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Because a Mp DCV system uses the same sensor for outside-air and inside-space measurements, any error is offset, just as in the case of two sensors that drift the same amount in the same direction.

NON-HUMAN POLLUTANTS

The issue of non-human indoor pollutants is addressed with additional discrete sensors in the central sensor array. Typically, non-human pollutants to be addressed in a commercial building include carbon monoxide (CO), volatile organic compounds (VOCs), and respiratory-size (less-than-2.5-µm diameter) particles.

CO in concentrations above 35 ppm can produce symptoms in humans. The Occupational Safety and Health Administration-permissible exposure limit for CO is 50 ppm.³ At least 19 states have a code requirement governing CO detection, particularly in residential structures.

VOCs are emitted from a variety of chemicals (both methane and non-methane hydrocarbons) and can be significantly higher indoors than outdoors. Common VOC-emitting products include paints and lacquers; paint strippers; cleaning supplies; pesticides; building materials and furnishings; office equipment, such as copiers and printers; correction fluids and carbon-less copy paper; graphics and craft materials, including glues and adhesives; permanent markers; photographic solutions; and stored fuels and automotive products.

According to the EPA, the health effects of exposure to VOCs include eye, nose, and throat irritation; headaches; loss of coordination; nausea; and liver, kidney, and central-nervous-system damage. Some VOCs can cause cancer in animals, and some are suspected or known to cause cancer in humans.⁴ Additionally, methane is a significant environmental hazard, with a global-warming potential of 25 averaged over 100 years, compared with a base value of 1 for CO₂.⁵

Constituents of fine-particle aerosols that are too small to be filtered by the nose and, thus, are inhaled directly into the lungs, respirable-fraction particles with diameters of less than 2.5 µm (PM2.5) are a significant contributor to nosocomial infections.⁶ Once a virus suspended on PM2.5 particles contaminates an air space, the degree of infection transmission is limited only by the survival of the virus and the ventilation in the space.

Small-particle aerosols also are a major factor in cross-contamination of cleanrooms and laboratories.

Other benefits of a Mp DCV system with PM2.5 monitoring include the ability to detect filter breakthrough. In applications using costly to maintain high-efficiency-particulate-air and/or high-minimum-efficiency-reporting-value filters, such as hospital surgical suites and cleanrooms, filter changes can be based on differential pressure and PM2.5 counts, rather than an arbitrary schedule. And because it is monitored for the same non-human pollutants, outside air that is of significantly poorer quality than indoor air can be kept out. Also, comfort control can be enhanced, as a dew-point sensor can be added to the central sensor array and, along with the local thermostat, provide an extremely accurate relative-humidity value to be used in controlling the latent load.

SUMMARY

Although conventional CO₂-driven DCV offers an opportunity for energy-cost savings and carbon-footprint reduction, it never has realized its full potential because of concerns involving non-human pollutants, control accuracy, and calibration and maintenance. Mp DCV satisfactorily addresses all of those issues by providing highly accurate differential sensing of multiple points and pollutants with a significantly reduced number of sensors, providing greater indoor-air quality and energy efficiency.

1. REFERENCES

   Roth, K.W., Dieckmann, J., & Brodrick, J. (2003, July). Emerging technologies: Demand control ventilation. ASHRAE Journal, pp. 91-92.

2. Fisk, W.J., Faulkner, D., & Sullivan, D.P. (2006). Accuracy of CO₂ sensors in commercial buildings: A pilot study. Berkeley, CA: Lawrence Berkeley National Laboratory. Retrieved from http://repositories.cdlib.org/lbnl/LBNL-61862/

3. OFR. (2006). Code of federal regulations (29 CFR table Z-1). Washington, DC: Office of the Federal Register.

4. EPA. (2009). An introduction to indoor air quality: Organic gases (volatile organic compounds - VOCs). Washington, DC: Environmental Protection Agency. Retrieved from http://www.epa.gov/iaq/voc.html

5. IPCC. (2007). Summary for policymakers. Cambridge, UK, and New York: Cambridge University Press. Retrieved from http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf

6. Mayhall, C.G. (2004). Hospital epidemiology and infection control (3rd ed.). Philadelphia: Lippincott Williams & Wilkins.

For past HPAC Engineering feature articles, visit www.hpac.com.

Larry Clark, LEED AP, is director of corporate business development for Hill York, an award-winning air-conditioning commercial contractor specializing in high-rise complexes. He can be contacted at lclark@hillyork.com.