Introduction to thermal comfort standards and to the proposed new version of EN ISO 7730

Energy and Buildings 34 (2002) 537±548

Introduction to thermal comfort standards and to the proposed new version of EN ISO 7730 B.W. Olesena,*, K.C. Parsonsb a

Wirsbo-VELTA GmbH, 22851 Norderstedt, Germany b Loughborough University, Loughborough, UK

Abstract This paper describes existing International Standards Organization (ISO) standards and current activity concerned with thermal comfort. It describes how an ISO standard is produced from a new work item proposal to publication as an International Standard. ISO Standards should be valid, reliable, useable, and with suf®cient scope for practical application. The existing thermal comfort standardÐEN ISO 7730Ðis considered in terms of these criteria as well as ISO 8996 (metabolic rate) and ISO 9920 (clothing). The work of ISO/TC 159 SC5, èrgonomics of the physical environment', is presented in Appendix A. The proposed revision of EN ISO 7730 is presented in detail. The revised standard will be based on requirements for general thermal comfort (predicted mean vote (PMV), operative temperature) and local thermal discomfort (radiant temperature asymmetry, draught, vertical air temperature differences, ¯oor surface temperatures). One critical issue is the effect of air velocity. Increased air velocity has a bene®cial effect at warm temperatures, but it may result in draught sensation in cooler temperatures. Another issue is the extent to which requirements of humidity need to be included in a standard for thermal comfort. Several recent research projects dealing with adaptation, in¯uence of air velocity and the effect of humidity have been responsible for keeping the standards up to date. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Indoor climate; Thermal comfort; Local discomfort; ISO; CEN; Standards; PMV; Adaptation; Draught

1. Introduction There are a number of national organizations whose standards have international in¯uence and some make a contribution to the creation of knowledge of thermal comfort and the application of that knowledge. For a standard to be truly international however, requires a co-ordination of countries and a process of common consent. The International Standards Organization (ISO) was set up in 1947 and has over 130 member countries. Its principles of a single representative organization from each country and a democratic system of voting support the notion of a democratic process of globalization and a fairly-operating world market in a world economy. It is interesting that one of the major issues concerning thermal comfort is the apparent con¯ict between a so called `western' approach, which attempts to `seal' a building and control the internal environment to constant levels of air temperature appropriate to western behavior and clothing, and an adaptive approach where people can adapt to a wider range of conditions that complement their culture and in hot (or even cold) climates are less energy demanding. The issue is more complex, however *

Corresponding author.

it demonstrates the advantage of international consensus over national trends and interests. That is, an international organization with democratic processes will allow a broad international perspective to be considered. The aim of this paper is to provide an introduction to ISO standards concerned with thermal comfort and to describe the proposed revision of thermal comfort standard EN ISO 7730. The standards are described in the context of the ISO system for standards production, existing ISO standards concerned with people in thermal environments, current thinking about future standards, and decisions that need to be made to bring thermal comfort standards into the 21st century. 2. How is an ISO standard produced? ISO standards are produced by experts from participating countries (P-members) according to agreed rules and a system of voting. There is a six-stage process from the initial idea for a standard to its ®nal publication. This is shown in Table 1. A thermal comfort standard is proposed (by a working group (WG), committee or other) and supported by a document that explains the requirement, the rationale and

0378-7788/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 7 7 8 8 ( 0 2 ) 0 0 0 0 4 - X

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B.W. Olesen, K.C. Parsons / Energy and Buildings 34 (2002) 537±548

Table 1 Six-stage process to the production of an International Standard (ISO) Stage

Process

Document

1 2 3 4 5 6

New work item proposal (TC, SC, WG, national, regional, organization, individual) Building expert consensus (WG) Consensus building across countries (TC, SC, WG) Integration of comments and preparation of DIS (WG) Integration of editorial comments and preparation of FDSI (WG) Preparation of International Standard (Secretariat)

New proposal (NP) (inquiry/voting) Working document (WD) Committee draft (CD) (inquiry/voting) DIS (inquiry/voting) FDIS (inquiry and Yes/No vote) International Standard (ISO)

KeyÐWG: working group ISO/TC 159 SC5 WG1 èrgonomics of the thermal environment'; Convener: Professor Bjarne Olesen, Denmark; SC: subcommittee ISO/TC 159 SC5 èrgonomics of the physical environment'; Chairman: Professor Ken Parsons, UK; Secretariat (BSI): Dr. Sina Talal; TC: Technical Committee ISO/TC 159 èrgonomics'; Chairman: Wolfgang Schultetus, Germany; Secretariat (DIN): Norbert Breutmann.

the scope of the proposed standard. The sub-committee (ISO/TC 159 SC5) then considers the proposal and submits it for international voting. On acceptance and an indication that suf®cient (5) member countries will participate in the work, the sub-committee allocates the work to a WG (ISO/ TC 159 SC5 WG1). The WG identi®es a project leader who develops working documents with advice and comments from the international experts under the guidance and coordination of the convener. A proposal to revise a standard would also be considered as a new proposal. All standards are reviewed every 5 years and, in the case of ISO 7730, it was considered that signi®cant developments had taken place since its adoption and that a revision was required. The work was allocated by ISO/TC 159 SC5 to its WG on thermal environments (ISO/TC 159 SC5 WG1) with Professor Bjarne Olesen as project leader. Working documents have been produced and discussed at meetings in Paris, Yokohama, Barcelona, Copenhagen and London. The document is now proposed as a Committee Draft (ISO CD 7730). It is important at this stage to have full international discussion about technical content so that the draft standard can be modi®ed to allow consensus (approval of two thirds of the P-members voting). The Committee Draft (ISO CD 7730) is circulated to member countries who will circulate it within their country and provide comments and a vote. Three months is allowed for voting. If accepted, the WG (ISO/TC 159 SC5 WG1) will respond to comments and revise the document accordingly. A Draft International Standard (ISO DIS 7730) will then be produced, circulated for voting and comments, and revised to form a Final Draft International Standard (ISO FDIS 7730). This will be subjected to a Yes/No vote and, if accepted, published as ISO 7730, probably in 2003. It should be noted that International Standard ISO 7730 and European Standard EN 7730 are developed together under the Standard EN ISO 7730. 3. ISO standards: ergonomics of the physical environment The following describes the current ISO ergonomics standards and activity concerned with thermal comfort.

