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Influence of Residential Hvac Duty Cycle on Indoor Air Quality

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Abstract: Measurements of duty cycle, the fraction of time the heating and cooling (HVAC) system was operating, were made in homes during the spring season of the RTP Particulate Matter Panel Study and the Tampa Asthmatic Children's Study. A temperature sensor/logger placed on an outlet vent monitored changes in temperature as the HVAC system cooled or heated the residence. Temperature data were compiled into a database and duty cycles were assessed from temperature step changes. Duty cycle measurements provided insight into the relationship between 24-hr integrated air exchange rate and indoor particulate matter (PM) concentrations as well as real-time indoor-outdoor PM ratios.

Statistically significant relationships between duty cycle-ambient temperature and duty cycle-air exchange rate were observed. Noticeable increases in duty cycle were noted in both studies as the mean ambient temperature increased or decreased around 65 degrees F. Differences in personal comfort preferences, building characteristics, and HVAC system operational efficiency contributed to the data variability. These same factors contributed to variability in the relationship between duty cycle and air exchange rate. HVAC heating mode duty cycle was positively correlated with the air exchange rate due to the positive temperature differential between inside and outdoors; whereas a negative temperature differential between inside and outdoors resulting from HVAC operation in cooling mode yielded a negative correlation with air exchange rate.

The relationship between duty cycle and 24-hour integrated PM indoor-outdoor ratio was confounded by the presence of strong indoor sources when the HVAC system was not operating and by the brief intervals (usually 10 minutes) of HVAC system operation. Individual cases when strong indoor sources were not present showed a definite correlation between duty cycle and integrated PM2.5 indoor/outdoor ratio (R2 = 0.36, p <0.01). Real-time indoor-outdoor PM data collected for a participant that operated their HVAC system for 30-minute intervals showed a maximum 90% reduction in indoor PM concentration at a rate greater than expected for deposition losses alone. The increased PM loss rate was attributed to the HVAC system. The usefulness of the duty cycle data in interpreting PM indoor-outdoor ratios would be improved if the HVAC system (filter efficiency, leaks) and building characteristics (penetration factor) were characterized in detail.

This work has been partially funded by the United States Environmental Protection Agency under contract 68-D-99-012 to the Research Triangle Institute. It has been subjected to Agency review and approved for publication.
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Citation:Thornburg, J., C. Rodes, P. A. Lawless, C. Stevens, and R. W. Williams. Influence of Residential Hvac Duty Cycle on Indoor Air Quality. Presented at 2003 AWMA Indoor Air Quality PM Meeting, Research Triangle Park, NC, July 21, 2003.
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Contact: Liz Hope - (919) 541-2785 or hope.elizabeth@epa.gov
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Division: Human Exposure & Atmospheric Sciences Division
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Branch: Exposure Measurements & Analysis Branch
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Product Type: Abstrct/Oral
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Presented: 07/21/2003
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Related Entries:
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Bullet Item Outdoor Vs. Human Exposure: NERL PM Exposure Panel Studies
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Last Updated on Monday, October 22, 2007
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