Ducts with rectangular cross-section are widely used in HVAC (heating, ventilation and air conditioning) distribution systems applied to residential, commercial and industrial buildings where round elements can't be used. Usual practical problem is calculating the required pressure drop to drive the air flow for a given duct geometry at a certain flow rate, for which the total hydraulic losses should be obtained. Friction losses in ducts depend on their size, flow capacity and construction material, while minor losses are caused by changes in flow stream direction, expansions or contractions, fittings and valves. The installation of elbow in a channel can induce larger losses due to local flow separation and swirling secondary flow. Viscous incompressible flow of air in the ducts was modelled and simulated. Hydraulic losses in cascade connected straight rectangular sections by three elbows are evaluated with the application of Computational Fluid Dynamics (CFD). Rectangular ducts which have the same hydraulic diameter but differ according to the lengths of their straight sections were being analysed. The numerically obtained loss coefficients were compared with available theoretical data and show good agreement especially in a selected range of Reynolds number. A conclusion is drawn from the comparative analysis that the lengths of the straight sections between the elbows significantly influence the duct hydraulic resistance because of the contribution to the velocity profile uniformity. Moreover, during the separate analysis of the elbows, it is detected that the elbow position in the system is an influencing parameter on the elbow loss coefficient.