Berkeley Model Human Comfort

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2 0 A S H R A E J o u r n a l a s h r a e . o r g A u g u s t 2 0 0 4

Predict Comfortubstantial progress in our understanding of human response

to thermal environments has been made since Standard 55-

1992, including the amendment 55-95a. Incorporating many of

these advances, Standard 55-2004, Thermal Environmental Condi-

tions for Human Occupancy , was recently published after complet-

ing four public reviews and receiving approval by both ASHRAE

and the American National Standards Institute (ANSI).

The standard specifies conditions of the indoor thermal environment thatoccupants will find acceptable. It is in-tended for use in design, commissioning,and testing of buildings and other occu-

pied spaces and their HVAC systems, and

for the evaluation of existing thermal en-vironments. Because of the inherentvariations in occupants metabolic ratesand clothing levels, and because it is not

possible to prescribe or enforce whatthese should be, this standard cannot

By Bjarne W. Olesen, Ph.D., Fellow ASHRAE, and Gail S. Brager, Ph.D., Fellow ASHRAE

practically mandate operating setpointsfor buildings.

The two most important additions in-cluded in this new standard are an ana-lytical method based on the PMV-PPDindices and introduction of the conceptof adaptation with a separate method for naturally conditioned buildings. Theadaptive model and several other changesare based on various ASHRAE sponsored research projects. This article provides anoverview of the key features and limitsof applicability of Standard 55-2004.

SSSSS

A Better Way to

About the Authors

Bjarne W. Olesen, Ph.D., is professor and director of the International Center for Indoor Envi-ronment and Energy at the Technical University of Denmark. Gail S. Brager, Ph.D., is professor andassociate director of the Center for the Built Envi-ronment at the University of California, Berkeley.

The following article was published in ASHRAE Journal, August 2004. Copyright 2004 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or inpaper form without permission of ASHRAE.


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PMV/PPD Method

One of the most significant changes to Standard 55-92 wasthe inclusion of the PMV-PPD method of calculation to deter-mine the comfort zone. Hopefully, this will result in engineers

being more likely to use the calculation method to estimate theacceptable range of thermal conditions for their particular situ-ation, rather than defaulting to the simpler graphic comfort zone,where the assumptions might not match their conditions. Stan-dard 55 is now more consistent with other international stan-dards, such as ISO EN 7730. 1

PMV (Predicted Mean Vote) is an index that expresses thequality of the thermal environment as a mean value of the votesof a large group of persons on the ASHRAE seven-point ther-

mal sensation scale (+3 hot, +2 warm, +1 slightly warm, 0 neu-tral, 1 slightly cool, 2 cool, 3 cold). PPD (Predicted Percentage Dissatisfied) is an index expressing the thermalcomfort level as a percentage of thermally dissatisfied people,and is directly determined from PMV. The PPD index is based on the assumption that people voting 2 or 3 on the thermalsensation scale are dissatisfied, and the simplification that PPDis symmetric around a neutral PMV (=0). Both PMV and PPDare based on general (whole body) thermal comfort.

For specified values of the other four thermal comfort fac-tors (humidity, air speed, clothing insulation and metabolicrate), a comfort zone can be defined in terms of a range of operative temperatures that result in a specified percentageof occupants who will find those conditions acceptable. Op-erative temperature is related to the dry heat exchange by

both convection and radiation, and is often approximated bythe simple average of the air temperature and mean radianttemperature (Appendix C of the standard provides more in-formation about calculating operative temperature).

Section 5.2.1 offers both a simplified graphic method for determining the comfort zone for limited applications, and alsoa computer program that allows the user to run the PMV-PPDmodel for a wider range of applications. When using either

method, one needs to assess or make assumptions about the

occupants metabolic rates and clothing insulation levels for the space being considered.

