The operation of circulated cooling systems (CCS), the conditions of solid deposits formation and their influence on the head transfer coefficient of heat exchangers are analyzed. Attention is drawn to the danger of jelly-like (mainly organic) deposits with thermal conductivity factor of less than 1 W/(m∙K). Deposits of such kind with the thickness of (1.0-1.5)mm reduce thermal conductivity of the walls of piping systems more than ten-fold though outside they seem to look quite secure. Deposits begin their formation mainly as calcium carbonate where microorganisms are fixed at. The example of turbine condensers shows how deposits reduce the efficiency of steam turbine units. Intensity of deposits formation and their growth are determined by the quality of cooling water and water mode of circulated cooling systems operation. Nowadays, while developing suggestions for deposits minimization, stationary mode of CCS operation is considered. However, the mode of CCS operation is changeable in time due to scarce water resources. The mode of CCS operation with variable water volume is considered in the work. It is shown that at V= var – CCS filling two variants of dependence of salts concentration on time are possible – their increase and decrease. In general, the operating mode of CCS (with respect to water flows) is described by three different dimensionless parameters that are interconnected. Regulatory documentation (SBC B.2.5-74:2013), on the requirements to quality of cooling water and water exchange mode uses models based on assumptions about stationarity of the process and the constant CCS water volume which is not always true. Therefore, the predictive reliability of such documentation is questionable.