Gas-fired tankless water heaters are gaining popularity as a means of producing domestic hot water for residential applications. As they become more widespread, these devices are increasingly becoming important to researchers who are seeking models of these devices that can be used to make reasonable predictions of their energy consumption. This is especially true for the case of condensing tankless water heaters as they have been shown to be the most efficient method of producing domestic hot water. Current models of these devices do exist, however, the uncertainties of their predictions are unclear and require data collected from onerous field-trials for calibration.This article makes a contribution by introducing a new model of a condensing tankless water heater whose uncertainties are well described that can be calibrated by a less-onerous experimental program in a laboratory. The model was calibrated with data spanning a range of conditions: inlet temperatures between 10 and 23 °C, water flow rates between 0.08 and 0.27 L s-1 and outlet temperatures between 36 and 48 °C.The model predictions were validated against data emanating from a previous field-trial. Above domestic hot water consumption values of 15 MJ day-1, the coefficient of determination (r2 value) was 0.98. The average error (difference between the average model predictions and the measured values) was 3.0% (in relative terms) while the root mean square error was 4.2%. The maximum error for a single point was 8.7% at a domestic hot water consumption of approximately 30.2 MJ day-1.

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Applied Energy
Department of Mechanical and Aerospace Engineering

Johnson, G. (Geoffrey), & Beausoleil-Morrison, I. (2016). The calibration and validation of a model for predicting the performance of gas-fired tankless water heaters in domestic hot water applications. Applied Energy, 177, 740–750. doi:10.1016/j.apenergy.2016.05.130