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ANSI/ASHRAE Standard 55 (Thermal Environmental Conditions for Human Occupancy) is a standard that provides minimum requirements for acceptable thermal 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 an ASHRAE technical committee comprised of industry experts. The most recent version of the standard was published in 2013.

Organization of standard
The body of the standard is currently comprised of a forward (describing changes made in the current version), eight sections and two normative appendices: Normative Appendix A: Methods for determining operative temperature
 * 1) Purpose
 * 2) Scope
 * 3) Definitions
 * 4) General requirements
 * 5) Conditions that provide thermal comfort
 * 6) Design compliance
 * 7) Evaluation of comfort in existing buildings
 * 8) References

Normative Appendix B: Computer program for calculation of PMV-PPD

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 body of the standard, as well as a bibliography, and a description of the addenda incorporated into the current version.

Purpose and scope
This standard establishes the range of thermal environmental factors and personal factors that create indoor thermal environmental conditions acceptable to a majority of occupants. It 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) 3000 m, and for indoor spaces designed for occupancy of at least 15 minutes.

Graphic comfort zone method
The graphic method utilizes an overlay on a psychrometric chart to indicate the  operative temperatures and humidity at which thermal comfort is achieved in the winter (1.0 clo) and summer (0.5 clo). It is based on the Predicted Mean Vote (PMV) model which predicts thermal sensation and acceptability based on a series of controlled laboratory tests. The graphic comfort zone model is limited in applicability to conditions when the metabolic rate of occupants is 1.0-1.3 met and the  humidity ratio is below 0.012 kg H20/kg dry air (0.012 lb H20/lb dry air). If these requirements are met and the environmental conditions inside the building fall within the indicated ranges, then comfort is achieved.

Analytical comfort zone method
For humidity ratios above 0.012 kg H20/kg dry air (0.012 lb H20/lb dry air), or for metabolic rates up to 2.0 met, the analytical model must be used to determine thermal comfort sensation. Also based on the PMV model, this method uses tools such as the ASHRAE Thermal Comfort Tool or the online CBE Thermal Comfort Tool, to evaluate thermal comfort. Users provide operative temperature (or air temperature and mean radiant temperature), air speed, humidity, met rate, and clo value, and the tool evaluates predicted thermal sensation on a scale from -3 (cold) to +3 (hot). Compliance is achieved if the conditions provide thermal neutrality, measured as falling between -0.5 and +0.5 on the thermal sensation scale.

Elevated air speed
The section sets provisions for increasing the upper air temperature limit at elevated air speeds above 0.20 m/s. The methodology is based on the SET (Standard Effective Temperature) model, which provides a way to assign an effective temperature (at a standard met rate and clo value) to compare thermal sensations experienced at a range of thermal conditions, met rates, and clo values. The upper limit of air speed is based on whether occupants have local control or not. To evaluate compliance, the ASHRAE Thermal Comfort Tool may be used, or a computer model validated against the code provided in Informative Appendix G of the standard.

Local thermal discomfort
This section is applicable when clothing insulation is less than 0.6 clo and metabolic rates are below 1.3 met. Radiant temperature asymmetry between ceiling and floor, and air and walls must be limited to reduce discomfort. To reduce draft risk at temperatures below 22.5 C, air speed due to the HVAC system must be 0.15 m/s or below. The vertical air temperature difference between ankle and head (defined as 0.1 m and 1.1 m above the ground) must be less than 3 C-change. If occupants' feet will be in contact with the floor, the temperature must be 19 - 29 C.

Temperature variations with time
When occupants do not have control over the cyclical variation or drifts in indoor environmental conditions, the conditions within this section must be met. Operative temperatures may not fluctuate more than 1.1 C-change within 15 minutes, nor change more than 2.2 C-change within 1 hour.

Acceptable thermal conditions in occupant-controlled naturally conditioned spaces
This method is known as the adaptive comfort model and was developed based on field research which found that occupants in naturally ventilated buildings were comfortable at wider ranges of temperatures. As described by the standard, the adaptive comfort model is applicable in buildings without mechanical cooling (and no operating heating system) where occupants’ met rates are 1.0-1.3 met and their clothing levels are 0.5-1.0 clo. For this model the standard provides a graph of acceptable indoor temperature limits at prevailing mean outdoor temperatures (a mean of the daily mean outdoor temperatures of the previous 7-30 days). The graph is valid for prevailing mean temperatures between 10 - 33.5 C. It provides 80% and 90% acceptability ranges, indicating the percentage of occupants expected to be comfortable at the indicated indoor and prevailing mean outdoor temperatures.

History of the standard
ANSI/ASHRAE Standard 55 was first published in 1966. It was revised in 1974, 1981, 1992, 2004, 2010, and 2013. Starting in 2004, it is now updated based on ASHRAE’s standard maintenance procedures. These periodic revisions are based on publicly-reviewed addenda to the previous version available on ASHRAE’s website. In 1992 the standard was updated with more extensive information on measurement protocols and an expanded definitions section. In 2004 the standard underwent significant changes with the addition of two thermal comfort models: the PMV model and the adaptive comfort model. The 2010 edition of the standard included multiple significant changes. It re-introduced the Standard Effective Temperature (SET) as a method to calculate the cooling effect of air movement. It also added a general satisfaction survey to section 7 intended to evaluate general thermal comfort in an occupied space, bringing the standard in line with current survey-based post-occupancy evaluation (POE) practices. In 2013 the body of the standard was rewritten for organization and clarity, with informative language moved from the body of the standard to informative appendices. The applicability of the cooling effect of air movement was expanded to apply to naturally conditioned spaces. Section 7 underwent major revisions for measuring thermal comfort in existing spaces including procedures for physical measurements and survey methods, and how to evaluate and report results. The last major change concerns measuring air speed and air temperature experienced by the occupant, which now must be an average across three heights and over a period of time.