Graphical Method. Using the PMV-PPD model, the ac-ceptable range of operative temperature is shown in a psy-chrometric chart for people wearing two different levels of clothing: 0.5 clo (0.08 m 2K/W) (typical for summer or cool-ing season) and 1.0 clo (0.155 m 2K/W) (typical for winter or heating season). The graphical comfort zones ( Figure 1 ) cor-respond to a PPD of 10% (general thermal discomfort). Thegraphic zones are simple to use, but are only applicable for limited situations where metabolic rates are between 1.0 to1.3 met (58.15 to 75.6 W/m 2), and air speed is less than 0.20

m/s (40 fpm). If clothing values are in-between 0.5 to 1.0 clo(0.08 to 0.155 m 2K/W), one can determine the acceptableoperative temperature range by linear interpolation betweenthe limits found for each zone. While the separate comfortzones reflect the fact that people usually change clothing ac-cording to outside temperature or season, this is not alwaysthe case for workplaces that have a fixed dress code, or ingeographical regions that have small seasonal variations, or where people might dress for an indoor climate that is con-sistent year-round (i.e., a constant setpoint temperature thatdoesnt change with the seasons).

It is important that people use this figure carefully, con-firming that the selected clo levels associated with a comfortzone are appropriate for the building they are designing or evaluating.

Computer Method. The computer program for calculat-ing the PMV and PPD indices is in Appendix D of Standard 55-2004. Although more complex than the graphical method,it can be applied to a wider range of conditions and allowsthe user to see the effects of altering the various factors af-fecting thermal comfort. The computer model itself is appli-cable for situations where clothing insulation is less than 1.5clo (0.23 m 2K/W), metabolic rates are between 1.0 to 2.0

Figure 1: Acceptable range of operative temperature and humidity (for spaces that meet criteria specified in Section 5.2.1).

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Data Based on ISO 7730 and ASHRAE Standard 55

Upper Recommended Humidity Limit, 0.012 Humidity Ratio

1.0 Clo 0.5 Clo

0.5PMV Limits

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10 13 16 18 21 24 27 29 32 35 38Operative Temp., C

Data Based on ISO 7730 and ASHRAE Standard 55

Upper Recommended Humidity Limit, 0.012 Humidity Ratio

1.0 Clo 0.5 Clo

0.5PMV Limits

No RecommendedLower Humidity

Limit

No RecommendedLower Humidity

Limit

908070605040302010% RH

908070605040302010% RH


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A u g u s t 2 0 0 4 A S H R A E J o u r n a l 2 3

Figure 2: Air speed required to offset increased temperature from Section 5.2.3.

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Limits for Light, PrimarilySedentary Activity

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met (58.15 to 75.6 W/m 2), and air speed up to 1 m/s (200fpm). In Standard 55-2004 the use of the model is limited toair speeds less than 0.20 m/s (40 fpm). Higher air speeds can

be used to increase the upper operative temperature limit incertain circumstances, as described in a later section. By in-

putting specific values of humidity, air speed, clothing, and metabolic rate, the model can be used to determine the op-erative temperature range that will produce a PMV withinthe range 0.5 < PMV < +0.5, which corresponds to a PPD of 10%. As described earlier, the 80% overall acceptability as-sumes 10% dissatisfaction for general thermal comfort (PPD),

plus an additional 10% dissatisfaction that may simulta-neously occur on average from local thermal discomfort.

Humidity The scope of Standard 55 clearly states that its criteria are

based only on thermal comfort. Therefore, Section 5.2.2 does

not specify a minimum humidity level since no lower humid-ity limits relate exclusivelyto thermal comfort. Thestandard acknowledges,however, that there may benon-thermal factors that af-fect the acceptability of verylow humidity environments,and designers should beaware of this even if it is be-yond the scope of this stan-dard.

The form of the upper limit of humidity haschanged throughout thestandards history. It was ex-

pressed in terms of a humid-ity ratio in 55-1981 (based originally on indoor air quality considerations), a relative hu-midity in 55-1992, and a wet-bulb temperature in 55-1995a.Regardless of which form was used in the past, the influenceof humidity on preferred ambient temperature within the com-fort range is relatively small. The committee decided to usethe more simple absolute humidity as the limiting parameter and return to the upper limit used in 1981, namely a humidityratio of 0.012. This upper humidity limit applies only to situ-ations where there is a system in place designed to controlhumidity.

