ANSI/ASHRAE 55-2023: Thermal Environmental Conditions for Human Occupancy

Small wooden houses following ANSI/ASHRAE 55-2023 for thermal environmental conditions.

An American National Standard, ANSI/ASHRAE 55-2023: Thermal Environmental Conditions For Human Occupancy, specifies indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to occupants.

Occupants Heat Indoor Spaces

The human body, albeit minimally, is a biological space heater, and, while Homo sapiens has a lesser surface area to project energy than many other homeothermic creatures, its members idiosyncratically inhabit buildings and other structures. This distinguishes the human thermal environmental conditions from that of other members of the animal kingdom. Under these conditions, each square foot of the human body emits heat equal to about 19 matches per hour, providing an endless indoor heat source and influencing building occupants’ thermal comfort.

On average, our heat exchange with our environment is equal to 58.2 w/m2 (18.4 Btu/h/ft2).  However, the thermal energy given off by the human body varies from an assortment of conditions, and greater activity leads to more heat emitted. Personal factors like this are paired with environmental factors, such as humidity and vapor pressure, to shape overall conditions. ANSI/ASHRAE 55-2023 utilizes the environmental factors of temperature, thermal radiation, humidity, and air speed, as well as the personal factors of activity and clothing.

About ANSI/ASHRAE 55-2023

ANSI/ASHRAE 55-2023 is used for specifying combinations of these personal and environmental factors to produce thermal environmental conditions that will be acceptable to a majority of the occupants within a space. Standard 55 is oriented toward providing thermal comfort, addressing the following six factors: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, and humidity.

Since comfort in the indoor environment is complex and responds to the interaction of all of the factors that are addressed herein, all criteria in ANSI/ASHRAE Standard 55-2023 is intended to be applied together. Specifically, it covers thermal environmental conditions acceptable for healthy adults at atmospheric pressure equivalent to altitudes up to 3000 m (10,000 ft) in indoor spaces designed for human occupancy for periods not less than 15 minutes.

The ANSI/ASHRAE 55-2023 document also features methods for determining other thermal environmental conditions, and it offers guidelines for demonstrating that the design of an occupied space or building meets the comfort levels established as acceptable for occupants at a specified metabolic rate and clothing level. It does not, however, address nonthermal environmental conditions (e.g. air quality, acoustics, illumination) or override any safety, health, or critical process requirements.

Metabolic Rate (met) Unit

Integral to the ANSI/ASHRAE 55-2023 standard, as well as the general concept of thermal conditions, is the met unit. This is a unit of metabolic rate of waste heat, and it is equal to the 58.2 w/m2 value recently alluded to. When you are seated quietly, you are producing about 1 met. However, this value varies by activity, from the extreme of heavy machine work (about 3 met) to the seemly minimal variance of conducting sedentary office work (about 1.2 met).

People with red shoes occupying building according to ANSI/ASHRAE 55-2023.

ANSI/ASHRAE 55-2023 features this unit heavily in its section devoted to the method for determining occupant metabolic rate. It specifically states, however, that averaged metabolic rate not be used to represent multiple occupants whose metabolic rates differ by more than 0.1 met. Therefore, in an office setting, an occupant who is seated and reading (at 1.0 met) should be considered separately from one who is seated and filing (at 1.2 met).

Changes to ANSI/ASHRAE 55-2023

ANSI/ASHRAE 55-2023 is the current edition of the American National Standard for thermal environmental conditions for human occupancy, being the most recently updated in a long line of publications beginning with the standard’s initial publication back in 1966. The 2023 edition incorporates eleven addenda to the previous, 2020 version of the standard, which it revises. Major changes made to ANSI/ASHRAE 55-2023 include:

  • A new method was added for the assessment of local thermal discomfort with vertical air temperature gradient between the head level and ankle level
  • The applicability of the standard was widened, as it now covers metabolic rates up to 4 from 2
  • Calculation methods were consolidated and simplified in the standard, which are now limited to two
    methods—standard and adaptive—and a new flow chart was added to provides guidance on when to use each calculation method
  • Informative references were updated
  • The documentation requirements of the standard (in Section 6) were overhauled. This included additions, clarifications, and simplifications, along with a new example spreadsheet compliance form that replaces the prior example form

The alterations made to ANSI/ASHRAE 55-2023 carry a renewed focus on organizational clarity. These updates are detailed further in Informative Appendix N of the standard.

ANSI/ASHRAE 55-2023: Thermal Environmental Conditions For Human Occupancy is available on the ANSI Webstore.

Changes to ANSI/ASHRAE 55-2020

The previous edition of this American National Standard was also a substantial revision. The 2020 edition incorporates eight addenda to the previous, 2017 version of the standard, which it revises. Major changes made to ANSI/ASHRAE 55-2020 include:

  • A new method was added for the avoidance of draft risk at the ankle region.
  • Thermal Environmental Control Classification Levels were added as part of the required documentation in Section 6, “Design Compliance.” These levels document how much control occupants have over their thermal environmental conditions in a given space.
  • The Graphical Method was removed and replaced with normative graphical examples of specific conditions using the analytical method and elevated airspeed methods. The previous Graphical Method was incorrectly more lenient than the analytical method, with significant error possible in certain conditions.
  • Applicability was expanded of the adaptive model used for naturally conditioned spaces. The model is now applicable to buildings that have a mechanical cooling system installed, as long as the system is not running.

In all, the alterations made to ANSI/ASHRAE 55-2020 carry a renewed focus on the application of the standard by practitioners and the use of clear, enforceable language. These updates are detailed further in Informative Appendix N of the standard.

Changes to ANSI/ASHRAE 55-2017

The previous edition of this standard was also significant. The 2017 edition replaced the 2013 edition, incorporating seven addenda. Noteworthy changes to ANSI/ASHRAE 55-2017 include:

  • Clarification of the three comfort calculation approaches in Section 5.3.3, “Elevated Air Speed”. This section also addresses an Elevated Air Speed Comfort Zone Method.
  • Simplification of Normative Appendix A, “Methods for Determining Operative Temperature,” to a single procedure for calculating operative temperature.
  • Removal of permissive language throughout the standard.
  • Modification of Section 2, “Scope,” to clarify that the standard should not be used to override any safety, health, or critical process requirements.
  • Addition of a new requirement to calculate the change to thermal comfort resulting from direct solar radiation affecting occupants.
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