The aim of this paper is to present a transient one-dimensional (1D) radial immersed coil heat exchanger model that accounts for the effect that geometry and operating conditions have on heat transfer performance. Insights gained through its use in both an analysis of experimental data and an implementation in the simulation environment TRNSYS are shown and discussed. While variation in the external convection coefficient of immersed coil heat exchangers has little effect on the annual solar fraction of a generic solar domestic hot water system, variation in collector side flow can influence the solar fraction as great as ±5%, in particular low collector side flow improves stratification inside the store.