One of the advantages of an Ondol is that it can maintain heat for an extended period. In a traditional Korean house, people usually extinguish the fire before going to sleep at night, since it can stay warm until the morning. An ondol conducts heat evenly throughout the whole room, although the part of the room closest to the "agungi" is much warmer. Comparing the Ondol with the Western radiator: the heat from the radiator rises towards the ceiling, but Ondol keeps both the floor and the air in the room warm. The advantage of the Ondol is that people do not have to worry about breakdown and repair of the Ondol. The Ondol is part of the house, therefore, it is less likely to run into problems. Any combustible materials can be used as fuel for the Ondol; there are no special fuel requirements. In contrast to heaters, such as fireplaces or charcoal-based heaters that leave ash in the room, Ondol does not cause pollution in the room leaving it clean and warm.