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Thermal Comfort — A Key to Occupant Satisfaction and Productivity

Mike Opitz PE, LEED AP
Certification Manager, LEED for Existing Buildings
U.S. Green Building Council

Intro

If there’s one thing facility managers agree on across the board, it’s probably that occupant comfort complaints are their biggest routine operational headache. If one person is too hot, someone else nearby is too cold, and tomorrow both complaints may be reversed. Even a young building may have trouble keeping most of the people happy most of the time, and if your building is old, well, good luck.

But all is not lost. Even though comfort is an intensely personal experience for each occupant, there’s lots of science illuminating the best ways to provide it. Sadly, the best techniques don’t often get implemented in practice, possibly because this knowledge is not widely known or understood. Keeping your occupants more comfortable will have the obvious benefit of reducing trouble calls, and therefore O&M costs. More subtle are the other benefits that can accrue: higher workforce productivity, better health, and in some cases lower HVAC energy costs.

All of these benefits are goals of the green building movement, which only reinforces the growing realization that having a green building is one way of saying you have a good building.

What Is Thermal Comfort?

As facilities managers know, most occupant comfort complaints will boil down to “I’m too hot” or “I’m too cold”, because typically we think of thermal comfort only in terms of temperature. Those vague but common perceptions actually only scratch the surface of a much larger set of factors that combine to drive the fundamental physical phenomenon: the human body’s heat balance. Our bodies are constantly generating metabolic heat at rates that depend on who we are and what we’re doing, and we constantly lose that heat depending on what we’re wearing and what our surrounding conditions are like. If those rates of heat gain and loss get imbalanced for some reason, we feel uncomfortable. Comfort researchers have identified several separate drivers of the heat balance:

Traditional Environmental Comfort Factors
(all are well-known, and facility managers can readily influence them)

  • Temperature of the room air, ignoring moisture content (i.e., the dry-bulb temperature)
  • Relative humidity of the room air
  • Speed of any room air that’s hitting the occupant
  • Average temperature of the solid surfaces surrounding the occupant (i.e., the mean radiant temperature)
  • The amount of solar heat directly hitting the occupant through windows

Other Comfort Factors
(relatively new options, &/or areas where facility managers have little influence)

  • Occupant activity level and clothing level
  • Adaptive factors, such as the amount of control we have over our comfort conditions
  • Variability, i.e., whether letting space conditions fluctuate somewhat is more comfortable than rigidly constant conditions

All of these elements are intuitive, but they interact in complex ways that are imperfectly understood. Further, some elements can be traded off with others, but only within certain limits. This complexity has led specialists to develop industry standards and guidelines to help building professionals sort through the issues and provide high comfort to building occupants.

LEED and Standards for Thermal Comfort

The LEED for Existing Buildings rating system has four elements with strong links to thermal comfort for building occupants:

  • Energy & Atmosphere prerequisite 1—existing building commissioning
  • Indoor Environmental Quality credit 6.2—controllability of systems: temperature & ventilation
  • Indoor Environmental Quality credit 7.1 —thermal comfort: compliance
  • Indoor Environmental Quality credit 7.2 —thermal comfort: permanent monitoring system

Commissioning is all about inspecting critical building systems and ensuring they meet the designer’s and owner’s performance intent. That has obvious ramifications for comfort, system performance, and operation costs, and commissioning is so important overall that LEED-EB requires it in order to become certified. IEQ credit 6.2, optional in LEED-EB, is about providing individual occupants with some local control over the comfort in their space to suit their needs and preferences. IEQ credits 7.1 and 7.2, also optional, both center on providing at least a minimum prescribed level of overall thermal comfort to occupants and maintaining that comfort over time. Both of these credits reference the main thermal comfort technical standard: ASHRAE Standard 55-2004, Thermal Comfort Conditions for Human Occupancy.

ASHRAE 55’s basic intent is to satisfy most of the people, most of the time. It combines four of the traditional comfort factors (air temperature, air humidity, radiant temperature, and air speed) to define comfort ranges for occupants. ASHRAE 55 is mainly intended for zoned HVAC systems designed to provide uniform conditions throughout a space. If you can show compliance with ASHRAE 55 at one point in time you get one LEED-EB point, and you get a second point if you confirm compliance over time.

