Comfortable heating of rooms with large areas of external glazing is one of the most challenging issues in cold climate zones. The rule of thumb, in this case is locating the heating unit under the window. Modern architectural trends lead to an increase in the number of facilities with panoramic glazing. The common practice is to locate trench units in close proximity to glass for such facilities. This analysis shows that even fan-assisted trench heating cannot always cope with cold air screening of large windows. A huge volume of cold air from the surface of the glass "outweighs" the warm rising flows, displacing the air from the convector towards the internal space areas. This can cause unwanted surface temperatures and uncomfortable conditions for people due to increased air velocity, i.e., draughts. Also, too low temperatures can lead to condensation and ice formation on glazing, especially in rooms with high humidity, such as swimming pools. In the present work, CFD analysis was used to evaluate equipment performance with increasing glazing height. Research results showed examples of trench heating implementation, leading to adverse consequences. The recommendation of using additional air supply along the glazing was tested. The simulation results showed that slot diffusers, accelerating the flow from trench heating, play a crucial role in the heating of floor-to- ceiling glazing and prevent draughts at floor level. In this work, the features of CFD modeling of trench units were considered.