Understanding Thermal Comfort Factors

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Clothing Level (CLO)

CLO is a numberical value describing thermal insulation provided by clothing, ranging from 0.5 to 1.5, and is referenced by ASHRAE 55-2004.

There are two basic methods of determining an individual’s CLO value. The first is based upon entire outfit; the second is by summing the total CLO values for each article of clothing. The first method is likely to be a more time-effective approach to estimating a building occupant’s attire.

The CLO valuation assumes that a person is standing. If an individual spends most of the day sitting, the CLO value may need to be increased depending upon the type of chair. You will need to estimate a CLO value for the average occupant for each season on a space-by-space basis. A general list of clothing ensembles, clothing articles and chair insulation can be found as Tables B1–B3 in Normative Appendix B in ASHRAE 55–2004 (pages 18–20).

Metabolic Rate (MET)

The use of metabolic rate in the LEED Online credit form seeks to estimate the typical level of activity of the occupants within a given space.

The decimal scale represented in ASHRAE 55-2004 ranges from 0.7 to 8.7. The 0.7 level represents sleeping or resting, while above 1.0 is light activity; greater than 2.0 represents moderate activity and perspiration. When values rise above 1.0, evaporation of perspiration becomes a factor in an individual’s level of comfort. You will need to estimate the average metabolic rate of the occupants in a given space. A complete list of MET rates can be found in Normative Appendix A of ASHRAE 55–2004 (page 15).

PMV-PPD: A Computer Calculation

An abbreviation for “Percent Mean Vote – Predicted Percent Dissatisfied,” PMV-PPD is a calculation typically conducted using a computer program.

ASHRAE 55–2004 provides programming code to assist with performing the calculation. The calculation only applies to building occupants with metabolic rates of 1.0 to 1.3 and clothing levels between 0.5 and 1.0; it is designed to simulate a typical office environment. Space usage outside of this will not generate usable results. PMV is a range of seven values from -3 to +3 representing a person’s thermal scale from cold to hot. This is the same rating system recommended for the Occupant Survey required for LEED IEQc7.2 for NC, CI, and Schools. The PMV-PDD model incorporates the inputs from metabolic rate, clothing level, air temperature, radiant temperature, air speed, and humidity. Typically these inputs and the computer program are handled by the mechanical engineer.

A Better Understanding of Thermal Comfort

The ASHRAE 55–2004 description of the six elements of personal thermal comfort may lack the level of detail required for non-practitioners to understand. Innova Airtech Instruments offers a website is nearly as in-depth as ASHRAE 55-2004 but with less technical jargon.

Reading a Psychrometric Chart

Psychrometric charts can look intimidating at first but are simple to use once you understand their structure and principles. These charts are a compilation of several different values of air and humidity properties, with different lines representing different ways to observe how changing conditions change several values at once. The charts are the results of predictable formulas and using them can save you valuable time which would otherwise be required to calculate the same results.

Visit this website to get a break-down of how to read and use a psychrometric chart.

Trane and Krueger have downloadable applications which can be use to quickly create your own customized psychrometric charts:

Natatorium Design

These documents, in addition to Chapter 4 of the ASHRAE HVAC Applications Handbook, provide handy references for natatorium design:


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Jul 27 2017
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