The DUOPASS® SYSTEM has been designed to deliver generous productions of D.H.W. This system, patented by Radiant in year 1996, lifts the temperature of the water to approximately 32° C., before it goes into the tank. This leads to a high well being for the end user, who enjoy remarkable flow rates of D.H.W., combined with reduced mechanical strains. These benefits are guaranteed even by means of small capacities, thus smaller overall dimensions, because the water is pre-heated and therefore requires a lesser amount of kcals. This revolutionary system has made it possible the combination of the continuity of the instantaneous boilers, together with the combort of the storage units, all in one product.