In recently, the importance of energy saving and, accordingly, optimization studies are increasing day by day. In addition, energy saving, and optimization studies are seriously discussed in heating and cooling applications. Fan coil devices, known as cooling devices, have also been seriously included in the studies in this context. With the developed regulation conditions, this development in both the combi boiler (heating) used, the air conditioner and larger cooling devices are mostly shifting towards water systems. Global warming has brought heating and cooling applications to the fore in aqueous systems due to gases formed because of combustion and other atmospheric polluting factors. In this study, the effect of the number of circuits on the heating process and the circuit methods in fan coil devices designed as 2 pipes in water systems were examined. By including different fin types and pipe diameters, the effect of both fin type and pipe diameter has been directly revealed in the experimental studies. In addition, with this study, the effect of the change in the number of circuits on the total heating capacity, the effect on the outlet air temperature and the change in water side pressure loss are conveyed. All experimental studies were carried out under the same conditions and air and water side conditions specified by Eurovent. Considering three different air flow rates, the relationship between air flow and heating capacity is also revealed. The pump power was compared according to the capacity obtained and the air flow rate, and its effect was correlated in energy saving and optimization studies. All these factors were brought together and the number of circuits, circuit method, fin type, pipe diameter, air velocity and pump power were evaluated together in the heat exchanger design in the heating process.