An Ondol heating system is a low temperature radiative heating system. Both radiation and convection are main heat transfer mechanisms in the Ondol heating room. In the present study an enclosure analysis was employed to evaluate the amount of thermal energy transferred from the Ondol surface to its room. Heat transfer through windows is considered in the evaluation. Radiation was found to be more dominant than convection on the Ondol surface of the test room. Under the current conditions the prefabricated floor heating type of Ondol system was also found to be more energy-efficient than the conventional type.