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How’s the Weather Inside?

The relationship of the thermal environment inside a building to the comfort, welfare, and productivity of people is a significant and sometimes overlooked component of facility design. Typically, when we think of thermal comfort, we most likely consider temperature alone. However, while temperature does have the most dramatic effect on our thermal environment, it is only one of several important components. Building surface temperatures, humidity, and air velocity (drafts) all bear directly on a person’s comfort. Since each of these elements plays an important role, we must address each of them in the design of heating, ventilating, and air conditioning (HVAC) systems.

“…while temperature does have the most dramatic effect on our thermal environment, it is only one of several important components.”

Because it is difficult to directly measure its impact, one of the more misunderstood aspects of HVAC design is the fact that the temperatures of the building surfaces surrounding a person can have a dramatic impact on their thermal comfort. This phenomenon is called the “radiant effect.” For instance, a person may feel very comfortable seated in the center of 75 degree room in the middle of winter. However, if that same person is seated near a window in that same room they may feel quite chilly. This is due to the fact that the person’s body is literally radiating, and losing, heat to the colder window. To overcome the “cold panel” effect of the window, the HVAC designer can, for example, incorporate baseboard heat which will provide a buffer of warm air between the person and the window. This same phenomenon can occur when nearby warm surfaces affect a person’s comfort. In these instances, the HVAC designer would address the situation by again creating a buffer, possibly with properly placed air conditioning diffusers, between the person and the offending surface.

Providing proper ventilation is essential to creating an environment where people are comfortable and productive. However, appropriate ventilation requires introducing outside air, which brings with it outdoor humidity. If left untreated, the humidity level inside a building will begin to mirror the humidity level outside the building. This can result in poor indoor air quality, with spaces becoming too humid in the summer or too dry in the winter. To prevent this, outdoor-air preconditioning systems, which condition the air to a “space-neutral” state before it enters the facility, are encouraged. These systems act to cool and dehumidify warm sticky air in the summer and heat and humidify cold dry air in the winter so the building temperature and humidity remain at acceptable levels.

Drafts are yet another condition that must be considered and controlled to maintain comfort. A draft is defined as any excessive or undesirable movement of air and we are all familiar with the feel and unpleasant effect of a cold draft. Inside a building, drafts are most often caused by poorly placed HVAC diffusers or by diffusers that have not been balanced properly. When locating diffusers, a designer must be careful to place them so they cannot blow directly on people. The goal is to orient diffusers to mix the air within the space before it reaches the occupants. Additionally, properly balancing the HVAC system is crucial so that the diffusers are not blowing too much air at too high a velocity.

The success of any HVAC system can be measured by the comfort, well-being, and productivity of the people the system serves. By carefully considering not only air temperature but also surface temperatures, humidity, and air movement, it is possible to create an environment that benefits all those who use it.

– Timothy J. Scharf, PE, LEED AP

Tim is a Principal, Licensed Mechanical Engineer, and LEED Accredited Professional. Please feel free to contact Tim for further details regarding the above information.