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Energy conservation measures such as air-to-air heat exchangers provide financial savings but also may produce contamination of fresh air by the exhaust. An investigation was conducted to determine the existence and extent of reentrainment of gases and vapors from building exhaust into the fresh-air supply of a rotary air-to-air heat recovery system, the heat wheel (a revolving cylinder made up of a porous media). The air duct connections are arranged so that the fresh inlet air and exhaust air axially flow through one-half of the wheel in a counterflow pattern. A tracer gas, sulfur hexafluoride (SF6), was introduced into the exhaust system and monitored in each compartment of the heat recovery unit. Air samples were collected in gas sampling bags using sampling probes placed in the center of the airstream. Analysis of the air samples was accomplished using infrared spectrophotometry. Using the concentration of SF6, measured in each compartment and the volumetric flow rates, the percent reentrainment was ...