Steady and 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 design. 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 hot water from the significantly colder bulk water of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the heated water to a helical flow path along the tube axis which induces additional secondary flow in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different heat exchanger side flow rates, inlet temperatures and two gap size between the outer wall of the helical tube and the flow distributor have been investigated to describe the impact of the modification of these parameters 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. Calculated results of the inner side heat transfer intensity (Nu) have been compared to existing empirical formulas to test the validity of the numerical results in case of common helical tube heat exchanger. 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) 2013 Elsevier Ltd. All rights reserved.