HEALTHY INDOOR ENVIRONMENTS

A study focusing on the healthfulness of indoor environments¹ found that exceeding ASHRAE-recommended minimums achieved reductions in short-term absentee rates. The study reported that the cost of providing additional ventilation was more than offset by the savings resulting from reduced sick leave. In fact, it noted that the recommended levels of outdoor-air supply may be associated with significant morbidity and lost productivity. On a national scale, this could cost as much as $22.8 billion per year.

Another study² reported that a 1-percent increase in productivity in an average office building could justify a doubling of the building's maintenance and energy budgets. This result stems from the fact that people-related costs, typically about $300 per square foot, far exceed energy costs, about $3 per square foot.

These problems lead to an important question: Is the intended amount of ventilation actually being achieved? Accurately monitoring and reviewing CO₂ concentrations in an occupied space and comparing them with outdoor conditions can provide an answer. In addition to this diagnostic determination, monitoring concentration percentages can be used to automatically adjust the amount of ventilation provided in variable-occupancy locations. This technique is called demand-controlled ventilation (DCV).

While more buildings are starting to include CO-₂monitoring equipment, they are not necessarily achieving the potential benefit that can accrue from this investment. Steps need to be taken so that accurate ventilation-performance data are collected and reviewed and ventilation performance can be fine-tuned to achieve a healthy balance between indoor environmental quality and energy use. As stated earlier, if you are not measuring ventilation, you cannot expect to manage it. Conversely, if you want to do a good job of managing energy, you need to do a good job of measuring it.

MEASURING AND MANAGING ENERGY

Achieving this full benefit involves several important steps, including appropriate placement of CO₂ sensors, proper monitoring methods, suitable calibration procedures, and a timely review of CO₂-monitoring data.

The biggest error in the placement of CO₂ sensors is locating them too far from occupied areas. When this occurs, sensors typically are placed in the return airflow at an air-handling unit (AHU). By measuring the air at this location, a ventilation-performance assessment at best reflects an average of the conditions in the locations from which the air is returning. In addition, as pointed out in ASTM Standard D6245-2002, Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation,³ sampling in the return airflow allows for the possibility that the supply air will short-circuit to the return, resulting in a low CO₂ concentration in the return air relative to the occupied portion of the space.

Measuring air too close to occupants also can be a problem. The inclusion of even a small portion of a concentrated exhaled breath can erroneously indicate a ventilation deficiency. Therefore, it is recommended that this parameter be measured at the periphery of an occupied space.

Two methods of CO₂ monitoring are available: distributed individual sensors and the more accurate shared-sensor approach. The accuracy of the use of individual sensors was called into question in a paper published by Lawrence Berkeley National Laboratory,⁴ which stated that the accuracy of CO₂ sensors in commercial buildings frequently is less than is needed to measure peak indoor/outdoor CO₂-concentration differences with less than 20-percent error. Ventilation rates are measured more accurately with a shared-sensor approach because measurements are based on the difference between indoor and outdoor CO₂ disparities. Further, the same sensor is used for the indoor and outdoor locations, rather than different sensors that might have various errors associated with them.

In addition to the continuous ventilation-performance commissioning that can be achieved with CO₂ monitoring, dew point should be assessed constantly. This information provides diagnostic feedback that assesses the effectiveness of a building's moisture management. Checking on moisture-management performance includes assessments of dehumidification and humidification operations, as well as the identification of localized elevated humidity caused by outdoor water intrusions or indoor water leaks.

Expected dehumidification may not be achieved because the chilled-water flow to a cooling coil may be restricted. However, localized humidification can be assessed by examining a plot comparing dew points at multiple monitoring locations. Monitoring dew points also can serve as an early warning system to rapidly identify the presence of elevated moisture levels in a building, which can be caused by a leak in the building envelope or plumbing. Early detection can help prevent uncontrolled mold growth.

Airborne-particulate concentrations also should be measured periodically to assess another aspect of IAQ conditions. One key thing to consider when assessing airborne-particulate concentrations is that much of a building's particulate matter comes in with, and is generated by, the building's occupants, not its HVAC system. Therefore, even if the HVAC system has filters with high Minimum Efficiency Reporting Values and its ductwork has been cleaned recently, there still may be elevated particulate levels in the air being breathed in by the occupants because of their presence and activities.

According to a document on cleanroom protocol,⁵ the particle-generation rate of a person standing motionless is 100,000 per minute in the 0.3-micron-and-larger size range. For a person walking at 2 mph, the particle-generation rate jumps to 5 million per minute in the 0.3-micron-and-larger size range.

Another aspect of IAQ measurement is assessing any volatile organic compounds (VOCs) that are present. Unfortunately, this can be difficult. Real-time measurement tools, such as photoionization detectors, merely are generalized VOC sensors, and their results are difficult to interpret. More accurate VOC determinations, such as laboratory analyses of pre-evacuated polished-aluminum canisters or sampling media, tend to be expensive and yield results only for a limited space and time. The most cost-effective approach may be to review whether occupant-generated bioeffluents are being diluted effectively and removed overnight. If this is not occurring, then the VOCs probably are not being diluted and removed effectively.

CONCLUSION

The bottom line regarding measuring parameters to improve IAQ while minimizing the associated energy use comes down to maximizing the quality of the data monitoring of ventilation and moisture-management performance. In many buildings, this is best achieved by installing a shared-sensor monitoring system that measures CO₂ concentrations and dew-point temperatures and having these IAQ-related measurements reviewed in a timely fashion.

Just as more people are realizing the benefits of integrated design, integrated performance assessments that consider how all of the components of a building and its HVAC system work together to achieve a healthy and productive indoor environment are needed.

REFERENCES

1. Milton, D., Glencross, M., & Walters, M. (2000). Risk of sick leave associated with outdoor air supply rate, humidification, and occupant complaints. Indoor Air, 10, 212-221.

2. Fisk, W.J., & Rosenfeld, A.H. (1997). Estimates of improved productivity and health from better indoor environments. Indoor Air, 7, 158-172.

3. ASTM. (2002). Standard guide for using indoor carbon dioxide concentrations to evaluate indoor air quality and ventilation. ASTM D6245-2002. West Conshohocken, PA: ASTM International.

4. Fisk, W., Faulkner, D., & Sullivan, D. (2006). Accuracy of CO₂ sensors in commercial buildings: A pilot study. Berkeley, CA: Lawrence Berkeley National Laboratory.

5. EE407 introduction to microfabrication: Clean room protocol. (n.d.). Retrieved from www.coe.montana.edu/ee/tjkaiser/ee407/notes/EE407-03CleanroomProtocol.pdf

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David W. Bearg, PE, CIH, holds a bachelor's degree in chemical engineering from Northeastern University and a master's degree in environmental health sciences from the Harvard School of Public Health. An accomplished author and speaker, he wrote the book “Indoor Air Quality and HVAC Systems.” He can be contacted at sagefarm@comcast.net.

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