An ondol, also called gudeul, in Korean traditional architecture, is underfloor heating

Underfloor heating

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...

 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

Firebox

In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name.-Railway locomotive firebox :...

 or stove

Stove

A stove is an enclosed heated space. The term is commonly taken to mean an enclosed space in which fuel is burned to provide heating, either to heat the space in which the stove is situated or to heat the stove itself, and items placed on it...

 (agungi; 아궁이) accessible from an adjoining (typically kitchen or master bedroom) room, a raised masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 floor underlain by horizontal smoke passages, and a vertical, freestanding chimney

Chimney

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...

 on the opposite exterior providing a draft.