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ANSI/ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy) is a standard that provides minimum requirements for acceptable thermal indoor environments. It establishes the ranges of indoor environmental conditions that are acceptable to achieve thermal comfort for occupants. It was first published in 1966, and since 2004 has been updated periodically by a technical committee of industry experts in ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). The most recent version of the standard was published in 2013.

The body of the standard currently consists of a forward (describing changes made in the current version), eight sections and two normative appendices:

After the body of the standard there are 11 informative appendices. These are not part of the standard, but provide additional information about terms and methods described within the standard, as well as a bibliography, and a description of the addenda incorporated from the previous version in the current version.

As described within the standard: "The purpose of the standard is to specify the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space".

The standard addresses the four primary environmental factors (temperature, thermal radiation, humidity, and air speed) and two personal factors (activity and clothing) that affect thermal comfort. It is applicable for healthy adults at atmospheric pressures in altitudes up to (or equivalent to) 3,000 m (9,800 ft), and for indoor spaces designed for occupancy of at least 15 minutes.

Adaptive model is a model that relates indoor design temperatures or acceptable temperature ranges to outdoor meteorological or climatological parameters.

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.

A occupant-controlled naturally conditioned space is where the thermal conditions of the space are primarily regulated by occupant-controlled openings.

Predicted mean vote is an index that predicts the mean value of the thermal sensation votes (self-reported perceptions) of a large group of persons on a sensation scale expressed from -3 to +3 corresponding to the categories "cold," "cool," "slightly cool," "neutral," "slight warm," "warm," and "hot."

Normative Appendix A: Methods for determining operative temperature
Normative Appendix B: Computer program for calculation of PMV/PPD
  • Method of compliance (e.g. Graphic Comfort Zone Method)
  • Design operative temperature and humidity, heating and cooling design outdoor conditions, total indoor loads, and design exceedance hours
  • Assumed values for environmental factors (operative temperature, humidity, and average air speed) and personal factors (clothing insulation and metabolic rate) for heating and cooling design conditions; spaces where personal factors are outside the specified limits should be indicated as not within the standard’s scope
  • A description of how local thermal discomfort will be addressed, including calculation methods, inputs and results
  • System equipment capacities for each space demonstrating that thermal loads will be met under heating and cooling design conditions
  • Where occupant-controlled elevated air speed is provided, a description of control type
  • Air speed, radiant temperature asymmetry, vertical radiant temperature asymmetry, surface temperatures, and temperature variations in time must be calculated per engineering industry standards (e.g. Chapter 57 of the ASHRAE Handbook-HVAC Applications)


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