Economizer is a thermal device that provides heat transfer between two fluids at different temperatures. Heat exchangers are classified according to criteria such as construction, transition mechanism, heat transfer type, flow type, flow order, number of flows, flow structure. In facilities with very high heat production and consumption, high amounts of heat energy are released as waste. In order to use this heat, heat recovery practices are carried out from waste hot liquid and waste hot gas fluids in many businesses and facilities. With this energy obtained, fresh air is mostly heated or cooled according to the desired temperatures before being delivered to the environment. In this study; The design, manufacturing and installation of an economizer for the recovery of energy loss from the heat released from the waste flue gas of a hot oil boiler fed with wood waste fuel was examined under appropriate conditions. The thermal oil inlet and outlet temperature differences of this boiler, which has a capacity of 8,000,000 kcal/h with a thermal oil temperature of 250-280 °C, are 30 °C. The temperature in the combustion chamber is 550 °C and the oil temperature heated in the boiler is a maximum of 300 °C.In this study, the first 234 °C waste gas coming out of the boiler reaches the chimney as 110 °C, the fresh air taken from outside as -20 °C with the help of economizer, fan, pipeline and vent channels, is heated with an economizer to the area to be used and sent to 24 °C, and the form line and office areas were heated with hot and inert gas using this method to provide energy gain.