# Downdraft Assessment of Glass Curtain Wall Buildings with a Radiant Floor Heating System
## 3 Assessment Parameters  
### Mechanism of Downdraft Prevention  
A downdraft occurs when air is cooled on the inside of a window as the heat transferred to the cold outside surroundings is enhanced, and the air in close proximity to the inside of the window is cooled as it loses heat to the cold window surface. According to [PERSON]'s study [26], the heat transfer phenomenon that occurs when heat is supplied from the floor below a cold window is different from the case when heat is supplied from a general vertical surface because of the heat exchange between the cold window and the floor. Moreover, RFHS supply heat to the cold air indirectly, in others words, it slowly supplied heat through the principle of natural convection. Thus, to assess whether an RFHS can actually prevent a downdraft, the interior heat transfer phenomenon caused by the heat exchange between an RFHS and cold windows must be analyzed and an RFHS application method that can prevent downdrafts must be developed. To prevent downdrafts when using an RFHS, the influx of the airflow should be blocked by raising the surface temperature of the RFHS and supplying heat to the cold airflow formed near the windows [27]. In other words, when cold air forms near a cold window and moves along the floor surface, the RFHS should raise the temperature of the air over a large radiating surface to block the cold airflow influx into the occupied area by forming an ascending flow caused by a warm airflow of RFHS. However, because the radiant heat resulting from the higher surface temperature of the RFHS relative to indoor thermal equilibrium leads to indoor overheating and occupant discomfort, the RFHS must be applied according to the required heating load in the space. The radiation area must be reduced to increase the surface temperature of the RFHS according to the heating load required in the room. That is, for the same thermal output, the narrower the radiation area, the higher the surface temperature, the wider the radiation area, and the lower the surface temperature. Because downdraft is generated by the effects of the indoor air temperature and airflow velocity, the indoor air temperature and the airflow velocity formed by the radiation area and surface temperature of the RFHS are important. Thus, it is necessary to analyze the effects of the radiation area and surface temperature of the RFHS on the temperature of the indoor air and the airflow velocity. If the cold airflow generated by the cold surface of a window in a space where an RFHS has been applied descends and flows into the occupied zone, heat is transferred to the cold airflow. The temperature rises and the warm airflow generated from the RFHS forms an ascending flow, blocking the indoor influx of cold airflow. Because an RFHS is applied to a narrow area with a high surface temperature, the indoor influx of cold air is blocked and a high air temperature in the vicinity of the inner floor surface results as shown in Figure 1. As the temperature difference between the cold air that flows along the floor surface and the warm air generated from the surface of the RFHS becomes larger, the airflow velocity at the floor surface increases. On the other hand, if heat is supplied according to the required heat demand in the space by widening the radiant heating area of the RFHS, the air temperature near the floor surface is relatively low compared to the case shown in Figure 2. However, the airflow velocity at the floor surface can be reduced because the temperature difference between the warm floor surface and the cold window surface is reduced. Figure 1: Airflow by application of radiant floor heating systems (RFHS) (narrow area/high surface temperature). Figure 2: Airflow by application of RFHS (wide area/relatively low surface temperature). The downdraft is intensified with decreasing air temperature and increasing air velocity.