For more detail on ergonomics and standardization the reader is referred to a special issue of the journal, Applied Ergonomics 26 (4) 1995. International Standards in ergonomics have been developed since 1974 when ISO/TC 159 was established at the request of the International Ergonomics Association (IEA). Sub-committee ISO/TC 159 SC5 Ergonomics of the Physical Environment was established at the same time and is responsible for over 30 work items which are requests, by international voting, to produce a standard. The sub-committee has three WGs that develop the standards. These are concerned with thermal environments, lighting and danger signals, and communication in noisy environments. Countries involved are Australia, Belgium, Czech Republic, Denmark, Finland, France, Germany, Italy, Japan, Korea, Mexico, Netherlands, Poland, Slovakia, Sweden, Thailand, UK, and USA. Some countries are more active than others, with some taking an observer role. A list of published standards and current work is provided in Appendix A. Standards concerned with thermal comfort are produced by ISO/TC 159 SC5 WG1. The main thermal comfort standard is ISO 7730 which is based upon the predicted mean vote (PMV) and predicted percentage of dissatis®ed (PPD) thermal comfort indices (Fanger, 1970). It also provides methods for the assessment of local discomfort caused by draughts, asymmetric radiation and temperature gradients. Other thermal comfort standards include a technical speci®cation, thermal comfort for people with special requirements (ISO TS 14415), responses on contact with surfaces at moderate temperature (ISO 13732, Part 2), and thermal comfort in vehicles (ISO 14505, Parts 1±4). Standards that support thermal comfort assessment include ISO 7726 (measuring instruments), ISO 8996 (estimate of metabolic heat production), ISO 9920 (estimation of clothing properties), and ISO 10551 (subjective assessment methods). 4. Present position and future options for ISO thermal comfort standards ISO thermal comfort standards should provide the best internationally agreed methods and data available. They can be judged on a number of criteria as discussed by Delleman


et al. [1]. ISO standards should be valid, reliable and useable with suf®cient scope for practical application. Validity: This is concerned with whether the assessment method or prediction accurately represents the phenomenon of interest. For example, does an index that predicts thermal comfort, accurately predict the thermal comfort perceived by people? Reliability: This is concerned with whether a standard used to assess thermal comfort would give the same prediction if repeatedly used to assess exactly the same conditions. If a procedure is ambiguous or non-specific (where to measure, what to measure, when to measure, etc.), it will reduce reliability. Note that reliability does not imply validity but validity does imply reliability. Usability: This is concerned with whether the users of a standard can use it correctly. A standard may be valid and reliable but if it is not presented such that the users can use it, it will be limited in its application. There are, of course, other criteria for assessing measurement methods (e.g. sensitivityÐthe standard needs to be able to distinguish between conditions of interest where a practical difference exists), however the three criteria above will be considered in this paper. Thermal comfort standards can therefore be judged in terms of the above criteria, as can revisions of standards. Criticisms and future options for standards can also be considered in terms of these criteria. 5. ISO 7730 moderate thermal environmentsÐ determination of the PMV and PPD indices and specification of the conditions for thermal comfort This standard describes the PMV and PPD indices and speci®es acceptable conditions for thermal comfort. The PMV predicts the mean value of the votes of a large group of people on the ISO thermal sensation scale (3 hot; 2 warm; 1 slightly warm; 0 neutral; 1 slightly cool; 2 cool; 3 cold). The PPD predicts the percentage of a large group of people likely to feel `too warm' or `too cool'. This was de®ned as those voting 3, 2, 2, or 3 on the scale. The indices are exactly as described by Fanger [2]. A draft rating index is provided in the standard as an equation involving air temperature, air velocity and turbulence intensity. It is applicable to mainly sedentary people wearing light clothing with a whole-body thermal sensation close to neutral. Recommended thermal comfort requirements are provided in Annex D of the standard (informativeÐnot a formal part of the standard). This includes optimum operative temperature; vertical air temperature gradient; mean air velocity; ¯oor temperature; and relative humidity. 5.1. ISO 7730: validity The PMV/PPD indices have been extensively investigated throughout the world and mostly in terms of validity. Does

539

the PMV accurately predict the actual mean vote (AMV) of people? Empirical research has led to mixed results and discussion of interpretation. Laboratory studies have often supported the validity of ISO 7730 whereas ®eld studies have not. However, the interpretation of results often involves discussion of the sensitivity of the method to estimates of variables such as metabolic heat and clothing insulation which are dif®cult to estimate and, in practical situations (along with the other parameters), often vary. Other issues are concerned with the sensitivity of the method. How well can the method distinguish between comfort conditions? How well does it need to? If we achieve statistical signi®cance between AMV and PMV of 0.1 of a scale value, does this have practical signi®cance? Practical signi®cance will depend upon context, but what difference between AMV and PMV would encourage standards makers to revise the standard on the grounds of validity? ISO 7730 has been criticized because of its lack of theoretical validity. The PMV/PPD indices were established in 1970. Since then there have been improvements to the human heat balance equation. There are also dynamic models of human thermoregulation that offer more accurate representations of physiological measures such as mean skin temperature and sweat rate. The prediction of sensation away from neutrality (towards warm or cool) is based upon the principle of thermal load. This has been criticized [3]. A more valid approach may be to predict deviation from neutrality using predictions of body state, such as skin temperature, sweat rate, or skin wettedness [4]. The question for the standards maker is do these limitations have practical signi®cance? Related to the validity of ISO 7730 is the validity of ISO 8996Ðmetabolic rate, and ISO 9920Ðclothing. The estimation of metabolic heat production and clothing insulation (two parameters to which the PMV is particularly sensitive) and other properties are dif®cult, especially when considering practical, dynamic contexts. This raises the question as to whether a valid rational thermal index or model is possible. Why continue to improve the heat balance equation or develop thermal models when the complexity of `reality' will undermine any improvement? The adaptive model of thermal comfort has questioned the validity of ISO 7730 and this is also related to scope. The question of validity is related to how well the methods in the standard (PMV, etc.) relate to the actual thermal comfort responses of people. If people change their thermal comfort response with prevailing outside climatic conditions (for exactly the same clothing, activity and indoor climate) the standard will not respond to this and it will have reduced validity. If different populations and cultures differ in thermal comfort responses (to identical clothing, activity and indoor climate) then the standard will have reduced validity for some populations. If the standard does not include those populations in its scope (e.g. people from Africa, Asia, etc.) then the standard may be valid but should it be accepted as a universal International Standard?

