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An Ondol, also called Gudeul, in Korean traditional architecture, is underfloor heating which utilizes 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 is supported by stone piers or baffles to distribute the smoke, covered by stone slabs, clay and an impervious layer such as oiled paper.
from: http://en.wikipedia.org/wiki/Ondol