Sizing a storage type water heating system

Sizing of the storage type hot water heater is based upon estimation of the maximum hourly hot water demand and duration of the peak load. Almost all designers assume 70 percent of the stored water is usable regardless of the fact that the usability factor depends on several parameters, e.g., the entering cold water temperature, heat recovery rate and heat loss from storage tank and piping. In this article, the storage type heater is analyzed in the transient state to investigate the actual performance of the heater under various load conditions with respect to time. An equation is derived for the enthalpy of the stored water as a function of time and system parameters. The equation can be applied for proper sizing of new storage type heaters and for evaluation of the performance characteristics of existing heaters.