# Downdraft Assessment of Glass Curtain Wall Buildings with a Radiant Floor Heating System
## 1 Introduction  
Currently, the control of thermal comfort in indoor building environments has become increasingly important as people look for ways to improve their quality of life. The demand for efficient use of interior space is also increasing. Consequently, interest in radiant heating systems has increased and applications of this technology are becoming more widespread [1].  
Radiant heating systems are advantageous in terms of thermal comfort because this system can set the air temperature to be a little low while maintaining comfort conditions through the radiation heat exchange between the occupants and the radiation surface [2]. The radiant heat transfer of a radiant heating system also covers 50% of heat exchange within a conditioned space [3], hence, it has an advantage of the temperature being uniformly supplied and maintained. Moreover, a radiant heating system is embedded in the floor or ceiling and primarily supplies radiant heat, and can, therefore, solve some problems associated with a forced convection perimeter systems, such asoccupying a large amount of space, unpleasant discharge airflow, and noise problems caused by fans [4; 5].  
Recently, radiant heating systems have been used in office or commercial buildings in the form of radiant floor heating systems (hereinafter RFHS) because of their thermal comfort advantages [6; 7]. RFHS can also be used for direct heating in the vicinity of occupants in large, high-story spaces such as the lobbies of buildings. Because recently constructed buildings must have superior thermal performance commensurate with regulations, these windows generally have an improved \(U\)-value [8; 9]. As a result, use of RFHS is increasing, reducing the heating load required for indoor heating.  
However, the RFHS also has disadvantages, including its struggle to respond quickly to load changes. Although the RFHS stores heat in the thermal mass, the thermal output encounters a delay after the RFHS is heated [10; 11]. The heat in the thermal mass may not be sufficient to prevent the downdraft. Thus, if the cold airflow caused by the heat transfer from the building enveloping the window side of a building is not immediately blocked, this cold airflow will invade the occupied area. A body is locally cooled, hence, the problem of local discomfort, such as in the presence of a downdraft, can occur. Until now, the method used to prevent downdraft is to use a forced convection system, such as FCU or convector. Therefore, a downdraft assessment should be conducted when an RFHS is installed in a building. The RFHS does not directly supply heat to cold air, but does so via heat exchange with a cold airflow according to the principle of natural convection. As such, assessing whether a downdraft is actually prevented is necessary.  
This study aims to evaluate whether a downdraft can be prevented by an RFHS in glass curtain wall buildings, such as office buildings, restaurants, and atrium spaces etc. The RFHS does not directly supply heat to cold air, but does so via heat exchange with a cold airflow. Hence, assessing whether a downdraft is actually prevented is necessary. The downdraft assessment herein is conducted in a space where an RFHS is installed by analyzing the downdraft prevention principles according to the RFHS application. An RFHS is applied to perimeter zones with different window types modeled using computational fluid dynamics (CFD) simulation. The relationship between the RFHS and the windows is examined via an assessment of the resulting downdraft by considering the radiant heat exchange effect between the window and the floor. Based on these results, the downdraft assessment is conducted by utilizing a CFD simulation of a case, in which an RFHS is installed in a building space.