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An illustration of the ondol system
Hangul 온돌 / 구들
Revised Romanization ondol / gudeul
McCune–Reischauer ondol / kudŭl

An ondol (Hangul온돌), also called gudeul (Hangul구들), in Korean traditional architecture, is underfloor heating which uses direct heat transfer from wood smoke to the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or a hotel or 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.



The earliest use of ondol has been found at an archaeological site in present-day North Korea. A Bronze Age archaeological find, circa 1000 BC, discovered in Unggi, Hamgyeongbuk-do, in present-day Korea, shows a clear vestige of gudeul in the excavated dwelling (Hangul움집) unearthed at the archaeological site.[citation needed]

Early ondols began as gudeul that contained the furnace/cooking stove in the building. With the evolution of the structure, the furnace was moved entirely outdoors to prevent overheating in the summer and reduce indoor smoke. As well, entire rooms would be built on the furnace flue to create ondol floored rooms.[1]


The term gudeul has been colloquially spoken for over two thousand years, and called by many alternate names (janggaeng (Hangul장갱; hanja長坑), hwagaeng (Hangul화갱; hanja火坑), nandol (Hangul난돌; hanja暖突), yeondol (Hangul연돌; hanja烟突)); the term ondol was introduced around the end of 19th century.[2] According to a Korean folkloric historian Son Jintae (1900 - missing during the 1950-53 Korean War), gudeul originated from guun-dol (Korean), which means "heated 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.[citation needed]


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.[3] 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.

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