An algorithm has been developed to calculate the maximum temperature reached by water drawn from a hot water tank, and the duration of time this hot water spent above 60oC. A transient simulation model for the hot water system is first used to calculate nodal tank temperatures for one day of operation, given user-specified ambient conditions and hot water load profiles.

The new algorithm then post processes this data to calculate temperature-exposure statistics for each parcel of fluid that is drawn from the tank.

Mixing between layers of fluid in the tank occurs during heating operation when the temperatures of adjacent layers are similar. This mixing phenomenon can cause degradation of temperature-exposure history, and is one of the major complexities dealt with in this new algorithm. Another key complexity of the calculation is how results are rounded to deal with the coarse numerical discretization of the tank.

Applications for this methodology include assessing and reducing the risk of microbial contamination. Obtaining detailed temperature-exposure history for a specified load profile allows the user to design control strategies to limit unsafe draw-offs, or control an inlet valve that allows a non-potable water source to enter the tank when it is possible to be adequately treated through heating in combination with other processes. These strategies improve water quality, especially for sensitive users, and water usage efficiency, allowing non-potable water to be used in the tank without compromising water quality for the end user.