ABSTRACT In this study@ the applicability of a linear input/output model to gas-fired@ tankless water heaters has been evaluated. This simple model assumes that the relationship between input and output@ averaged over both active draw and idle periods@ is linear. This approach is being applied to boilers in other studies and offers the potential to make a small number of simple measurements to obtain the model parameters. These parameters can then be used to predict performance under complex load patterns. Both condensing and non-condensing water heaters have been tested under a very wide range of load conditions. It is shown that this approach can be used to reproduce performance metrics@ such as the energy factor@ and can be used to evaluate the impacts of alternative draw patterns and conditions.