ASHRAE 55.1-2004

Center for Environmental Design Research Center for the Built Environment (University of California, Berkeley) Year  Paper OlesenBrager comfort A Better Way to Predict Comfort: The New ASHRAE Standard 55-2004 B. W. Olesen G. S. Brager Technical University of Denmark University of California, Berkeley This paper is posted at the eScholarship Repository, University of California. http://repositories.cdlib.org/cedr/cbe/ieq/OlesenBrager2004 comfort Copyright c©2004 by the authors. 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 Comfort ubstantial 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 that occupants 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 inherent variations in occupants’ metabolic rates and clothing levels, and because it is not possible to prescribe or enforce what these should be, this standard cannot By Bjarne W. Olesen, Ph.D., Fellow ASHRAE, and Gail S. Brager, Ph.D., Fellow ASHRAE practically mandate operating setpoints for buildings. The two most important additions in- cluded in this new standard are an ana- lytical method based on the PMV-PPD indices and introduction of the concept of adaptation with a separate method for naturally conditioned buildings. The adaptive model and several other changes are based on various ASHRAE sponsored research projects. This article provides an overview of the key features and limits of applicability of Standard 55-2004. SSSSS A Better Way to About the Authors Bjarne W. Olesen, Ph.D., is professor and direc- tor of the International Center for Indoor Envi- ronment and Energy at the Technical University of Denmark. Gail S. Brager, Ph.D., is professor and associate 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 in paper form without permission of ASHRAE. A u g u s t 2 0 0 4 A S H R A E J o u r n a l 2 1 Purpose and Scope of Standard 55 Standard 55 deals exclusively with thermal comfort in the indoor environment. The scope is not limited to any specific building type, so it may be used for residential or commercial buildings and for new or existing buildings. It also can apply to occupied spaces such as transportation means (e.g., cars, trains, planes and ships). The standard specifies conditions acceptable to a majority of a group of occupants exposed to the same conditions within a space. The body of the standard clearly defines “majority” such that the requirements are based on 80% overall accept- ability, while specific dissatisfaction limits vary for different sources of local discomfort. A space that meets the criteria of the standard likely will have individual occupants that are not satisfied due to large individual differences in preference and sensitivity. The standard does not cover hot or cold stress in thermally extreme environments, or comfort in outdoor spaces. It also does not address non- thermal environmental conditions (e.g., air qual- ity or acoustics), or the effect of any environmental factors on non-thermal human responses (e.g., the effect of humidity on health). Predicting Thermal Comfort Thermal comfort is essentially a subjective response, or state of mind, where a person expresses satisfaction with the thermal environment. While it may be partially influenced by a variety of contextual and cultural factors, a person’s sense of thermal comfort is primarily a result of the body’s heat exchange with the environment. This is influenced by four parameters that constitute the thermal environment (air temperature, radiant temperature, humidity and air speed), and two personal parameters (clothing and activity level, or metabolic rate). Methods for estimating clothing and meta- bolic rate are presented, respectively, in Appendix A and B of Standard 55-2004. New values for thermal insulation of chairs have been added in Appendix A. With only limited exceptions, existing prediction tools for thermal comfort assume steady-state conditions. However, the standard does provide some requirements for non-steady state environments, although these requirements are based on limited laboratory data. While the methods are based on data derived primarily from studies with activity levels typical of office work (1 to 1.3 met [58.15 to 75.6 W/m2]), some of the requirements may be applicable to moderately elevated activities. People may be dissatisfied due to general (whole body) thermal comfort and/or due to local (partial body) thermal discomfort parameters (radiant asymmetry, draft, vertical air temperature difference, and floor surface temperature). Pres- ently, no method exists for combining the percentages of dis- satisfied people due to various factors to give an accurate prediction of the total number of people finding the environ- ment unacceptable. For example, we don’t know if the dis- satisfaction resulting from general thermal discomfort is additive with the percentages of those who are dissatisfied due to local discomforts, or whether the total dissatisfied may be less than the sum of the individual percent- ages (i.e., some people complaining about more than one particular problem simultaneously). To simplify the situation, Standard 55 has tradi- tionally defined an acceptable thermal environ- ment as one in which there is 80% overall acceptability, basing this on 10% dissatisfaction criteria for gen- eral thermal comfort, plus an additional 10% dissatisfaction that may occur on average from local thermal discomfort. The standard also specifies separate percent acceptability levels for the various physical variables that may cause local discomfort. These range from 5% to 20%. The requirements for providing thermal comfort are all con- tained in Section 5 of Standard 55-2004. Section 5.2 repre- sents the primary methodology for determining acceptable thermal conditions for most applications. It includes the PMV- PPD method for determining acceptable operative tempera- ture for general thermal comfort (5.2.1), followed by additional requirements for humidity (5.2.2), air speed (5.2.3), local dis- comfort (5.2.4), and temperature variations with time (5.2.5). When Section 5.2 is used, all of the requirements of these sub- sections must be met. Section 5.3 presents a new alternative compliance method applicable for naturally conditioned build- ings, based on an adaptive model of thermal comfort. Each of these sections gives specific requirements for thermal com- fort, and defines the relevant limitations of applicability. The New ASHRAE Standard 55 2 2 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 PMV/PPD Method One of the most significant changes to Standard 55-92 was the 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 the acceptable 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 the quality of the thermal environment as a mean value of the votes of 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 thermal comfort 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 thermal sensation scale are dissatisfied, and the simplification that PPD is symmetric around a neutral PMV (=0). Both