A proposal for rational energy saving using wasted heat is showed in the present paper. Thermodynamic-mathematical model is presented like an effort for water purification from waste heat. This paper describes computing results of heat transformer operation for water purification using low grade waste heat. Equations, parameters and simplifications used in the model are briefly described. The main parameter of the carried out study is the coefficient of performance (COP) defined for reversed heat pumps and the second main parameter is absorber temperature, both parameters has been showed and correlated between them. Main objective of this work is to show the optimal operating condition for different process which deliver low grade waste heat and requires water purification. Assisted computing simulation was used for obtain these results. The main conclusion is an ecological proposal for optimal recover of low grade waste heat. Many operating conditions are showed in graphical form and discussed for different environment conditions.