540


5.2. ISO 7730: reliability De®ning the PMV/PPD in an International Standard provides the major advantage of ensuring that when it is calculated anywhere in the world the same result will be obtained. However, if two assessments were made of identical conditions, by different users, or the same user on a different occasion, identical outcomes may not be achieved. Methods for estimating metabolic rate (ISO 8996), clothing insulation (ISO 9920), and environmental parameters (ISO 7726) will in¯uence reliability as will ambiguities in the standard about where and when to assess the environment. 5.3. ISO 7730: usability It is not clear who the users of ISO 7730 are intended to be and this clouds a discussion of usability. However it is probably reasonable to assume that those involved in environmental design and assessment, building services, engineering and ergonomics, would be users. The PMV/PPD indices provide clear predictions of likely discomfort and dissatisfaction. It could be argued, however, that exactly how to measure or estimate input parameters for the model are technical and not exact. Examples, in an annex to the standard, of how it can be used would aid usability. Usability testing where groups of users were observed using the standard and feedback provided would also improve the usability of the standard. 5.4. ISO 7730: scope The scope of a standard is concerned with to what it does and does not apply. ISO 7730 can be considered in terms of to whom it applies and over what range of environmental conditions. The PMV/PPD index was developed using North American and European people. The standard notes that deviations may occur due to ethnic and national-geographic deviations and for people who are sick or disabled. It applies to healthy men and women. Children are not considered. The standard applies to indoor environments where steady state thermal comfort or moderate deviations from comfort occur. This allows for interpretation and judgment. Does the standard apply to environments where conditions vary? Can the PMV-index be used as an adaptive model as it can account for changes in clothing, activity, posture and environmental conditions? The draught rating model is limited in scope to a narrow range of conditions and for people in thermal neutrality. People who are hot or cold may respond differently from the predictions of the draught rating model.

ISO 7730 and the assessment of thermal comfort. The methods are divided into three levels according to accuracy. Level I provides tables of estimates of metabolic rate (assumed identical to metabolic heat production) for kinds of activity and occupation. This is 'rough information where the risk of error is great'. Level II presents tables of estimated metabolic rate based upon group assessment, speci®c activities, and measurement of heart rate. This is `high error risk accuracy 15%'. The most accurate measure (5%) is a method of estimating metabolic rate by analysis of expired àir' from the lungs (indirect calorimetry). The principle is that energy is produced from burning food in oxygen. Comparison of the oxygen content of expired air (collected in a Douglas bag or other methodÐa typical value will be around 16±18%) with that of inspired air (20%) provides the rate of oxygen used by the body. With adjustments for type of combustion (from CO2 output), temperature, and pressure the metabolic rate can be derived from the calori®c value of food. The units are presented as Watts per square metre of the body surface of a standard person (70 Kg, 1.8 m2 male; 60 Kg, 1.6 m2 female). For an activity, such as walking up hill, the weight of the person will be important and adjustments may need to be made. The validity of ISO 8996 is, in principle, high as oxygen consumption clearly relates to energy production. However, there are limitations. How metabolic rate relates to heat production for a given activity is not clear. Heart rate is affected by a number of factors (including psychological) as well as metabolic rate. The accuracy presented is an estimate and is not justi®ed. The use and calibration of instrumentation in indirect calorimetry is important, and the estimation of metabolic heat production from tables is applicable to the context and population measured in the production of the tables. This will reduce reliability. There are also limitations to scope (e.g. to which populations does the standard apply?) and usability (who are the intended users and can they use it as intended?). Despite the limitations, ISO 8996 probably provides the best available methods and data. The importance of the estimate of metabolic rate can be demonstrated in an example calculation of the PMV for conditions: air temperature ta mean radiant temperature tr 24 8C; partial vapor pressure Pa 1000 Pa; air velocity v 0:15 m s 1; clothing insulation 1.0 Clo; and metabolic rate estimate of 100 W m 2 provides a PMV 0:9. However with a 15% accuracy adjustment, a metabolic rate value of 85 W m 2 provides PMV 0:7 and 115 W m 2, PMV 1:1. Further discussion of estimation of metabolic heat production is provided in Havenith et al. [5] in this Special Issue.

6. ISO 8996 ergonomicsÐdetermination of metabolic heat production

7. ISO 9920 ergonomics of the thermal environmentÐestimation of the thermal insulation and evaporative resistance of a clothing ensemble

This standard describes six methods for estimating metabolic heat production, an essential requirement in the use of

ISO 9920 provides an extensive database of the thermal properties of clothing and garments. The properties are

B.W. Olesen, K.C. Parsons / Energy and Buildings 34 (2002) 537±548 Table 2 The influence of accuracy of estimate of metabolic rate and clothing insulation on PMV and PPD values M (W m 2)

Clo (m2 8C W 1)

50 58 66 85 100 115

0.130 0.155 0.180 0.130 0.155 0.180

PMV 1.0 0.0 0.4 0.5 0.9 1.2

PPD (%) 27.7 5.0 8.8 10.5 22.6 36.4

ta tr 24 8C; Pa 1000 Pa; v 0:15 m s 1.

