burned mainly rice paddy straws, agricultural crop waste, biomass or any kind of dried firewood. For short-term cooking, rice paddy straws or crop waste was preferred, while long hours of cooking and floor heating needed longer-burning firewood. Unlike modern-day water heaters, the fuel was either sporadically or regularly burned (two to five times a day), depending on frequency of cooking and seasonal weather conditions. With the traditional ondol heating, the floor closer to the furnace was normally warm enough, and the warmest spots reserved for elders and honored guests. Ondol had problems such as environmental pollution and carbon monoxide poisoning resulting from burning coal briquettes. Thus, other technology heats modern Korean homes. The famous American architect, Frank Lloyd Wright, was building a hotel in Japan, and was invited to a Japanese family's house. The homeowner had experienced the ondol in Korea, and he built an ondol room in his house. Wright reportedly was so impressed that he invented radiant floor heating which uses hot water as the heating medium. Some of Wright's buildings employed this system. Instead of ondol-hydronic radiant floor heating, modern-day houses such as high-rise apartments have a modernized version of the ondol system. Many architects know the advantages and benefits of ondol, and they are using ondol in modern houses. Since the ondol has been introduced to many countries, it is beginning to be considered as one of the systems of home heating. Modern ondol are not the same as the original version. Almost all Koreans use modern versions, so it is hard to find the traditional ondol system in Korean houses. North Korea still utilizes the basic traditional design of the Ondol that use mostly coal instead of biomass to survive the harsh winters. Advantages and disadvantages 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