Air SpeedThe operative temperature limits in Section 5.2.1 are based

on a limit of air speed less than 0.20 m/s (40 fpm). However,higher levels of air movement can be beneficial for improvingcomfort at higher temperatures. Section 5.2.3 presents a graph( Figure 2 ) showing the relationship between elevated air speed

and the temperature rise above the upper limit of the comfortzone (i.e., for a given air speed, what upper temperature limitwould be acceptable; or for a given temperature rise, what air speed would be required). This figure is applicable for lightlyclothed people with clothing insulation between 0.5 to 0.7 clo

(0.08 to 0.1 m 2K/W) and metabolic rates between 1.0 to 1.3met (58.15 to 75.6 W/m 2), and in situations were occupants areindividually able to control the air movement.

Limited data is available that shows the precise relationship between increased air speed and improved comfort, so the re-lationship is derived from theoretical calculations of equiva-lent heat loss from the skin, combined with professional

judgment about reasonable limitations that should be placed on this allowance. Recent research sponsored by ASHRAE 2

has experimentally verified the diagram for occupants havingindividual control. This graph is especially important for com-mercial buildings that are primarily in cooling mode because

of high internal loads, where there may be an opportunity toreduce energy use while im- proving comfort. This can beachieved by allowing thetemperature to rise slightlytowards the higher end of thecomfort zone, while giving

people the opportunity to in-dividually control air move-ment through task/ambientconditioning systems, per-sonal or ceiling fans, or op-erable windows.

Local DiscomfortThe PMV and PPD indices

express warm and cold dis-comfort for the body as a

whole. However, thermal dissatisfaction also may be caused byunwanted cooling (or heating) of one particular part of the body(local discomfort). Local thermal discomfort may be caused bydraft, high vertical temperature difference between head and ankles, too warm or too cool a floor, or by too high a radianttemperature asymmetry. The requirements for local thermal dis-comfort in Section 5.2.4 apply to lightly clothed people with cloth-ing insulation between 0.5 to 0.7 clo (0.08 to 0.1 m 2K/W), and metabolic rates between 1.0 to 1.3 met (58.15 to 75.6 W/m 2).The effect of local discomfort is greatest at lower activity or lighter clothing, so therefore, the risk of discomfort is lower for met >1.3 (m 2K/W > 0.1) and clo > 0.7 (W/m 2 > 75.6), and the require-ments are conservative and also may be applied for these cir-cumstances. While these requirements apply to the entire comfortzone, they are based on exposures where people are close to ther-mal neutrality (not cool, not warm). The allowable percent dis-satisfied varies from 5% to 20%, based on the source of local


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discomfort. Future studies will be required to develop infor-mation on the combined effectof general thermal comfortand local thermal comfort, or

the combined effect of severallocal thermal discomfort pa-rameters.

The forms of local discom-fort discussed in Standard 55-2004 are:

Radiant TemperatureAsymmetry. This refers to thenon-uniform thermal radiationfield around the body due tohot and cold surfaces and di-rect sunlight. Allowing for 5% dissatisfied, the standard provides

separate physical limits for a warm or cool ceiling and a warm or cool wall to reflect the bodys different sensitivities to thesesources. For example, people are most sensitive to radiant asym-metry caused by warm ceilings or cool walls (such as windows).

Draft. Air motion within a space may improve comfort un-der warm conditions, but also may produce a draft sensation in

cooler conditions. Draft isdefined as the unwanted lo-cal cooling of the bodycaused by air movement. The

pr ed ic ted pe rc en ta ge of

people dissatisfied due toannoyance by draft is a func-tion of local air temperature,air speed and turbulence in-tensity. The requirements inthe standard for maximumallowable air speed are based on 20% dissatisfied. Themodel is based on the great-est sensitivity to draft thehead region with airflow

from behind and so the requirements may be conservative

for other locations on the body and other directions of airflow.The criteria in this section do not apply to the use of elevated air speed under individual control for offsetting warm condi-tions, as presented in Section 5.2.3.

