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Relationship Between Hvac System Operation, Air Exchange Rate, and Indoor-Outdoor Particulate Matter Ratios

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Abstract: Measurements of duty cycle , the fraction of time the heating and cooling (HVAC) system was operating, were made in each participant's home during the spring season of the RTP Particulate Matter Panel Study. A miniature temperature sensor/data logger combination placed on the return air vent monitored changes in temperature as the HVAC system cooled or heated the residence. The temperature data were compiled into a database, and the duty cycle assessed from step changes in temperature using a series of macros and subjective analyses. The duty cycle measurements will provide insight into the relationship between 24-hr integrated air exchange rate (AER) and indoor particulate matter concentrations. The mild climate conditions during the Spring season created large variability in the duty cycle data due to the large number of days the HVAC did not operate or operated in both heating and cooling modes. The group average HVAC duty cycle was 0.061 (operational only 6.1% of the time) with a standard deviation of 0.054. The median number of days the HVAC system operated during a sampling week was 2.5 days, and the maximum and minimum 24-hr duty cycles were 45% and 0%, respectively. Almost 31% of the participants kept their HVAC off 6 or more days during their 7 days of sampling. A significant relationship between the 24-hr duty cycle means and air exchange rates existed only if the HVAC system operated for 6 or more days per week and the range in the 24-hr duty cycles exceeded 0.10. In those cases, an increase in duty cycle caused an increase in the 24-hr average duty cycle. The magnitude of the increase depended on the outdoor-indoor temperature differential. The duty cycle correlated with 24-hr indoor-outdoor particulate matter ratios only if their was a significant relationship between duty cycle-AER and there were not any strong indoor particle sources. The usefulness of the duty cycle data in interpreting particulate matter indoor-outdoor ratios would be improved if HVAC system particle removal efficiency, air flow recirculation rates, and integrity (leak assessments) were characterized in detail for both heating and cooling mode. Also, the infrequency of HVAC system use during the mild Spring and Fall "transition" seasons limited the effect on the indoor particle concentrations. However, the HVAC system should have a larger influence on indoor particle concentrations in Winter (heating) and Summer (cooling) seasons when the system operates more than 10% of the time.

This work has been funded wholly by the United States Environmental Protection Agency under contract #68-D5-0040 and 68-D-99-012 to RTI. It has been subjected to Agency review and approved for publication.
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Citation:Thornburg, J., C. Rodes, E. Rodes, and R. W. Williams. Relationship Between Hvac System Operation, Air Exchange Rate, and Indoor-Outdoor Particulate Matter Ratios. Presented at International Society of Exposure Analysis 2002 Conference, Vancouver, Canada, August 11-15, 2002.
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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: Human Exposure Analysis Branch
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Product Type: Abstrct/Oral
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Presented: 08/11/2002
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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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