based upon measurements on heated manikins where basic (or intrinsic) thermal insulation is measured as well as vapor permeation properties of garments and ensembles. The major question of validity is therefore whether measurements on manikins represent the `true dynamic' properties of clothing as worn by people. Although the in¯uence of air penetration (and pumping) is discussed, it is not suf®ciently quanti®ed in detail. The scope of the standard excludes the effects of absorption of water, buffering, textile comfort, rain, snow, and special protective clothing such as heated clothing. It also `does not deal with the separate insulation on different parts of the body and discomfort due to the asymmetry of a clothing ensemble'. The reliability of the manikin measures is generally considered to be high for repeated measures on the same manikin. However results may vary between manikins. How well a clothing ensemble of interest can be matched with values in the database is debatable. This is also relevant to usability. Another issue of usability is whether users have suf®cient training in how to interpret and use the information provided. It is important to have a view of how accurately the standard can predict clothing insulation properties. No guidance is provided on this. If we assume around 15% accuracy and combine it with metabolic rate (15% accuracy) the results in Table 2 show how the PMV/PPD indices vary for sitting at rest in a business suit and light activity in a business suit. It can be seen that predictions of discomfort will vary within the accuracy of metabolic rate and clothing insulation estimates. Inaccuracies in estimates of environmental variables will increase this uncertainty. 8. Revision of EN ISO 7730: analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort As for the current Standard, the criteria for an acceptable thermal climate are speci®ed in the proposed revision of EN ISO 7730 as requirements for general thermal comfort (PMV/PPD or operative temperature (air and mean radiant temperature), air velocity, humidity) and for local thermal discomfort (draught (mean air velocity, turbulence intensity, air temperature) vertical air temperature differences, radiant

541

temperature asymmetry, surface temperature of the ¯oor). The proposals have been in¯uenced by standards in Europe and the USA [6,7]. For most thermal parameters it has been possible to establish psychophysical relationships between the intensity of the parameter and a predicted percentage of people ®nding the conditions unacceptable. People may be dissatis®ed due to general thermal comfort and/or dissatis®ed due to local thermal discomfort parameters. At the present time there is no method for combining the percentages of dissatis®ed people to give an accurate prediction of the total number of people ®nding the environment unacceptable. For example, we do not know if the dissatisfaction resulting from general thermal discomfort is additive with the percentages of those who are dissatis®ed due to local discomforts, or whether the total dissatis®ed may be less than the sum of the individual percentages (i.e. some people complaining about more than one particular problem simultaneously). Thus to simplify the situation, in American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 55 the approach has been to specify criteria corresponding to 10% dissatisfaction with general thermal comfort, to which a net increase of another 10% dissatis®ed due to local discomforts has been added, bringing the total level of thermal acceptability down to 80%. In real buildings, it may be desirable to establish different target levels of thermal dissatisfaction based on what is technically possible, what is economically viable, energy considerations, environmental pollution, or occupant performance. Therefore, it is suggested in the revision of EN ISO 7730 (and ASHRAE 55) to specify different levels of acceptability as in CR 1752. Individual countries or contracts between client and designer can then specify which levels must be used. Table 3 gives recommended levels of acceptance for three classes of environment [6]. 9. General thermal comfort For general thermal comfort, Table 4 lists the criteria for operative temperature and air velocity corresponding to the three classes of comfort, for three typical spaces. The optimal temperature is the same for all three classes but the acceptable range will change as the allowed percentage of dissatis®ed changes. For the design of heating systems and heat load calculations, the lower value in the range should be used, and for cooling, the upper value is relevant. The recommendations in ASHRAE Standard 55-92 are mainly given for occupants performing light, primarily sedentary activity (<1.2 Met). The acceptable range of operative temperature corresponds to class B for people in typical summer (cooling season) clothing (0.5 Clo) and typical winter (heating season) clothing (1.0 Clo). Separate comfort zones for two seasons re¯ect the fact that people usually change clothing according to outside temperature

542


Table 3 Three categories of thermal environment: percentage of dissatisfied due to general comfort and local discomfort (CR 1752, 1998) Category

A B C

Thermal state of the body as a whole

Local thermal discomfort

PPD (%)

Draught rate, DR (%)

Vertical air temperature difference (%)

Warm or cool floor (%)

Radiant temperature asymmetry (%)

<15 <20 <25

<3 <5 <10

<10 <10 <15

<5 <5 <10

<6 <10 <15

PMV 0.2 < PMV < 0.2 0.5 < PMV < 0.5 0.7 < PMV < 0.7

Table 4 Example criteria for operative temperature and mean air velocity for typical spacesa Type of building/space

Clothing cooling season (summer; Clo)

Clothing heating season (winter; Clo)

Activity (met)

Category

Operative temperature cooling season (summer; 8C)

Operative temperature heating season (winter; 8C)

Mean air velocity cooling season (summer; m s 1)

Mean air velocity heating season (winter; m s 1)

Office

0.5

1.0

1.2

A B C

24.5 0.5 24.5 1.5 24.5 2.5

22.0 1.0 22.0 2.0 22.0 3.0

0.18 0.22 0.25

0.15 0.18 0.21

Cafeteria/restaurant

0.5

1.0

1.4

A B C

23.5 1.0 23.5 2.0 23.5 2.5

20.0 1.0 20.0 2.5 20.0 3.5

0.16 0.20 0.24

0.13 0.16 0.19

Department/store

0.5

1.0

1.6

A B C

23.0 1.0 23.0 2.0 23.0 3.0

19.0 1.5 19.0 3.0 19.0 4.0

0.16 0.20 0.23

0.13 0.15 0.18

a

Relative humidity is assumed to be 60% for `summer' and 40% for `winter'.

(summer±winter). It, of course, creates some problems in some work places that have a ®xed dress code or in geographical regions that have very small seasonal variations in outdoor temperature. In the currently proposed revisions of EN ISO 7730 (and ASHRAE 55-92) the in¯uence and use of the following factors are being considered for inclusion.

