In this paper, the fabrication and optimization of a spiral-tube heat exchanger (STHE) were considered for improving the heat transfer rate and efficiency of traditional instantaneous water heaters. The large number of instantaneous water heaters exported from the customers of the “Garman Gas Toos” company, which was mainly due to corrosion and leakage, imposed a lot of cost and credit reduction for this company. The high energy consumption was the second reason that justified working on a new STHE. The main innovation of this research is the design and construction of a new heat exchanger with a smaller size and higher efficiency with the help of identifying the factors affecting its efficiency and heat transfer rate. In order to optimize the responses, three variables were considered, including fin number (per unit area), exhaust outlet diameter, and water flow rate. Implementing face-centered central composite design (CCD), the proposed levels of factors and the corresponding response variables were measured in the “Garman Gas Toos” laboratory. Using the design of experiments (DoE), the effects of the three factors and their mutual interaction effects were evaluated. Response surface methodology (RSM) was devised to build a prediction model and obtain the values of the factors for which the responses were optimal. Based on the results, optimum conditions for the STHE were found to be an exhaust diameter of 4 cm and a water flow rate of 6 L/min coupled with six fins. At this optimal point, the values of efficiency and heat transfer rate, as response variables, were obtained as 85% and 8480 W, respectively.