The available data from ASHRAE and CIBSE on pressure losses in HVAC duct fittings are of unsatisfactory accuracy and insufficient quantity, which may lean to inaccurate sizing and poor energy efficiencies in air supply systems. This situation is receiving increasing international attention and much effort is being spent in its rectification, especially using computation fluid dynamics (CFD). This paper presents an investigation of the accuracy of the CFD approach. It was found that the combination of the k-epsilon model and the higher order QUICK scheme produces the highest accuracy (with a relative error of 10%). Grid dependency tests showed that a relatively low grid density in the straight sections upstream/downstream of the duct fitting is sufficient but a higher density in the duct axis direction is required in the section containing the duct fitting. In addition, a computational domain set-up with short upstream/downstream sections is adequate. The examination of CFD accuracy and its maximisation should be extended to other fittings. This paper illustrates the important factors involved and the procedures to follow.