Solar energy is an alternative to fossil fuel as a source of electrical and thermal energy. The simplest way of solar energy use is the conversion of solar radiation into heat. Arranged according to this principle are the solar collectors that convert solar radiation into heat and transfer it to water or some low-boiling coolant to heat water in a heat exchanger. One of the advantages of flat solar collectors is their cheapness, so they are used for domestic and industrial purposes. They transform solar energy with the help of absorbers which envelope coolant containing tubes of a collector and transfer heat to the coolant. Such collector is easy to design and to maintain. This work presents the results of a thermohydraulic calculation of a flat solar collector using CFD to analyze heat transfer processes between a feather-type absorber and water. The simulation was made with in CFD Solidworks Flow Simulation using standard solar radiation modeling tools. The results of the paper are the pictures of the coolant flows inside the collector, the graphs of the temperature distribution, and the incident heat radiant (solar) flux over time. The results of the calculations were approximated and demonstrated a high level of compliance (within 10-12 %) with the characteristics obtained during field experiments. The latter indicates the reliability of the chosen mathematical model of the solar collector and the calculation method.