9.1. Increased air speed The proposed revision of EN ISO 7730 (and ASHRAE 55-92) will include a diagram to estimate the air speed required to offset an increase in temperature (Fig. 1). The ®gure is based on a theoretical calculation, but an ASHRAE sponsored research project (843-TRP: human response to air

Fig. 1. Air speed required to offset increased temperature. The air speed increases in the amount necessary to maintain the same total heat transfer from the skin. This figure applies to increases in temperature above those allowed in the summer comfort zone with both tr, and ta increasing equally.


movements, Part 1Ðpreference and draught discomfort) has recently veri®ed the relation based on experimental tests with human subjects. This research project also investigated whether people achieved the same level of environmental acceptability at higher temperature/velocity combinations as they do at lower temperature/velocity combinations. Toftum et al. [8] experimentally veri®ed the diagram in ASHRAE 55-92 (Fig. 1) for occupants having individual control (ceiling fans, operable windows). This study also showed that the requirement of personal control of the increased air speed is essential for this acceptance. Therefore, it may not be appropriate to offset a temperature increase by increasing the air speed within a centrallycontrolled air system. 9.2. Adaptation Several extensive ®eld studies summarized by de Dear and Brager [9] have shown that, in buildings running with centrally-controlled HVAC systems, the PMV-model approximates the observed thermal comfort of occupants quite closely. Fig. 2 shows a comparison between the optimum comfort temperatures observed in a database of building ®eld experiments [9] and the predicted optimum comfort temperatures in those same buildings based on building-averages of the six comfort parameters. As seen in the left-hand panel of Fig. 2, there is very close agreement between predicted and observed comfort temperatures in the database's centrally-controlled HVAC buildings and this holds true across a broad swathe of external climatic contexts (represented as mean monthly outdoor effective temperature). The right-hand panel of Fig. 2 shows a comparison between PMV-based predictions of optimum thermal comfort and the actual comfort temperatures observed in naturally ventilated (free-running, no mechanical cooling) buildings. In such buildings the occupants seem capable of adapting to a broader range of conditions and can accept higher indoor temperatures than predicted by the PMVmodel. In free-running buildings located in warmer climates

543

the occupants typically wear lighter clothing, and this in turn causes the comfort temperature predicted by the PMVmodel in those buildings to be elevated. The PMV-model can also account for the impact of higher air velocities, created perhaps by operable windows or fans. The observed comfort temperature at 30 8C outside temperature is approximately 1.5 K higher. The reason for this difference is not yet documented. A building occupant's perception of the indoor climate in that building at a given point in time may be in¯uenced by what he or she expects to ®nd there. Some of the difference may also be explained by an average daily lower activity level (posture, moving slowly) which is dif®cult to evaluate in a short time ®eld study. It is also not clear how the performance or productivity is in¯uenced by the higher temperatures. In both the EN ISO 7730 and ASHRAE 55-92 revisions it is discussed how these results can be integrated into the standards. In EN ISO 7730 it will be recommended to use a wider PMV-range (class C) for design and/or evaluating of the thermal environment in such buildings. The proposed revision of ASHRAE 55-92 will include a diagram for predicting the comfort range from the outside temperature. It should be noted that even as far back as 1936, ASHRAE took into account thermal adaptation when specifying requirements for the indoor temperature. The following text can be found in ASHRAE Handbook, 1936, Chapter 3. It should be kept in mind that southern people, with their more sluggish heat production and lack of adaptability, will demand a comfort zone several degrees higher than those given here for the more active people of northern climates. 9.3. Humidity Recommended humidity limits have caused a lot of discussion during revision of standards. The in¯uence of humidity on preferred ambient temperature within the comfort range is relatively small. In EN ISO 7730, a humidity range of 30±70% RH is recommended, but mainly for indoor

Fig. 2. Comparison of the RP-884 adaptive models' predicted indoor comfort temperatures with those predicted by the PMV-model for both air-conditioned and naturally ventilated buildings (reproduced from [9]).

544


air quality reasons. In ASHRAE 55-92 no lower limit, and an upper limit at 17 8C dew-point temperature (humidity ratio 12 g kg 1) will be proposed. Requirements for acceptable indoor air quality [10,11] will, however, specify a more narrow range for the humidity. But should this be taken into account in a thermal comfort standard?

The summation of the product `weighting factor time' is called `weighted time' (in h). The values may be used for the evaluation of long term comfort conditions. An acceptable weighting time of, for example, 100±150 h may be speci®ed.

9.4. Long term evaluation of the general thermal comfort conditions

Non-steady state conditions often occur in the form of temperature cycles, temperature ramps and temperature transients. Some requirements are listed in ASHRAE Standard 55-92, but only limited research has been done on this subject to date. In many buildings there may be large energy savings if the indoor temperature is permitted to ¯oat (ramp) during day. But what is still acceptable and what the impact is on productivity, remain largely unanswered questions in the thermal comfort literature to date. It is also unclear whether peoples acceptance of non-steady state conditions is in¯uenced by whether they have personal control over those conditions, such as with a task-ambient control system, or with operable windows. An ASHRAE research project, 1161-TRP [12] has just begun to examine this very issue by conducting ®eld studies of of®ce buildings with operable windows. Perhaps the ®ndings of this research may eventually be incorporated into future revisions of the standards.

If criteria have to be met 100% of the time of occupancy, including under extreme weather conditions, the amount of heating and/or cooling capacity of any HVAC installation would be prohibitive. Economic and/or environmental considerations lead to a more pragmatic position of allowing the thermal conditions to exceed the recommended ranges for a limited time. By computer simulation, comfort conditions are often tested over longer periods of time, for different types of buildings and/or HVAC design. There is a need to quantify with some index the long term comfort conditions so that we may compare alternative designs. For these purposes, we recommend the following method for consideration in future revisions of the standards. The time during which the actual PMV exceeds the comfort boundaries is weighted with a factor that is a function of the PPD. Starting from a PMV-distribution on a yearly basis, and the relation between PMV and PPD, the following is calculated: weighting factor wf

PPDactual PMV PPDPMV limit

The weighted time is totalised for a characteristic working period during 1 year. X warm period : wf time; where PMV > PMVlimit X cold period : wf time; where PMV > PMVlimit

9.5. Non-steady state thermal environments

10. Local thermal discomfort The PMV and PPD indices express warm and cold discomfort for the body as a whole. But thermal dissatisfaction may also be caused by unwanted cooling (or heating) of one particular part of the body (local discomfort). Local thermal discomfort may be caused by draught, high vertical temperature difference between head and ankles, too warm or too cool a ¯oor, or by too high a radiant temperature asymmetry. Persons engaged in light sedentary activity are the most sensitive to local discomfort. The diagrams (Figs. 3±6) and Table 4 apply to this group of people with a

Fig. 3. Mean air velocity as a function of local air temperature and turbulence intensity for the three categories of the thermal environment.


