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3.3Criteria for Thermal Comfort


In the previous two Chapters, different physical parameters have been addressed in the context of physiological reactions to the environment. These parameters - temperature (air, radiant, surface), air velocity and humidity - are also the basis for defining criteria for an acceptable thermal environment. The criteria result in requirements for general thermal comfort (PMV-PPD-index or operative temperature) and for local comfort disturbance (draft, radiant asymmetry, vertical air temperature differences, surface temperatures). They can be found in international standards and guidelines such as EN ISO 7730:2005, CR 1752, EN 15251:2007-08 and ASHRAE 55:2004-04 or in their national derivate respectively.

Further environmental parameters, e.g. air quality, visual or aural environment, can interact with the thermal environment, and therefore, influence thermal comfort or overall satisfaction in a space. As there is not enough proof for quantitative correlations, their evaluation is only possible through a direct assessment after the building went under operation.

Operative Room Temperature

The most important criterion for the thermal environment is the indoor temperature in the form of the so-called operative temperature. As a sufficient approximation for most cases, the operative temperature can be calculated as the arithmetic mean of the air temperature and the mean radiant temperature of surrounding surfaces in an occupied zone. Air temperature refers to the average value of the temperature in space and time in an occupied zone [ASHRAE 55:2004-04].

For a first general thermal comfort evaluation, a simplified calculation of the mean radiant temperature can be carried out with surface temperatures weighted by the different surface areas. This is for example the case in building simulation programs without a geometrical model for spaces. Taking the example in Figure 2 , this calculation would lead to a mean radiant temperature of 21 °C (for the center of the room). The general equation for the mean radiant temperature combines the different surface temperatures with the view factor between a person and the respective surfaces:

Numbers of view factors can be found in the literature for different situations [Rietschel et. al 2008]. The equation allows to assess thermal comfort for each position in a space and to consider the (local) influence of warm and cold surfaces appropriately. Taking into account the person's position in Figure 2 with a view factor of 0.3, the mean radiant temperature for this position accounts to 19 °C.

Figure 2 Example of a room with a person sitting close to one colder surface with a temperature of 12 °C. All other surfaces have a temperature of 22 °C.

Figure 2 shows the influence of the glazing quality of a window on the operative temperature, considering the accurate mean radiant temperature. Obviously, higher discomfort is experienced close to a colder window surface as might occur in non-refurbished buildings. Assuming an air temperature of 22 °C for the example in Figure 2 , the operative temperature at the person's position amounts to 20.5 °C whereas the simplified calculation would give an operative temperature of 21.5 °C (for the center of the room). For air velocities higher than 0.2 m/s and metabolic rates of the occupants higher than 1.3, the operative temperature has to be calculated according to [ASHARE 2009]. The difference between air and mean radiant temperature is within 6 K in most field measurements and it is not limited by standards and guidelines. For designing indoor climate, it might be interesting to consider lower air temperatures to compensate warm surfaces (glazing or shading system) to extend the comfort zone of the operative temperature.

Figure 2 Simulation results for the operative temperature at a level of 1.2 m in a room with one window (location Munich, January 1, 9 a.m.). Upper picture: single pane glazing with U-value of 6.3 W/m²K (single pane). Lower picture: triple low-e-glazing with U-value of 0.9 W/m²K. The room air temperature is 20 °C and the air velocity is 0.1 m/s. Assumption for calculation: met value of 1.2 and a clo-value of 1.0.

Parameters for local discomfort

Besides the operative temperature, there are further temperature-related criteria to describe the thermal environment, particularly to assess local discomfort. The temperature asymmetry in a space is also based on the radiant temperature of surfaces and is defined as the temperature difference between either two vertical (walls) or horizontal (ceiling and floor) surfaces. In the example shown in Figure 2 , the temperature asymmetry for the person close to the cold surface amounts to a difference of 8 Kelvin, based on a view factor of 0.8 and a radiant temperature of the half space in the direction of the window of 14 Kelvin.

Another criterion is the absolute surface temperature, mostly important for the floor to which the body has constant contact for long periods in many situations. Finally, the stratification of the air temperature in a space has to be taken into account as a criterion for local comfort. Accepted temperature ranges for these three criteria can be found in [EN ISO 7730:2005] and [ASHRAE 55:2004-04]. Accepted PPD-values are given in Table 2 .

