What is "ondol"

n. A Korean form of underfloor heating that uses direct heat transfer from wood smoke to the underside of a thick masonry floor.

Ondol , also called gudeul , 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.