545

Table 1 are listed in Table 4 for possible inclusion in the current round of revisions to the standards. 10.1. Draught

Fig. 4. Local discomfort caused by vertical air temperature difference. Applies when the temperature increases with height from the floor.

Fig. 5. Local thermal discomfort caused by warm or cold floors.

thermal sensation for the whole-body close to neutral. At higher activities, people are less thermally sensitive and consequently the risk of local discomfort is lower. The local discomfort criteria based on the three classes in

The in¯uence of air velocity on general thermal comfort and on local thermal comfort has been studied quite extensively in the USA, Europe and Japan. The effect of air velocity has not only been studied with sedentary people in general thermal comfort, but also with people at higher activity levels and who are on the warm or cold side of comfort. Air motion within a space can lead to draught sensation, but may also lead to improved comfort under warm conditions. The draught model, which is included both in ASHRAE Standard 55 and EN ISO 7730, is listed as DR 34

ta v

0:050:62 0:37 v Tu 3:14

where DR is the draught rating, i.e. the percentage of people dissatis®ed due to draught; ta the local air temperature in 8C; v the local mean air velocity in m s 1; and Tu is the local turbulence intensity in percent. The model of draught is based on studies comprising 150 subjects exposed to air temperatures of 20±26 8C, mean air velocities of 0.05±0.4 m s 1 and turbulence intensities of 0± 70%. The model applies to people at light, mainly sedentary activity with a thermal sensation for the whole-body close to neutral. The sensation of draught is lower at activities higher than sedentary, and for people feeling warmer than neutral. Recent studies by Griefhahn [13] indicate that this model must be modi®ed to take into account length of exposure and activity level. Studies by Toftum and co-workers [8,14±16] show additional in¯uence of the velocity directions. The two studies do not agree completely with the above draught model. According to Griefhahn the models predict DR percentages which are too low, while according to Toftum et al. [8], it predicts values which are too high. Further studies are needed before the draught model and Fig. 4 from the proposed standards will be changed. The above criteria show that some restrictive requirements on air velocity are necessary to avoid the sensation of draught in cool environments. In warm environments it may, however, be bene®cial for the total comfort to increase the air velocity above these levels. This effect is partly included in the use of the PMV-index. New studies indicate that, if the occupant is allowed to select their own air speed higher air temperatures are accepted [8]. 10.2. Vertical air temperature difference

Fig. 6. Local thermal discomfort caused by radiant temperature asymmetry.

A high vertical air temperature difference between head and ankles may cause discomfort. In Fig. 4, the percentage of those dissatis®ed as a function of the vertical air temperature difference between head and ankles, is shown (1.1 and 0.1 m above the ¯oor). The ®gure applies when the temperature increases upwards. People are less sensitive for decreasing temperature.

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Table 5 Recommended categories for local thermal discomfort parameters Category

Vertical air temperature difference (K)

Floor surface temperature (8C)

Radiant temperature asymmetry (K) Warm ceiling

Cool ceiling

Cool wall

Warm wall

A B C

<2 <3 <4

19±29 19±29 17±31

<5 <5 <7

<14 <14 <18

<10 <10 <13

<23 <23 <35

The studies have mainly been made with seated people in general thermal comfort so there is a need to extend research to human subjects at other activity levels before revisions of the indoor thermal environment standards can be proposed. The criteria in Table 5 and Fig. 4 will be included in the proposed revision. 10.3. Floor temperature If the ¯oor is too warm or too cool, the occupants may feel uncomfortable due to warm or cool feet. For people wearing light indoor shoes, it is the temperature of the ¯oor rather than the material of the ¯oor covering which is important for comfort. In Fig. 5, the percentage of those dissatis®ed as a function of the ¯oor temperature, is shown for spaces which people are with bare feetÐsee ISO/CD 13732-2. In this case, not only the ¯oor temperature but also the ¯oor material is important for comfort. The studies on ¯oor temperatures and comfort have been made for the situation where only the feet are in contact with the ¯oor. In Asia, it is common to sit or lie on the ¯oor. This may have an effect on the preferred ¯oor temperature. Several studies have been made in Japan and Korea, but often with a very limited number of subjects. It is also very important to distinguish between electrically heated ¯oors and water based ¯oor heating systems. This has not been taken into account in Table 4 proposed for the revision of EN ISO 7730 and ASHRAE 55. 10.4. Radiant asymmetry Radiant asymmetry may also cause discomfort. People are most sensitive to radiant asymmetry caused by warm ceilings or cool walls (windows). In Fig. 6, the percentage of those dissatis®ed as a function of the radiant temperature asymmetry caused by a warm ceiling, a cool wall, a cool ceiling or by a warm wall, is shown. Radiant asymmetry is rarely a problem in ventilated/air-conditioned spaces, except at high illumination levels and at large window areas. Direct solar radiation should be avoided in the occupied zone, by means of building design or solar shading devices. In industrial work places, it is mainly the radiant asymmetry from overhead radiant heaters or a hot roof, which cause problems. The values established for sedentary persons are too conservative for the higher activity levels and higher ceilings in industry. A study by Langkilde et al. [17]