To satisfy the people who fall outside ASHRAE’s standardized comfort bands you will likely need individualized comfort controls at each workstation. It’s important to remember that these people aren’t necessarily “outside the normal range” because they’re pickier or complain more; often they just happen to have different physiologies, and they really are suffering when they complain. That’s one reason LEED-EB offers another point for individualized control systems.

Strategies for Facility Managers

To some degree facility managers’ hands are tied when it comes to thermal comfort: they usually inherit the buildings they run, and building systems are not necessarily designed and installed to provide optimum thermal comfort conditions. Within those constraints facilities staff have some latitude, however, and can take action in either operational areas or as part of facility renovations.

Operational strategies typically focus on air temperatures: setpoints can be adjusted, and the HVAC distribution system can be repaired, adjusted, or rebalanced as needed to ensure the supplied heat or cool match the space loads. Beyond that are more approaches that should be explored, including adjustment of sequences of operations to change the humidity of the supply air, which can correct space humidity problems as well as reduce mold in building structures. Leaks in the building shell can also be repaired, and terminal registers can be repaired or adjusted to avoid stratification or undesired air circulation within rooms. Retro-commissioning your systems periodically is a great way to discover unknown problems and prioritize maintenance needs. For buildings implementing setback or setup temperatures during off-work hours, think carefully about how far in advance of the early morning arrivals to have your occupied setpoints kick in: the closer the building’s thermal mass is brought to the target temperature, the more comfortable the occupants will be (because of radiant temperature effects).

Renovations bring several other opportunities into play. One recent trend is “breathable” office chairs that allow air movement through the chair seats and backs. Individual temperature controls can be explored if the HVAC system is within the renovation scope. Building thermal mass (concrete or steel structure) can be exposed, coupling it to the room air, which reduces temperature swings (be cautious if considering this in conjunction with a setback strategy, as the two approaches can conflict with each other). A complete gutting and refit can be especially beneficial for perimeter areas if you install new windows: they can dramatically improve the shell’s airtightness, they can be opened during mild weather if appropriate, and they can have high “insulation” value. The comfort benefits of a high insulation value (i.e., a low U-factor) are definite but easy to overlook; it happens because the glass surface temperature becomes closer to the room air temperature, which allows comfort to be improved and likely maintained with reduced HVAC setpoints (cooler in winter, warmer in summer).

Conclusion

Comfortable building occupants are happy, healthy, and productive occupants. Thermal comfort, cleverly and robustly provided, can also lower your operating energy costs. Though improved operations practices can clearly help, major renovations offer a true golden opportunity to specify a building shell and mechanical systems designed to optimize comfort and allow more operational flexibility.

The interactions between the drivers of thermal comfort are a great illustration of the importance of maintaining a broad, whole-building perspective when operating your buildings. Analyzing and addressing all the pieces of the puzzle in an integrated manner will almost always lead to a better, more reliable solution at lower cost. Improving thermal comfort is an obvious area where by being green you, your owners, and your occupants all profit.

Resources

American Society of Heating, Refrigeration, and Air Conditioning Engineers (www.ashrae.org)
Publishes Standard 55-2004, Thermal Comfort Conditions for Human Occupancy and the Handbook of Fundamentals.

U.S. Green Building Council (www.usgbc.org)

The USGBC’s LEED for Existing Buildings program addresses thermal comfort in several of its requirements and credits.

Simplified Design of HVAC Systems, William Bobenhausen, John Wiley & Sons, 1994. A superb introduction to thermal comfort issues and HVAC in general.

Headquartered in Washington, D.C., the U.S. Green Building Council http://www.usgbc.org is the nation’s leading coalition for the advancement of buildings that are environmentally responsible, profitable, and healthy places to live and work. Established in 1993, the Council offers various products and services to include the LEED Green Building Rating System, an annual International Green Building Conference and Exposition, membership summits, information exchange, education, and policy advocacy.

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