This article numerically examines a Trombe wall for a mild winter climate. A number of semicircular fins are mounted on the Trombe wall. The inside of the fins is filled with phase change material (PCM). Graphene nanoparticles are also used in PCM. The wall is positioned at the south side of the building and is installed to heat a 4 × 4 room. Room and Trombe walls are simulated at 100, 300, and 500 W/m2 using ANSYS FLUENT software. The effect of factors such as Trombe wall height, distance between the glass wall and absorber plate, number of fins, presence or absence of PCM in fins, and flux quantity on average room temperature (TRA) is investigated. The findings reveal that the quantity of heat flux provided to the wall has a considerable influence on the room temperature (TRO), therefore increasing the applied heat flux raises the TRA. An increase in the air gap of the Trombe wall reduces the TRA. Adding fins by up to 11 enhances the TRO, but further increasing reduces the TRO. The use of PCM in the fins lowers the TRO during the day but stores the energy for the times with low solar flux and heats the room in the afternoon.