ASHRAE 55


ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy is an American National Standard published by ASHRAE that establishes the ranges of indoor environmental conditions to achieve acceptable thermal comfort for occupants of buildings. It was first published in 1966, and since 2004 has been updated every three to six years. The most recent version of the standard was published in 2023.

Organization of standard

The body of the standard consists of a foreword, eight sections and several normative appendices:
  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
The informative appendices 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.

Purpose

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".

Scope

The standard addresses the four primary environmental factors and two personal factors that affect thermal comfort. It is applicable for:
  • healthy adults at atmospheric pressures in altitudes up to ,
  • people whose clothing insulation is between 0.0 and 1.5 clo, who are not wearing highly impermeable clothing,
  • metabolic rates between 1.0 and 2.0 met,
  • people who are neither sleeping nor reclining,
  • for indoor spaces designed for occupancy of at least 15 minutes.

    Definitions

Adaptive model

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

Thermal comfort

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

Occupant-controlled naturally conditioned spaces

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

Predicted mean vote (PMV)

Predicted mean vote is an index that predicts the mean value of the thermal sensation votes 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." The value of PMV can be calculated either using the code provided in Appendix B of the standard or can be freely calculated with either the CBE Thermal Comfort Tool for ASHRAE 55, with the Python package pythermalcomfort and with the R package comf.

Comfort zone

Comfort zone refers to the combinations of air temperature, mean radiant temperature, and humidity that are predicted to be an acceptable thermal environment at particular values of air speed, metabolic rate, and clothing insulation

Clothing insulation (Icl)

Clothing insulation is the resistance to sensible heat transfer provided by a clothing ensemble

Metabolic rate (met)

Metabolic rate is the rate of transformation of chemical energy into heat and mechanical work by metabolic activities of an individual, per unit of skin surface area equal to 58.2 W/m2, which is the energy produced per unit skin surface area of an average person seated at rest.

Exceedance hours

Exceedance hour is the number of occupied hours within a defined time period in which the environmental conditions in an occupied space are outside the comfort zone.

Methods to evaluate thermal comfort

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 and summer. It is based on the Predicted Mean Vote model. 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 H2O/kg dry air. If these requirements are met and the environmental conditions inside the building fall within the indicated ranges, then compliance is achieved.

Analytical comfort zone method

For humidity ratios above 0.012 kg H2O/kg 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 to evaluate thermal comfort. Users provide operative temperature, air speed, humidity, metabolic rate, and clothing insulation value, and the tool evaluates predicted thermal sensation on a scale from -3 to +3. Compliance is achieved if the conditions provide thermal neutrality, measured as falling between -0.5 and +0.5 on the PMV scale.

Elevated air speed

The section sets provisions for increasing the upper air temperature limit at elevated air speeds above. The methodology is based on the SET model, which provides a way to assign an effective temperature to compare thermal sensations experienced at a range of thermal conditions. 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 D of the standard.

Local thermal discomfort

Radiant temperature asymmetry between ceiling and floor, and air and walls must be limited to reduce discomfort. To reduce draft risk at temperatures below, air speed due to the HVAC system must be or below. The vertical air temperature difference between ankle and head is limited to for seated occupants and for standing occupants. If occupants' feet will be in contact with the floor, the temperature must be.

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 within 15 minutes, nor change more than within 1 hour.

Acceptable thermal conditions in occupant-controlled naturally conditioned spaces

This method, also known as the adaptive comfort model, is applicable in buildings without mechanical cooling 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. An accompanying table lists provisions for higher operative temperatures at air speeds above and up to. The graph is valid for prevailing mean temperatures between. 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.

Demonstrating design compliance

This section of the standard is applicable for the design of buildings. All of the building systems must be designed to maintain the occupied spaces at the indoor conditions specified by one of the described evaluation methods at design conditions. The systems must be able to maintain these conditions within the expected range of indoor and outdoor operating conditions.
For demonstrating design compliance, the following are the core requirements that must be documented:
  • Each unique space. Spaces excluded from compliance documentation must be clearly identified with a rationale.
  • The method of design compliance: Determining Satisfactory Thermal Environment in Occupied Spaces or Determining Acceptable Thermal Conditions in Occupant-Controlled Naturally Conditioned Spaces.
  • Each representative occupant and their location within the space, along with clothing insulation and metabolic rate values for each design comfort condition: When selecting a single value, the rationale must be explained. If occupants are considered nonrepresentative, this must be documented with an explanation.
  • Description of the design comfort conditions: The conditions should combine indoor and outdoor factors at which occupant thermal comfort shall be evaluated and cover the most challenging scenarios for occupant comfort. Every unique combination of space and representative occupant must be evaluated under at least two conditions: cooling and heating. The design comfort conditions should be carefully considered as they may not align with the room peak heating and cooling load conditions.
  • The operative temperature to and its expected range used in the comfort calculation.
  • Solar radiation impact: If direct-beam solar radiation affects a representative occupant, the documentation must include the compliance method used, along with all applicable calculation inputs, methods, and results.
  • Thermal environmental control classification level: The level must be documented for each space, including the control measures, the means of control, and the degree of change of the environmental factor.
  • Compliance status: The compliance or non-compliance for each combination of space, representative occupant, and design comfort condition must be clearly stated.

    Evaluation of comfort in existing buildings

Although the evaluation of comfort in existing buildings is not mandatory in ASHRAE 55, it can be used as a guideline when required by other standards. Occupant surveys and environmental measurements are primarily used for evaluation. When the building automation system is added, the relevant characteristics must be described.