Buildings are not created to use energy, but to provide healthy and productive indoor environments. However, efforts to save energy may cause a reduction in health and productivity by degrading indoor-air quality (IAQ). Therefore, maintaining buildings that provide healthy and productive indoor environments requires testing and measuring of key IAQ parameters. As often is said, if you do not measure energy, you cannot manage it — the same is true for ventilation performance.

IAQ-MEASUREMENT GOALS

Because IAQ can be considered a function of the interaction of contaminant sources and the effectiveness of ventilation utilized to dilute and remove air contaminants, measurement goals include assessing the source strength of air contaminants and determining the amount of ventilation provided to occupants. While contaminant sources in furnishings and finishes may be reduced through careful administration of maintenance operations, people-related sources, such as shed cold and flu viruses, always will be a concern. This makes the continual assessment of the actual amount of ventilation provided the most important aspect of achieving a healthy indoor environment.

The monitoring of ventilation performance also is important for energy conservation. Many building-energy conservation efforts will be able to decrease the amount of energy required to provide thermal comfort because of improvements in the thermal performance of building envelopes and lighting efficiencies. The energy required to condition outdoor air for ventilation will become a larger percentage of the energy consumed in ongoing building operations.

Continually assessing ventilation performance requires monitoring of the combined result of an HVAC system's components to ensure they achieve the intended amount of ventilation. Just assessing the quantity of outdoor air entering an HVAC system's intake does not show how much air is delivered to building occupants. Although the quantity of outdoor air entering an HVAC system may be enough to ventilate a building generously, imprecise distribution may not deliver ventilation to occupied building zones. Just as the ultimate result of a building's performance depends on the interaction of design decisions, operational performance depends on how the different aspects of a building and HVAC system work together. Therefore, it is necessary to conduct an integrated performance assessment over the life of a building.

The IAQ measurement approach that this article will address is the monitoring of carbon-dioxide (CO₂) concentrations at the periphery of an occupied space. People are a significant source of this gas, and its generation rate is far more predictable than any of the other bioeffluents generated by humans. The CO₂ concentration in one exhaled breath is about 40,000 ppm, which is 100 times greater than the 400 ppm found in relatively clean outdoor air. CO₂ concentrations measured at the periphery of an occupied space reflect the dynamic interaction between occupancy conditions and ventilation effectiveness.

Occupancy conditions include the number of people present, their metabolic activity levels, and how long they have been present in a space. Ventilation effectiveness includes how rapidly occupant-generated bioeffluents are diluted and removed.

VENTILATION RATES

The minimum recommended ventilation rates listed in American National Standards Institute (ANSI)/American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.1, Ventilation for Acceptable Indoor Air Quality, are based merely on perceived comfort among occupants and are not defined as achieving the healthiest indoor environment. According to ANSI/ASHRAE Standard 62.1, IAQ is defined as acceptable if the indoor air contains no known contaminants at harmful concentrations as determined by cognizant authorities and if a substantial majority (80 percent or more) of the people exposed to the air do not express dissatisfaction.

Sometimes, however, even this ASHRAE-recommended minimum ventilation rate is not achieved. As shown in Figure 1, an overzealous effort to save energy in an office building resulted in localized ventilation deficiencies. In this shared-sensor-monitoring approach, all of the CO₂ concentrations values are measured with the same sensor. Air samples are drawn to a central location by a vacuum pump, computer control, and sampling-line network.

Using the difference between indoor and outdoor CO₂ concentrations, a ventilation rate can be determined. For example, the FAS No. 2 location in Figure 1 has a peak indoor value of 1,050 ppm and an outdoor value of 410 ppm, resulting in a ventilation rate no greater than 16.5 cfm per person. In a situation in which it cannot be determined if equilibrium conditions were achieved before people started leaving for the day, the actual ventilation rate might be lower. Despite the uncertainty of the actual ventilation rate, it is clear that the ASHRAE-recommended minimum ventilation rate of 20 cfm per person was not being achieved. What also can be observed in this CO₂ data plot is that the operation of the HVAC system was not effective in achieving a complete overnight purge, as a residual level of CO₂ remained in the building the morning of reoccupancy.

The dampers controlling the outdoor-air percentage were readjusted, and an assumed value of 20 percent was dialed in, resulting in the CO₂ monitoring data shown in Figure 2. With this larger amount of outdoor air in the supply air, early morning residual bioeffluents from the previous day's occupancy, as well as peak CO₂ values observed in the building, were reduced, indicating that a much healthier indoor environment was being achieved.

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