shows that signi®cantly higher radiant asymmetry is acceptable. Based on criteria similar to the above requirements of less than 5% dissatis®ed, the recommended asymmetry limit is 10±14 8C. In these studies there were no differences between seated and standing persons. Also the in¯uence of wearing a helmet was insigni®cant. In previous studies simulating the conditions in residential and commercial buildings, the distance between the heated ceiling and the head of the subjects was only 1 m, while the distance between the infrared radiant heaters and the head was 4 m in the experiments reported by Langkilde et al. [17]. For the same asymmetry level in the two types of experiments, the difference in the radiant temperature level at feet and head height will be much larger when the distance from the person to the heaters is small (1 m). In industrial work places where the heaters normally are mounted more than 4 m from the occupants, a larger asymmetry can be accepted before the temperature difference felt between head and feet causes discomfort. That is also the reason why no difference was found between seated and standing persons. The change proposed for the standards is the inclusion of Table 5 with the three levels and Fig. 6. 11. Discussion and conclusion Some important issues are being discussed in the current round of standard revisions. Ful®lling the given criteria does not guarantee 100% thermal acceptability. Due to individual differences, it may be very dif®cult to satisfy everybody in a space but some forms of individual control of the thermal environment combined with individual adaptation (clothing, activity) will increase the level of acceptance. Field studies have shown that for heated and air-conditioned buildings the use of the PMV/PPD index agrees with the observations. But for `free-running' buildings in warm climates, where summertime reliance on natural ventilation occurs, there seems to be an additional adaptation which cannot be explained alone by behavioral adaptations, such as changes to clothing or air velocity. It may be due partly to the adaptation of the activity (metabolic rates), which is very dif®cult to measure in the ®eld. Most likely, it is primarily due to psychological adaptation in the form of shifting expectations which result from having personal control, and a history of more diverse thermal experiences. Another issue is whether the thermal environment must be inside the given range 100% of the time, or whether it is


more reasonable to allow conditions to vary outside the speci®ed criteria for some limited periods of time. For each of the thermal comfort factors, it may be possible from measurements or calculations to calculate a factor `% dissatisfied time' to take into account how long a time, and by how much, the conditions may deviate from the established comfort criteria. For such factor(s), additional criteria may be established or the values should be given in terms of an overall index of indoor environmental quality. Except for draught (air velocity), the local thermal discomfort parameters like vertical air temperature differences, ¯oor surface temperatures and radiant temperature asymmetry have mainly been studied for younger, sedentary people in general thermal comfort. The studies have in most cases only looked at one factor. There is a need to extend the study on local thermal comfort parameters for other types of activities and subject groups. There is very little information on the combined effect of general thermal comfort and local thermal comfort. Some studies have been made on the in¯uence of air velocity (draught) and general thermal discomfort. The in¯uence of the general thermal comfort on other local discomfort parameters like radiant temperature asymmetry, vertical air temperature differences, and ¯oor temperatures has not been studied. Moreover, very few studies have looked at the combined effect of several local thermal discomfort parameters. To be able to predict the combined in¯uence of the thermal environment on people, it is important to obtain more information on the combined effect of general and local comfort, and when there is exposure to several local discomfort parameters at the same time. As there are very large inter-individual variances in response to the thermal environment, it is essential to use relatively large samples of subjects to enable research results to achieve the statistical signi®cance necessary for generalization in practice. A key objective of standards is to transfer the latest scienti®c knowledge into practice. Standards governing indoor thermal environments at the international level (International Standard Organization (ISO); European Standard Organization (CEN)), and also the national level (ASHRAE) are on a constant cycle of revision, public review, and promulgation. Substantial progress in our understanding of human response to thermal environments has been made in numerous laboratory and ®eld research projects in the last decade. Many of these signi®cant advances are now informing the current round of standard revisions. Appendix A. A.1. ISO TC 159 SC5Ðergonomics of the physical environment: summary of work ISO TC159 SC5 produces International Standards in the area of the ergonomics of the physical environment. As this has a wide scope and standards are produced in other areas of

547

standardization (e.g. vibration) within ergonomics this has been con®ned to thermal environments (WG1), lighting (WG2) and danger signals and communication in noisy environments (WG3). WG1 produces standards concerned with heat stress, cold stress and thermal comfort as well as supporting standards concerned with the thermal properties of clothing and metabolic heat production due to activity. It also considers physiological measures, skin reaction to contact with hot, moderate and cold surfaces and thermal comfort requirements for people with special requirements. WG2 is concerned with the ergonomics of lighting and is strongly guided by the international lighting commission (CIE). WG3 considers communication in noisy environments including warning and danger signals and speech. Recent new work items have included the effects of combined stress environments and also the performance of glazing in terms of visual and thermal comfort. A.2. Published standards and standards in development ISO 7243 (1995): Hot environmentsÐestimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature). ISO 7726 (1998): Thermal environmentsÐinstruments and methods for measuring physical quantities. ISO 7730 (1994): Moderate thermal environmentsÐ determination of the PMV and PPD indices and specification of the conditions for thermal comfort. ISO 7731 (1986): Danger signals for workplacesÐauditory danger signals. ISO 7933 (1989): Hot environmentsÐanalytical determination and interpretation of thermal stress using calculation of required sweat rate. ISO 8995 (1989): Principles of visual ergonomicsÐthe lighting of indoor work systems. ISO 8996 (1990): ErgonomicsÐdetermination of metabolic heat production. ISO 9886 (1992): Evaluation of thermal strain by physiological measurements. ISO 9920 (1995): Ergonomics of the thermal environmentÐestimation of the thermal insulation and evaporative resistance of a clothing ensemble. ISO 9921-1 (1996): Ergonomic assessment of speech communication (Part 1)Ðspeech interference level and communication distances for persons with normal hearing capacity in direct communication (SIL method). ISO 10551 (1995): Ergonomics of the thermal environmentÐassessment of the influence of the thermal environment using subjective judgment scales. ISO 11399 (1995): Ergonomics of the thermal environmentÐprinciples and application of International Standards. ISO 11428 (1994): ErgonomicsÐvisual danger signalsÐ general requirements, design and testing. ISO 11429 (1994): ErgonomicsÐsystem of danger and non-danger signals with sound and light.

