The goal of this study was to better understand how to design guidance to flush chemically contaminated residential water heaters. The specific objectives were to identify factors that influence fixture flow rates and water heater turnover and develop and test mass balance models for various sizes of residential water heaters on the basis of real-world flow conditions. Flow monitoring was conducted for two residential buildings. Water pressure, aerators, and the number of fixtures running affected flow rates, which differed between hot and cold water systems.Heaters (19, 40, 50, and 80 gal) exhibited characteristics of both continuously stirred tank reactors (CSTRs) and plug-flow reactors. The ideal CSTR model was overly conservative in predicting contaminant concentration exiting water heaters. A variable-volume CSTR model more accurately predicted contaminant concentration for the heater types examined. Contaminated water characteristics (i.e., chemical volatility, toxicity) and potential exposures should be a primary consideration when choosing which procedures to implement. 2017