The internal cold and warm water supply networks do have the role to permanently assure the discharge and necessary pressure at the consumption points, being necessary important investments in the total cost of water supply system. At the same time, the big volume of building constructions and social-cultural edifices and the more reduced working time starting from design to execution involves the use in design of the means of bigger efficiency and precision for the calculus and dimensioning of internal water distribution networks. Classically, the pipes diameters are chosen according to the average economical velocities, which can’t consider the dynamics of the economic and energetic parameters of these networks. In this paper an improved model is developed for optimal design of networks with inferior distribution for the water supply of buildings, implemented in a computer program for IBM-PC compatible systems. This program might be applied at each significant change interfered in the economic policy.