Vertical Air Temperature Difference. A large vertical air temperature difference between the head and ankles may cause

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Mean Monthly Outdoor Air Temp., F50 59 68 77 86 95

Figure 3: Acceptable operative temperature ranges for naturally conditioned spaces.

90% Acceptability Limits80% Acceptability Limits

Advertisement in the print edition formerly in this space.


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A u g u s t 2 0 0 4 A S H R A E J o u r n a l 2 5

discomfort. Based on criteria of 5% dis-satisfied, the allowable temperature differ-ence is 3C (5.4F), which applies for situations where the temperature increaseswith height from the floor (i.e., the head

is warmer than the feet). People are lesssensitive to decreasing temperature, and no limits for this situation are included inthe standard.

Floor Surface Temperature. If thefloor is too warm or too cool, occupantsmay feel uncomfortable due to warm or cool feet. Based on criteria of 10% dis-satisfied, the allowable range of floor surface temperature is 19C to 29C(66.2F to 84.2F), which is based on

people wearing normal indoor shoes. The

standard does not address situationswhere people are barefoot, or are sittingon the floor.

Temperature Variations With Time Non-steady-sta te conditions in the

form of fluctuations in air temperatureand/or mean radiant temperature mayaffect occupants thermal comfort. How-ever, only limited research has been doneon this subject.

The standard specifies limits on cyclicoperative temperature variations (i.e.,where the operative temperature repeat-edly rises and falls within a period notgreater than 15 minutes), expressed as anallowable peak-to-peak variation of 1.1C (2.0F). For cyclic variations witha period greater than 15 minutes, or for monotonic temperature drifts (passivetemperature changes) and ramps (activelycontrolled temperature changes), thestandard provides a table that specifiesthe maximum operative temperaturechange allowed within a given time pe-riod. The requirements of this section areapplicable only for situations where thefluctuations are not under the direct con-trol of the occupant.

Adaptation and Naturally ConditionedBuildings

Section 5.3 is a new optional method for determining acceptable thermal con-ditions in naturally conditioned spaces.

It is applicable only for spaces where thethermal conditions are regulated prima-rily by the occupants through openingand closing of windows, there is no me-chanical cooling (mechanical ventilation

is allowed), and metabolic rates rangefrom 1.0 to 1.3 met (58.15 to 75.6 W/m2). The space may have a heating sys-tem in place, but this method does notapply when the heating system is in op-eration. The range of acceptable opera-tive temperatures ( Figure 3 ) is a functionof outdoor temperature, and is based onan adaptive model of thermal comfort de-veloped from an ASHRAE-sponsored re-search project. 3 The model is derived from a global database of 21,000 mea-

surements taken primarily in office build-ings from four different continents.The research showed that, when occu-

pants have control over operable win-dows and are accustomed to conditionsthat are more connected to the naturalswings of the outdoor climate, the sub-

jective notion of comfort and preferred temperatures change as a result of avail-ability of control, different thermal ex-

perience, and resulting shifts in occupant perceptions or expectations. Since themodel is based on field data that alreadyaccounts for peoples clothing adaptation,it is not necessary to estimate the clo val-ues for the space.

The field data also accounts for localthermal discomfort effects in typical

buildings, so it is not necessary to addressthese factors when using this option. If the user believes that more extreme localconditions might occur, then they are en-couraged to use the local thermal discom-fort criteria in Section 5.2.4. No humiditylimits are required (consistent with Sec-tion 5.2.2, which states that humidity lim-its are only applicable when there is asystem designed to control humidity),and no air speed limits are required whenthis option is used.

ComplianceSection 6 states that building systems

(combination of mechanical systems,control systems, and thermal envelopes)



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shall be designed so that, at design con-ditions, the thermal conditions of thespaces can be maintained within thespecifications and also at all combina-tions of less extreme conditions (i.e., ex-

ternal climate and internal loads) that areexpected to occur.

