2 -1 0 +1 +2 +3

SLIGHTLY NEUTRAL SLIGHTLY COLD COOL COOL COMFORT WARM WARM HOT

Figure 22. Thermal sensation scale for the PMV index.

The percentage of people dissatisfied (PPD) with the thermal environment at various conditions has beenmathematically related to PMV. Dissatisfaction is defined as anybody not voting either -1, +1 or 0. A PPD of 10%corresponds to the PMV range of -0.5 or +0.5. Even with PMV equal to zero, about 5% of the people remaindissatisfied. This practically implies that it is not possible to achieve thermal comfort for all people in a space,since most people have different dressing habits, different levels of activities, different metabolic rates anddifferent psychological influences, which also play a role in determining thermal comfort. The objective is toprovide thermal comfort for the majority of occupants in a space. Values ranging between -0.5 < PMV < 0.5 andPPD < 10%, are considered acceptable.

The PMV theory has gained wide acceptance, but still remains a rather simplistic simulation of complexphenomena. However, it can provide some indication of the anticipated thermal comfort conditions.

Human adaptation is also a factor that needs to be taken into account when assessing thermal comfortconditions. People living in hot climates have developed a tolerance to high ambient temperature, compared topeople from northern climates. For example, a heat wave for southern Europe is considered when the

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temperature exceeds 40oC while for northern climates temperature exceeding 30oC for most people areconsidered close to their tolerance limits. Tolerance limits are reversed in winter.

Thermal comfort conditions can be improved by adjusting one of the influencing parameters. It is preferable, inorder to achieve the desirable end effect, to give priority to the parameters which can be varied with no or lowenergy requirements.

Solar control can reduce direct solar gains, that may influence humans directly if they are exposed to solarradiation or it will increase the cooling load, by trapping excessive thermal radiation into the space. Airmovement around the human body can also influence thermal comfort. It determines the convective heat exchangeof the body and the evaporative capacity of the air. Convective losses are directly proportional to a power of the airvelocity and the temperature difference of the skin and air temperature. Higher air velocities increase evaporationrates and consequently enhance the cooling sensation and reduce the negative effect of high humidity.

During summer, natural ventilation or the use of ceiling fans to enhance and control indoor air movement, canshift the thermal comfort area to higher air temperatures. The ASHRAE recommended upper limit of indoor airmovement is 0.8 m/s. Above this value, loose papers may be disturbed. Such air speeds permit one to maintain aspace about 2oC warmer, at for example 60% relative humidity, and still maintain thermal comfort. Even in air-conditioned spaces, this will allow us to maintain the thermostat at a higher setting, which means a lower energyconsumption of the A/C system, while maintaining comfort conditions.

Humidity is another determinant factor of thermal comfort. It does not affect the thermal load from theenvironment on the body, but it determines the evaporative capacity of the air. Low relative humidity of ambient airaids the evaporation of perspiration from the human body, which in turn enhances the cooling sensation.

Thermal comfort is not an exact concept and human responses with regard to comfort do not occur as a simpleresponse to temperature. Continuous research activities on thermal comfort reveal the influencing parametersand their interrelations in order to better control the indoor environment in an energy conscious manner. Betterunderstanding of the complex processes, will provide accurate tools and means for defining HVAC operatingconditions that satisfy thermal comfort in an energy efficient way.