548


A.3. Technical reports ISO TR 11079 (technical report, 1993): Evaluation of cold environmentsÐdetermination of required clothing insulation, IREQ. A.4. Current work programme A.4.1. Ergonomics of the thermal environmentÐISO TC 159 SC5 WG1 ISO 15742: Ergonomics of the physical environmentÐ combined effects of thermal environment, air pollution, acoustics and illumination. Revision of ISO 7933 (1989): Hot environmentsÐanalytical determination and interpretation of thermal stress using calculation of required sweat rate. Revision of ISO 8996 (1990): ErgonomicsÐdetermination of metabolic heat production. Revision of ISO 9886 (1992): Evaluation of thermal strain by physiological measurements. Revision of ISO 7730 (1994): Ergonomics of the thermal environmentÐanalytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort. Revision of ISO TR 11079 (technical report, 1993): International StandardÐevaluation of cold environmentsÐ determination of required clothing insulation, IREQ. ISO DIS 11371: Ergonomics of the thermal environmentÐvocabulary and symbols. ISO DIS 12894 (1993): Ergonomics of the thermal environmentÐmedical supervision of individuals exposed to hot or cold environments. ISO/NP 13732 (Part 1): Ergonomics of the thermal environmentÐmethods for the assessment of human responses to contact with surfaces (Part 1: hot surfaces). ISO CD 13732 (Part 2): Ergonomics of the thermal environmentÐmethods for the assessment of human responses to contact with surfaces (Part 2: moderate surfaces). ISO/NP 13732 (Part 3): Ergonomics of the thermal environmentÐmethods for the assessment of human responses to contact with surfaces (Part 3: cold surfaces). ISO NP 14405: Ergonomics of the thermal environmentÐ evaluation of the thermal environment in vehicles. ISO NP 14415: Ergonomics of the thermal environmentÐ application of International Standards to the disabled, the aged and other handicapped persons. ISO NP 15265: Ergonomics of the thermal environmentÐ risk of stress or discomfort. ISO NP 15743: Ergonomics of the thermal environmentÐ working practices for cold indoor environments. A.4.2. LightingÐISO TC 159 SC5 WG2 Revision of ISO 8995 (1989): Principles of visual ergonomicsÐthe lighting of indoor work systems.

A.5. Danger signals and communication in noisy environmentsÐISO TC 159 SC5 WG3 ISO 9921: Ergonomic assessment of speech communication in noisy environmentsÐrevision of Part 1 and to include Parts 0, 2 and 3 (principles, criteria, prediction and assessment). Revision of ISO 7731 (1986): Danger signals for workplacesÐauditory danger signals.

References [1] N. Delleman, T. Itani, K.C. Parsons, K. Scheuermann, T. Stewart, User involvement in International Standardization, in: Proceedings of the XIVth Triennial Congress of the IEA and 44th Annual Meeting of the Human Factors and Ergonomics Society, Ergonomics for the New Millennium, 29 July±4 August 2000. [2] P.O. Fanger, Thermal Comfort, Danish Technical Press, Copenhagen, 1970. [3] M.A. Humphreys, J.F. Nicol, Conflicting criteria for thermal sensation within the Fanger predicted mean vote equation, in: Proceedings of CIBSE/ASHRAE Joint National Conference, Harrogate, UK, 29 September±1 October 1996. [4] A.P. Gagge, J.A.J. Stolwijk, Y. Nishi, An effective temperature scale based on a single model of human physiological temperature response, ASHRAE Transactions 77 (1971) 247±262. [5] G. Havenith, I. HolmeÂr, K.C. Parsons, Personal factors in thermal comfort assessment: the estimation of clothing properties and metabolic heat production, Energy and Buildings (2002). [6] CR 1752, Ventilation for Buildings: Design Criteria for the Indoor Environment, CEN, Brussels, 1998. [7] ASHRAE Standard 55-92, Thermal Environment Conditions for Human Occupancy, ASHRAE, Atlanta, 1992. [8] J. Toftum, A.K. Melikov, L.W. Rasmussen, A.A. Kuciel, E.A. Cinalska, A. Tynel, M. Bruzda, P.O. Fanger, Human Response to Air Movement. Part 1. Preference and Draught Discomfort, ASHRAE Project 843-TRP, Technical University of Denmark, Denmark, 2000. [9] R. de Dear, G.S. Brager, Developing an adaptive model of thermal comfort and preference, ASHRAE Transactions 104 (1a) (1998) 145± 167. [10] L.G. Berglund, Comfort and humidity, ASHRAE Journal 40 (8) (1998). [11] L. Fang, G. Clausen, P.O. Fanger, The impact of temperature and humidity on perception and emission of indoor air pollutants, in: Proceedings of Indoor Air'96, Tokyo, Institute of Public Health, Tokyo, 1996. [12] S.G. Brager, 1161-TRP: The Effect of Personal Control and Thermal Variability on Comfort and Acceptability, Contract Awarded by ASHRAE, September 2000. [13] B. Griefhahn, Bewertung von Zugluft am Arbeitsplatz, Fb 828, Schriftenreihe der Bundesanstalt fuÈr Arbeitsschutz und Arbeitsmedizin, Dortmund, 1999. [14] J. Toftum, R. Nielsen, Draught sensitivity is influenced by general thermal sensation, International Journal of Industrial Ergonomics 18 (4) (1996) 295±305. [15] J. Toftum, R. Nielsen, Impact of metabolic rate on human response to air movements during work in cool environments, International Journal of Industrial Ergonomics 18 (4) (1996) 307±316. [16] J. Toftum, G. Zhou, A. Melikov, Airflow direction and human sensitivity to draught, in: Proceedings of CLIMA 2000, Brussels, 1997. [17] G. Langkilde, L. Gunnarsen, N. Mortensen, 1985. Comfort limits during infrared radiant heating of industrial spaces. CLIMA 2000, Copenhagen.

Introduction to thermal comfort standards and to the proposed new version of EN ISO 7730

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