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Ondol (Hangul: 온돌; hanja: 溫突), also called gudeul (Hangul: 구들), in Korean traditional architecture, is underfloor heating that uses direct heat transfer from wood smoke to heat the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or to a hotel or a sleeping room in Korean (as opposed to Western) style.
The main components of the traditional ondol are a firebox or stove (agungi; 아궁이) accessible from an adjoining (typically kitchen or master bedroom) room, a raised masonry floor underlain by horizontal smoke passages, and a vertical, freestanding chimney on the opposite exterior providing a draft. The heated floor, supported by stone piers or baffles to distribute the smoke, is covered by stone slabs, clay and an impervious layer such as oiled paper.
Use of the ondol has been found at archaeological sites in present-day North Korea. A Bronze Age archaeological site, circa 1000 BC, discovered in Unggi, Hamgyeongbuk-do, in present-day North Korea, shows a clear vestige of gudeul in the excavated dwelling (Hangul: 움집) unearthed at the archaeological site.
Early ondols began as gudeul that contained the heating for a home, as well as heat for cooking. When a fire was lit in the furnace to cook rice for dinner, the flame would be sent horizontally. This was achieved by having a flue entry located beside the furnace. This was essential, as it would not allow the smoke to travel upward, which would cause the flame to go out too soon. As the flame would pass through the flue entrance, it would be guided through the network of passages with the smoke. Entire rooms would be built on the furnace flue to create ondol floored rooms.
The term gudeul used colloquially for the modern day ondol in Korea for over two thousand years, and called by many alternate names (janggaeng (Hangul: 장갱; hanja: 長坑), hwagaeng (Hangul: 화갱; hanja: 火坑), nandol (Hangul: 난돌; hanja: 暖突), yeondol (Hangul: 연돌; hanja: 烟突)); the term ondol was introduced around the end of the 19th century. According to a Korean folkloric historian Son Jintae (1900 - missing during the 1950-53 Korean War), gudeul originated from guun-dol (Korean), which means "warm stone", and its pronunciation has undergone some change from gudol or gudul to finally take the form of gudeul. Ondol was first written in Hanja by modern-day writers.
An Ondol had traditionally been used as a living space for sitting, eating, sleeping and pastimes in most Korean homes before the 1960s. Unlike the western style, Koreans are accustomed to sitting on the floor, sleeping on the floor, and working and eating at low tables instead of raised tables with chairs. The furnace 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 burning was either sporadically or regularly done (two to five times a day), dependent on frequency of cooking and seasonal weather conditions.
With the traditional ondol heating, floor spots closer to the furnace were normally warm enough, with warmer spots reserved for elders and honored guests. Ondol had problems such as carbon monoxide poisoning resulting from burning coal briquette, and environmental pollution. For these reasons, other technology heats modern Korean homes.
The Ondol, which is the radiant floor heating system, was suitable to the lifestyle of the Koreans. One famous American architect, Frank Lloyd Wright, was building a hotel in Japan. One day, he was invited to one Japanese family's house. The owner of the house experienced the ondol in Korea, and he could not forget it, so he built an ondol room in his house. Wright was so impressed by ondol, that he invented radiant floor heating which does not use hot air, but uses hot water to heat up the floor. This system applied to Wright's buildings. Instead of ondol-hydronic radiant floor heating, the modern-day houses such as high-rise apartments have a developed version of the ondol system. Many architects know the advantages and benefits of ondol, and they are using ondol in various different methods in modern houses. Since the ondol has been introduced to many countries in the world, it is beginning to be considered as one of the systems of home heating. The ondol which are used these days are not same as the original version. Almost all of the Korean use the developed version system, so it is hard to find the traditional ondol system in Korean houses.
Advantages and disadvantages
One of the advantages of an Ondol is that it can maintain heat for an extended period of time. 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 than other areas. 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. Ashes from an Ondol can be used as fertilizer, making the Ondol an environmentally friendly system. 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. 
On the other hand, the Ondol has some disadvantages. Mud and stones are one of the main materials that make up the Ondol. Such materials take quite a long time to heat up, therefore the room takes a long time to warm up. In addition, it is difficult to adjust the temperature of the room.
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