A simple methodology was developed to collect measurement of duty cycle, the fraction of time the heating and air conditioning (HAC) system was operating inside residences associated with the National Exposure Research Laboratory's Research Triangle Park Particulate Matter Study and the Tampa Asthmatic Children's Study. The primary purpose of the measurements was to assess whether HAC duty cycle was related to reductions in indoor particulate matter (PM) concentrations. A miniature temperature logger placed on HAC outlet vents was able to monitor the changes in temperature as the HAC system cooled or heated the residence. Parallel measurements of air exchange rates and indoor PM2.5 and PM10 mass concentrations were also collected as part of the studies. Results indicated that duty cycle exhibited a strong diurnal pattern commensurate with ambient temperature fluctuations. Duty cycles were positively associated with residence air exchange rates for heating-mode operations, but negatively associated when systems operated in the cooling mode.