Economizer is a type of heat exchanger through which the boiler feed water is passed and the heat of the hot exhaust gases is used to heat this water. In this way, the feed water is sent into the boiler at a higher temperature and requires less energy to evaporate or heat the water. In this article, the heat capacity and fluid flow of an economizer heat exchanger in the chimney of a boiler have been investigated using HTRI software. And the effect of fins with different materials, agitators and the number of passes has been investigated. Temperature, pressure drop and velocity graphs in the chimney and water pipe are presented. The results show that heat transfer is greater in the case of fins than without fins. The heat capacity of the economizer in the case of copper fins is greater than fins with other materials. By using agitators, the heat capacity of the economizer increases and the pressure drop is also increased. The fluid velocity along the pipe decreases and the temperature is increasing. The effect of the third and fourth passes on heat transfer is greater than the first and second passes.