The paper gives results of a study (in both occupied and unoccupied houses in the Tennessee Valley) of the impact of heating and air-conditioning (HAC) system operation and leakage on ventilation and intercompartment transport. Forced-air HAC systems caused an average and maximum increase in air infiltration rates of 1.8- and 4.3-fold, respectively, during brief whole-house studies of tracer gas decay in 39 occupied houses. An average increase in air infiltration rate of 0.33+/-0.37/h corresponded to an incremental air leak of 240 cu m/h, based on approximate house volume. More detailed tracer gas decay studies were performed in basements, kitchens, and bedrooms of six houses with low infiltration rates. The HAC mixed the indoor air efficiently between measurement sites. HAC operation also caused 1.1- to 3.6-fold increases in air infiltration rates, corresponding to absolute increases of 0.02 to 0.1/h. In an unoccupied research house, 3-fold increases in average air infiltration rate with HAC operation were reduced to 2-fold by sealing the external HAC unit and crawlspace ductwork system.