Credited to South Korea. Ondol belongs to the whole Korean peninsula. The earliest reported evidence, dated to about 1000 BCE, is at Unggi in present-day North Korea, so the first origin may lie outside South Korea. Sites and records in the south include a Baekje-era ondol site and Joseon court records from Seoul.
How it began
The earliest reported use of ondol is at an archaeological site at Unggi in North Hamgyeong, in present-day North Korea, dated to the Bronze Age, about 1000 BCE. The native Korean word for the system is gudeul, said to derive from words meaning heated stone; the term ondol was introduced around the end of the nineteenth century. In the south of the peninsula, an underfloor heating site of the Baekje era has been reconstructed at the Gyeonggi Provincial Museum.
How it works
A fire is lit in a firebox called an agungi, usually in the kitchen. Because the flue entry sits beside the fire, flame and smoke are drawn sideways into horizontal passages under a raised masonry floor and out through a freestanding chimney on the opposite side of the house. The floor rests on stone piers and is covered with stone slabs, clay and a sealed layer such as oiled paper. The flues spread under the whole floor so the hot air lingers, and the stone stays warm long after the fire used for cooking has gone out.
How it spread
For centuries the system was limited. From the end of the Goryeo dynasty heated rooms were used mainly by the wealthy and for the sick and elderly. A court record of 1417 describes an ondol room made for sick students at the Seonggyungwan academy, and only in the sixteenth century did all its rooms become ondol rooms. During the cold of the sixteenth and seventeenth centuries the system spread, and by late Joseon it was common in the thatched houses of ordinary people.
Who pushed back
The spread of ondol had costs. It consumed large amounts of firewood, and the rising demand contributed to deforestation on the Korean peninsula from late Joseon through the 1950s and 1960s. When coal briquettes replaced firewood from the 1960s, incomplete combustion caused many cases of carbon monoxide poisoning. Fire was also a risk: a 1563 fire in the king's bedroom was traced to a misplaced stone in a small ondol built to heat his bed.
What lasted
Ondol made the floor the warmest part of the house, and Koreans became accustomed to sitting, eating and sleeping on it, with the warmest spots reserved for elders and guests. The practice of removing shoes and sitting on the floor in homes and restaurants is linked to it. The smoke passing through the flues also helped protect wooden houses from insects and fungi.
Where it stands today
In 1962 hot-water boilers using the ondol principle were developed to cut fuel use and the risk of poisoning. Almost all Korean homes now use modern versions, and the traditional wood-fired system is hard to find in South Korea. In current usage the word covers any underfloor heating.
