Unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor. The primary purpose of the study is to improve the temperature stratification in hot water storage tanks operated with coiled-tube heat exchangers. The investigated operating mode is the extraction of thermal energy from hot water storage tanks. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the colder water from the significantly hotter bulk fluid of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the cooling tank fluid to a helical flow path along the tube axis which induces additional secondary flow pattern in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different inlet temperatures have been investigated to describe the impact for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers. (C) 2014 Elsevier Ltd. All rights reserved.