ondol

Ondol is a traditional Korean underfloor heating system that warms living spaces by radiant heat from beneath the floor. In its classic form, a heat source such as a wood- or coal-fired furnace sends hot exhaust through a system of clay flues embedded in the floor and sometimes in the walls. The floor above stores and receives the heat, radiating warmth upward and evenly across the room; the exhaust is vented by a chimney, and heat output is controlled by adjusting the furnace and vents.

Etymology and design: The name 온돌 (ondol) combines Korean 온, meaning warm, and 돌, meaning stone, literally "warm stone." The system is a defining feature of traditional Korean architecture, especially in hanok houses, where rooms are organized around central heating via the ondol flues.

History: Evidence of ondol-style heating appears in ancient Korean architecture; the system was well developed during the Goryeo and Joseon dynasties, and remained standard in many homes until being supplanted by modern central heating in the late 20th century. It influenced floor plans and interior materials, with stone or concrete floors acting as radiant heat surfaces.

Modern variations: Today, traditional flue-based ondol is relatively rare in new construction but is preserved in heritage sites and some rural homes. Many contemporary Korean buildings use hydronic or electric underfloor heating systems that circulate warm water or electric mats beneath the floor to provide radiant heat. The term "ondol" remains a general reference to underfloor heating in Korea.