Lebanon has been encountering an energy crisis for more than two decades and has a large energy supply deficit. Most of energy demand is for space cooling, heating, and domestic hot water heating. A mitigation measure for such use of conventional fossil fuel energy sources is the use of solar energy. Solar thermal collectors allow to harness solar energy for water heating use; however, the usual techniques lead to overcrowding building roofs. Façade integration technology such as, building integrated solar thermal (BIST) system allow the exploitation of the building’s large façade for solar heating. Aesthetic incorporation is important, being the façade an architecture feature. Heat exchange through the building’s envelope is also affected, particularly in the case when water is stagnant the pipes of the collector. Thus, the aim of this paper is to assess the impact on thermal comfort that arises from using a BIST system on the façade of a building and is under stagnant conditions using a simulation software TRNSYS-18. The model will evaluate the thermal performance of a room with a flat plate thermal collector (FPTC) directly integrated into the south wall and compare it to a control room. The main goal is to assess indoor air / surface temperatures and the thermal comfort parameters according to the Predicted Mean Vote (PMV) model of the room representing a Lebanese office building.