Table 2 Limits of PPD-values for the evaluation of local discomfort according to ISO 7730:2005. The building categories in ISO EN 7730:2003 present the closeness with which the indoor conditions in a building can be controlled. EN 15251:2007-08 uses a different approach for categorization with the (four) categories being based on the occupants' expectations [ISO EN 15251:2007]. Categories I to III however have the same limits for operative temperature like categories A to C in ISO 7730:2003.






thermal condition of human body

local discomfort

category

PPD [%]

PMW

DR [%]

PD [%]

vertical difference in air temperature

warm or cold floor

asymmetric radiation

A

< 6

-0.2 < PMV < 0.2

< 10

< 3

< 10

< 5

B

< 10

-0.5 < PMV < 0.5

< 20

< 5

< 10

< 5

C

< 15

-0.7 < PMV < 0.7

< 30

< 10

< 15

< 10

Humidity and Air Velocity

Two more criteria – humidity and air velocity - have to be considered in terms of thermal environment. Humidity is addressed only as a boundary condition for general comfort (an upper limit is given in [ASHARE 55]). On the other hand, air velocity in a space can be experienced either as draught sensation or may lead to improved thermal comfort under warm conditions. Draft may occur due to enforced air movement (open window/door, air outlet of ventilation system) or due to buoyancy effects (air falling down along a cold window surface). Allowable air velocities and acceptable limits for draft rates in terms of predicted percentage of people dissatisfied with draft are summarized in [ISO 7730:2005] and [ASHRAE 55:2004-04].

ISO 7730:2005 describes an allowance for higher air velocities in order to offset an increased operative room temperature which was adopted by ASHRAE 55:2004-04 and EN 15251:2005-07 standards. The effect is more pronounced for situations with higher radiant temperatures and lower air temperatures. ASHRAE 55:2004-04 suggests limits for a maximum temperature offset (3 K compared to a temperature without elevated air velocity) and a maximum air velocity (0.8 m/s). Additionally, individual control is advised. In contrast to these limitations higher air velocities for neutral-to-warm indoor conditions are suggested in [Zhang and Arens 2007]. Based on findings from field studies, the application of higher air velocities is even proposed in spaces with central air systems. Further research is needed to confirm these findings, which should include statements on the direction of the air with reference to a person (horizontal or vertical air flow, front-side flow, etc.).

Overall satisfaction with the thermal environment

For most parameters describing the indoor thermal environment, it has been possible to establish a relationship between the parameter itself and a predicted percentage of people rating the indoor condition as unacceptable or acceptable. People may be dissatisfied due to general thermal comfort and/or local thermal comfort parameters. However, today no method exists for combining these percentages of dissatisfied persons to give a good prediction of the total number of occupants finding the thermal environment unacceptable.

Comparable to the thermal environment there are a large number of criteria and requirements for other indoor environment qualities like air quality, visual comfort and aural comfort. On the one hand, there is a possible physical interference of the different comfort requirements, e.g., requirements for daylight and resulting solar heat gains through windows or recommended ventilation rates and noise from outside through open windows. On the other hand, the various comfort criteria have an impact on the (overall) occupants' satisfaction with the workplace and probably on thermal comfort. They also include social and architectural aspects related to a specific workspace. Figure 2 exemplary shows a survey result for German office buildings for which the subjective votes on the satisfaction with different environmental parameters are given with respect to their relevance for the occupants' overall satisfaction with their workplaces [Gossauer and Wagner 2008].

The lower left field shows parameters with high satisfaction but the weighting calculation shows that they are less important for the general satisfaction with the workplace. In the lower right field, occupants are satisfied with the parameters and they are more important for the general satisfaction. The upper left square shows parameters with higher dissatisfaction but with less importance for the general satisfaction whereas parameters in the upper right combine higher dissatisfaction with higher importance for the general satisfaction with the workplace.

Figure 2 Matrix with relevant satisfaction parameters found in 17 German office buildings in a field study. The parameters are weighted by their correlation coefficients against the overall satisfaction with the workplace (importance of the parameter). For details of the survey see [Gossauer 2008].Guidelines for Thermal Comfort

Human thermal comfort is defined as the state of mind that expresses satisfaction with the surrounding environment [ASHRAE 55:2004-04]. Thermal comfort is achieved when both thermal equilibrium is maintained between the human body and its surroundings, and the person’s expectations of the surrounding conditions are satisfied. Occupant satisfaction was first considered in the 1980s, when it was found that some chronic ill-heath was building-related (e.g. reported symptoms like lethargy, headaches, dry eyes and dry throat) [Bordass et al. 2001a]. Current national and international regulations draw on diverse - and partly controversial - results from thermal comfort studies carried out in laboratories or in the field. There are two main models to determine human thermal comfort and to predict the occupant’s satisfaction with the interior conditions: (i) the heat-balance approach, used in the standard EN ISO 7730:2005 and (ii) the adaptive approach described in the standards EN 15251:2007-08, ASHRAE 55:2004, and the Dutch Guideline ISSO 74:2005 [Boestra et al. 2005]. The discussion about thermal comfort and user satisfaction has mainly been concerned with non-residential buildings than dwelling, but has implication for the residential sector.

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