PMV and PPD are based on general (whole body) thermal comfort. For specified values of the other four thermal comfort fac- tors (humidity, air speed, clothing insulation and metabolic rate), a “comfort zone” can be defined in terms of a range of operative temperatures that result in a specified percentage of 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 by the simple average of the air temperature and mean radiant temperature (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 also a computer program that allows the user to run the PMV-PPD model 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 m2·K/W) (typical for summer or cool- ing season) and 1.0 clo (0.155 m2·K/W) (typical for winter or heating season). The graphical comfort zones (Figure 1) cor- respond to a PPD of 10% (general thermal discomfort). The graphic zones are simple to use, but are only applicable for limited situations where metabolic rates are between 1.0 to 1.3 met (58.15 to 75.6 W/m2), 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 m2·K/W), one can determine the acceptable operative temperature range by linear interpolation between the limits found for each zone. While the separate comfort zones reflect the fact that people usually change clothing ac- cording to outside temperature or season, this is not always the case for workplaces that have a fixed dress code, or in geographical 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 that doesn’t change with the seasons). It is important that people use this figure carefully, con- firming that the selected clo levels associated with a comfort zone 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 allows the 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 1.5 clo (0.23 m2·K/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). 0.016 0.014 0.012 0.010 0.008 0.006 0.004 0.002 H u m id it y R at io 70 65 60 55 50 45 40 35 30 10 D ew -P o in t Te m p ., °F 50 55 60 65 70 75 80 85 90 95 100 Operative Temp., °F Data Based on ISO 7730 and ASHRAE Standard 55 Upper Recommended Humidity Limit, 0.012 Humidity Ratio 1.0 Clo 0.5 Clo 0.5 PMV Limits 0.016 0.014 0.012 0.010 0.008 0.006 0.004 0.002 H u m id it y R at io 20 15 10 5 0 –5 –10 D ew -P o in t Te m p ., °C 10 13 16 18 21 24 27 29 32 35 38 Operative 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 0.5 PMV Limits No Recommended Lower Humidity Limit No Recommended Lower Humidity Limit 90 80 70 60 50 40 30 20 10% RH 90 80 70 60 50 40 30 20 10% RH 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. A ir S p ee d , f p m 0 2 4 6 8 Temp. Rise, °F 300 250 200 150 100 50 0 ( )ar tt − C10°− F18°− F9°− C5°− F9° C5° F18° C10° Temp. Rise, °C 0 1.1 2.2 3.3 4.4 Limits for Light, Primarily Sedentary Activity 0 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 A ir S p ee d , m /s met (58.15 to 75.6 W/m2), and air speed up to 1 m/s (200 fpm). In Standard 55-2004 the use of the model is limited to air speeds less than 0.20 m/s (40 fpm). Higher air speeds can be used to increase the upper operative temperature limit in certain 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 within the 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 exclusively to thermal comfort. The standard acknowledges, however, that there may be non-thermal factors that af- fect the acceptability of very low humidity environments, and designers should be aware of this even if it is be- yond the scope of this stan- dard. The form of the upper limit of humidity has changed throughout the standard’s 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 influence of humidity on preferred ambient temperature within the com- fort range is relatively small. The committee decided to use the more simple absolute humidity as the limiting parameter and return to the upper limit used in 1981, namely a humidity ratio of 0.012. This upper humidity limit applies only to situ- ations where there is a system in place designed to control humidity. Air Speed The 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 improving comfort 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 comfort zone (i.e., for a given air speed, what upper temperature limit would be acceptable; or for a given temperature rise, what air speed would be required). This figure is applicable for lightly clothed people with clothing insulation between 0.5 to 0.7 clo (0.08 to 0.1 m2·K/W) and metabolic rates between 1.0 to 1.3 met (58.15 to 75.6 W/m2), and in situations were occupants are individually 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 ASHRAE2 has experimentally verified the diagram for occupants having individual 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 to reduce energy use while im- proving comfort. This can be achieved by allowing the temperature to rise slightly towards the higher end of the comfort zone, while giving people the opportunity to in- dividually control air move- ment through task/ambient conditioning systems, per- sonal or ceiling fans, or op- erable windows. Local Discomfort The PMV and PPD indices express warm and cold dis- comfort for the body as a whole. However, thermal dissatisfaction also may be caused by unwanted cooling (or heating) of one particular part of the body (local discomfort). Local thermal discomfort may be caused by draft, high vertical temperature difference between head and ankles, too warm or too cool a floor, or by too high a radiant temperature 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 m2·K/W), and metabolic rates between 1.0 to 1.3 met (58.15 to 75.6 W/m2). 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 (m2·K/W > 0.1) and clo > 0.7 (W/m2 > 75.6), and the require- ments are conservative and also may be applied for these cir- cumstances. While these requirements apply to the entire comfort zone, 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 2 4 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 discomfort. Future studies will be required to develop infor- mation on the combined effect of general thermal comfort and local thermal comfort, or the combined effect of several local thermal discomfort pa- rameters. The forms of local discom- fort discussed in Standard 55-2004 are: Radiant Temperature Asymmetry. This refers to the non-uniform thermal radiation field around the body due to hot 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 body’s different sensitivities to these sources. 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 is defined as the unwanted lo- cal cooling of the body caused by air movement. The predicted percentage of people dissatisfied due to annoyance by draft is a func- tion of local air temperature, air speed and turbulence in- tensity. The requirements in the standard for maximum allowable air speed are based on 20% dissatisfied. The model is based on the great- est sensitivity to draft — the head 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 In d o o r O p er at iv e Te m p ., °F 32 30 28 26 24 22 20 18 16 14 In d o o r O p er at iv e Te m p ., °C