Engineers are used to the notion of design weather data, which representscertain percentages of exceedance of outdoor weather conditions (e.g., 1%design, four-month summer basis, 29hours of exceedance), and recognize thatit is impractical for HVAC systems tomeet all loads encountered in its lifetime.Therefore, it is also expected that the ther-mal comfort requirements of this stan-

dard may not be met during excursionsfrom those design conditions. Documen-tation requirements for comfort criteriahave been expanded in Standard 55-2004, and include information such astolerances, capacities and control of com-

fort variables, as well as building layoutand maintenance.

EvaluationIn addition to being used for design,

Standard 55 also can be used to evaluateexisting thermal environments. For this

purpose, Section 7 specifies the measure-ment positions, periods and conditionsunder which one should determine the ef-fectiveness of the building at providingthe environmental conditions specified inthe standard. Validation of comfort (7.6)has been added to the evaluation sectionfor 2004. There are two methods of evalu-ating compliance with comfort require-ments. The first method involves an

occupant survey (a sample is proved inAppendix E), and the second method analyzes environmental variables to de-termine comfort conditions.

ConclusionStanding Standards Project Committee

55 is approved to operate under continu-ous maintenance and the committee hasseveral goals for the next set of revisions.In real buildings, it may be desirable toestablish different target levels of thermaldissatisfaction based on what is technically

possible, what is economically viable, en-ergy considerations, environmental pollu-tion, or occupant performance. As such, itmight be desirable for building profession-als and clients to be able to make their own

judgment about what percentage of the oc-cupants should be satisfied in a given con-text for the environment to be deemed thermally acceptable.

Future changes to Standard 55-2004likely will include an appendix that givesrecommended levels of acceptance for three classes of environments (i.e., 6%,10% and 15% dissatisfied), as is currentlydone in many international standards.There are also plans to develop new ap-

pendices elaborat ing on methods for compliance and evaluation, as well asdevelop more clear guidance for code-making bodies. SSPC 55 and TC 2.1 alsoare planning to cosponsor a user manualto accompany Standard 55-2004.

While the standard specifies conditionsthat will satisfy 80% of the occupants, thatstill may leave 20% dissatisfied. The bestway to improve upon this level of accept-ability is to provide occupants with per-

sonal control of their thermal environment,enabling them to compensate for inter- and intra-individual differences in preference.The market is seeing an increasing num-

ber of new workplace-based HVAC prod-ucts that provide occupants with the abilityto individually control airflow, air tem-

perature and/or radiation. 4 To optimize thedesign and operation of such systems, aneed exists for more research about theindividual differences among occupants,as well as the combined effect of local

asymmetries that may be more likely tooccur in these environments. Ideally, suchresearch would lead to future revisions of Standard 55 with the aim of providingcomfort that reliably suits more than 80%of the occupants.

AcknowledgmentsThe authors gratefully acknowledge

the many past and current members of SSPC 55 who contributed to ASHRAEStandard 55-2004. Although too numer-ous to mention here, they are listed onthe inside cover of the standard. In par-ticular, we would like to express our ap-

preciation to the past and current Chairsof SSPC 55, Dan Int-Hout (19992001)and Wayne Dunn (2002present), and toStephen Turner who played a key role asmaster editor of the document.

References1. ISO EN 7730, 1994. Moderate thermal

environments - Determination of the PMV and PPD indices and specification of the condi-tions for thermal comfort . International Stan-dards Organization. Geneva

2. Toftum, J., et al. 2000. Human responseto air movement. Part 1: Preference and draft discomfort. ASHRAE Project 843-TRP, Tech-nical University of Denmark.

3. de Dear, R. and G.S. Brager. 1998. De-veloping an adaptive model of thermal com-fort and preference. ASHRAE Transactions104(1a):145167.

4. Bauman, F. 2003. Underfloor Air Dis-tribution (UFAD) Design Guide. Atlanta:ASHRAE.


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Berkeley Model Human Comfort