# Downdraft Assessment of Glass Curtain Wall Buildings with a Radiant Floor Heating System
## 3 Assessment Parameters
### Parameters for Downdraft Assessment  
To reduce cold airflow and assess the downdraft in a space where an RFHS is applied, the radiant heat condition such as thermal output and radiation area of the RFHS should be considered to transfer heat indirectly to the cold airflow and form an ascending airflow, additionally, the indoor environment, which affects the downdraft, should also be considered. Parameters that affect temperature and velocity of the airflow near the floor include the surface temperature and radiation heat area of the RFHS in accordance with the mechanism of preventing downdraft (3.1). Thus, these quantities were selected as assessment parameters that affect downdraft.  
When using an RFHS, to reflect the indoor environment conditions that should be considered in downdraft prevention, parameters affecting downdraft in indoor spaces were derived by considering existing research. [PERSON]'s study Eckert (1958) involved a theoretical analysis of the turbulent natural convection flow above the cold surface of windows. Based on [PERSON]'s study Eckert (1958), [PERSON] (1958) derived the maximum velocity of the airflow in Equations (2)-(4) by measuring the minimum air temperature near the floor surface when the cold air formed by the cold surface of the window moves along the  
Figure 3: Distribution of draft rate by application of RFHS.  
floor surface owing to the descending flow caused by natural convection. According to the obtained results, as the vertical height of the cold window surface increases and the temperature difference between the surface of the window and the occupied area increases, the maximum velocity of the airflow at ankle height increases. However, the area of interest in downdraft assessment is not the area close to the cold external surface, but rather the area in which people actually reside. Therefore, [PERSON] [15] evaluated the maximum velocity of the airflow and the minimum temperature close to the floor surface as cold airflow descends and flows along the floor to assess the risk of an occupant feeling a downdraft.  
According to experimental results, the maximum velocity and minimum temperature of the airflow were measured within 0.4 m of the cold surface. Then, the velocity of the airflow was reduced up to a distance of 2 m from the window, and the maximum velocity of the airflow was reduced by more than 25% at a point more than 2 m away and then remained constant.  
\[u_{max}=0.055\sqrt{(h\Delta t)}\ x<0.4 \tag{2}\]  
\[u_{max}=0.095\frac{\sqrt{(h\Delta t)}}{x+1.32}\ 0.4\leq x\leq 2.0 \tag{3}\]  
\[u_{max}=0.028\sqrt{(h\Delta t)}\ 2.0>x \tag{4}\]  
where \(u_{max}\) is the maximum velocity at the bottom of the cold surface, \(h\) is the height in meters of the vertical surface, \(\Delta t\) is the temperature difference between the cooled surfaces in reference to the occupied zone, \(x\) is the distance from the window (cold surface).  
According to these findings, a downdraft is mainly generated near the floor surface as the airflow formed by the cold surface of a window moves along the floor surface. The velocity of the airflow entering the occupied space along the floor surface is influenced by the descending velocity of the cold airflow formed near the window.  
The maximum velocity of the airflow in proximity to the window increases as the surface temperature of the window decreases and as the vertical height of the window increases. Therefore, the thermal performance of the window (i.e., the \(U\)-value of the window), which determines the surface temperature of the window and is related to its heat insulation properties, and the vertical height of the window, which affects the descending velocity of the cold airflow formed near the window, were selected as indoor downdraft assessment parameters.