Ondol

Ondol
Hangul	온돌
Hanja	溫突
Revised Romanization	ondol
McCune–Reischauer	ondol

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.

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