A case study was conducted to design construct and test a thermosiphon economizer for increasing the temperature of feed water by using waste hot water. The simulation of the thermosiphon economizer (heat exchanger) for the establishment of the optimum size was performed using the thermos-economical method. The thermosyphon was made of a total of 80 copper tubes (outside diam.19.05 mm) with evaporative adiabatic and condensation lengths of 300 10 and 300 mm respectively. The working fluid was R-134a filling ratio was 50% by evaporative volume and the thermosiphon was arranged in a staggered form. In testing performance flow rate of the inlet hot water was set at 10 L/min and the controlled inlet temperature of cold water at 25oC. The average flow rate of the cold water was set at 5 7 and 10 L/min respectively. Results showed that the maximum of rate of heat transfer from the condenser section occurred at the flow rate of the cold water of 10 L/min. When test the inlet temperature of hot water of 4555 and 65 o C were used at the inlet volume flow rate of cold water of 10 L/min maximum heat transfer rates from the condenser 4626 8260 and 8790 W were achieved. Moreover the outlet temperature of the cold water which was initially 25o C changed to 311 34 and 37o C respectively. Therefore increasing the outlet feed water temperature reduced energy consumption. The effectiveness of the thermosiphon economizer with the inlet temperatures of hot water of 45 55 and 65o C was 0.47 0.48 and 0.49 respectively.