Gas-fired combi boilers are commonly used to meet the need for heating and general-purpose hot water in developing countries. In this study, a thermoelectric combi boiler generator (TECBG) was developed. When the boiler is operated, cold water flows through the cold surface of TECBG and enters the boiler. In the same way, it is used by flows through the hot surface of TECBG in heated water. Thus, temperature difference occurrs between the surfaces of TECBG. The temperature difference is converted into electrical energy by Seebeck effect. The proposed system was implemented on a domestic combi boiler. The maximum temperature difference between the designed system and the TECBG surfaces was recorded as 48 °C. 12V9AH battery was used to charge through a DC-DC charge regulator. About 14.28 Wh power was generated by the TECBG when the temperature difference of the TECBG, current, and voltage were 48 °C, 1.18 A, and 13.8 V, respectively. The electrical energy consumed by the combi boiler was measured as 115 Wh. Although the obtained energy is not quite high, it is an important gain in terms of energy efficiency because according to data recorded in Turkey, there are 15 million combi boiler users. Assuming that a single heating boiler runs for 4 h, it is possible to produce 306.6 GWh/year, derived from 14.28 Wh × 4 h × 365 days × 15,000,000 heating boiler. The proposed system provides significant advantages and thus can decrease power consumption.