ASHRAE, Energy Saving, Sustainable Building, Thermal Comfort

Understanding Thermal Comfort & How To Predict It

Traditionally, interior conditioning strategies have only ever placed importance on the management of indoor temperature and humidity conditions with the use of mechanical conditioning systems to provide thermal comfort. Owners and designer may not have known the alternatives available to them or the impact thermal comfort has on the building and its occupants.

Well thought-out design that utilizes a wider array of available mechanisms and opportunities can be leveraged to achieve significant energy savings, whether through operational improvements on an existing system or evaluating options for a retrofit.

In some instances it is possible to achieve thermal comfort through an alternative low energy system such as a radiant cooling system combined with a Dedicated Outdoor Air System (DOAS). A case study done in Hyderabad, India on the Infosys building SDB-1, showed how advanced thermal comfort criteria can be used to address the local climate, predict the viability of the system and meet energy-saving goals.


What is thermal comfort?


Thermal comfort is defined as “that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation,” by ANSI/ASHRAE Standard 55-2010.  Thermal comfort is a subjective assessment by a person expressing their satisfaction with their local thermal environment and is a crucial consideration when designing any new building.

What is thermal comfort

In reality, a person’s perception of indoor comfort is influenced by a number of factors, including:

  • Occupants’ activity – What activities the building occupants are doing.
  • Clothing – The thermal factor of the clothing worn by the occupants.
  • Air temperature – Temperature of the air within the occupied space.
  • Relative Humidity – Percentage of water vapor in the air.
  • Radiant temperature – The average temperature of the surfaces within the occupied space.
  • Airspeed – The speed at which the air is moving.

Attaining optimal thermal comfort is no small feat but it can be improved by giving the influencing factors the attention they deserve in the design phase of a project. Sustainable building design can optimize energy use by focusing on the most important human comfort factors in a specific climate and for particular occupant activities. This will allow designers and owners to then select the appropriate systems for cooling, heating and ventilation, such as a radiant cooling solution with a Dedicated Outdoor Air System (DOAS), to deliver the desired comfort levels and reduce energy usage.


Setting Thermal Comfort Criteria


Architects, engineers and building owners need to make comfort a high priority for each project and design accordingly. According to the Berkeley National Laboratory, a comfortable interior ensures lower lease turnover rates, lower maintenance calls and operating costs and higher occupant productivity.

Setting thermal comfort criteria

In a study called “Guide to Setting Thermal Comfort Criteria and Minimizing Energy Use in Delivering
Thermal Comfort
” they outlined 3 main tasks when establishing criteria for thermal comfort:

  1. Set Goals: Communicate clear thermal comfort expectations to all building stakeholders (facilities,
    occupants, management, design team)
  2. Guide and Assess Design: Provide a quantifiable means for assessing the thermal comfort success of a building design
  3. Support Operations: Guide building operators in how thermal comfort is delivered and can be improved in operation


Predicting Thermal Comfort


There is no shortage of techniques that can be used when estimating the likely thermal comfort, including:

  • Effective temperature
  • Equivalent temperature
  • Wet Bulb Globe Temperature (WBGT)
  • Resultant temperature

Although all are relatively effective, BS EN ISO 7730, the standard regarding the ergonomics of thermal comfort suggests that it can be expressed in terms of Predicted Mean Vote (PMV) and Percentage People Dissatisfied (PPD).

Developed by Professor Ole Fanger, at Kansas State University and the Technical University of Denmark, the research was carried out to find out if people felt comfortable under different conditions. From their results, Fanger developed equations that would help predict comfort levels, taking into account the 6 factors that influence thermal comfort; clothing, activity level, air temperature, mean radiant temperature, air speed and relative humidity.

PMV is an index that predicts the mean vote of a group of people voting on how comfortable they are in an environment. While PPD is a function of PMV.


Predictive Mean Vote

Predicted mean vote sensation scale

The PMV refers to a thermal scale that runs from Cold (-3) to Hot (+3) where zero is optimal comfort. The research
subjects a large number of people to different conditions in a climate chamber and then having the subjects select a position on the scale that best described their comfort level. The result relates the thermal factors to each other through heat balance principles and produces the following scale.

The recommended acceptable PMV range for thermal comfort from ASHRAE 55 is between -0.5 and +0.5 for an interior space.

Experiment here with the levels of thermal comfort and test which factors influence comfort level most significantly. Try test your current environment and see if the thermal temperatures you are experiencing fall within acceptable comfort levels. Click here to try!

Thermal comfort calculator


Predictive Percentage of Dissatisfied


When it comes to PPD, the research tries to predict the percentage of occupants that will be dissatisfied with the thermal conditions. PPD increase as PMV moves away from 0. ASHRAE 55 recommends an acceptable PPD is less than 10% as it is impossible to account for everyone’s optimal thermal comfort as each person has their own unique influencing factors such as activity and clothing.


Designing for Human Comfort


Historically thermal comfort in buildings has been controlled by simple dry bulb temperature settings. As we move into more sophisticated low energy building systems that make use of alternate systems such as natural ventilation, mixed mode system and radiant thermal conditioning strategies, a more complete understanding of human comfort is needed for both design and control.

Understanding the factors that influence thermal comfort is a critical design consideration and will help guide architects and owners when selecting the right cooling, heating and ventilation systems. This, in turn, will ultimately lower lifecycle costs and increasing thermal comfort.

Radiant cooling vs all air cooling eBook

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