If you spend a winter in Korea, one of the first things you may notice indoors is the floor. Even in a modern apartment with beds, sofas, and dining tables, sitting or lying directly on a warm floor still feels completely natural to many Koreans.
That warmth has a long history, but the heating system under a modern apartment floor is not the same technology used in a traditional hanok. In the past, heat and smoke from a fire traveled beneath stone floors. Today, most Korean homes use hot water circulating through pipes under the floor, supplied by an individual boiler, central heating system, or district heating network.
The technology changed dramatically. What remained was the idea that the floor itself should be warm.
How Traditional Ondol Actually Worked
Traditional ondol was built around a relatively simple idea: instead of allowing the heat from a cooking or heating fire to escape immediately, send it beneath the room and use it to warm the floor.
A traditional system included an agungi, or firebox, channels beneath the floor called gorae, stone slabs known as gudeuljang, and a flue leading toward a chimney. When a fire was lit in the agungi, hot gases moved through the channels beneath the room. The stones and layers of earth above them absorbed heat and gradually transferred it to the floor.
The National Institute of Korean History’s overview of ondol explains the system as a form of heating that retains heat beneath the floor rather than simply warming the room directly from an open fire.
This also meant that different parts of a traditional room could have noticeably different temperatures. The area closest to the firebox was called araenmok and was usually the warmest. The area farther away, toward the flue and chimney, was called winmok and tended to be cooler.
In winter, that temperature difference mattered. The warmest part of the room naturally became the place where people wanted to sit, rest, or warm themselves after coming in from the cold.
Traditional ondol was not built in exactly the same way in every house. The shape and arrangement of the underground channels varied according to the building, region, and construction method. The builder had to make sure that heat moved effectively beneath the floor while smoke continued safely toward the chimney.
From Partial Gudeul to Whole-Floor Heating
Ondol developed gradually over a long period rather than appearing all at once in its later form.
Early forms included jjokgudeul, or partial-floor heating, in which only part of a room was heated. Archaeological and historical evidence from northern areas of the Korean Peninsula shows early systems in which heated channels ran along one or two sides of a living space rather than beneath the entire floor.
Over time, more extensive systems developed. Whole-floor ondol, known as ongudeul, appeared gradually and became much more widespread during the Joseon period. Even in early Joseon, however, not every room in every house had a heated floor. Historical records indicate that some upper-class homes initially reserved ondol rooms particularly for elderly or sick family members.
By the late Joseon period, ondol had spread much more broadly across social classes and regions. It became closely associated with Korean residential life, although its exact form continued to vary.
The cultural importance of ondol goes beyond the construction itself. Ondol Culture was designated as a National Intangible Cultural Heritage in 2018. The designation recognizes not only the heating structure but also the domestic customs and ways of living that developed around a heated floor.
How a Warm Floor Shaped Everyday Life
Once the floor itself becomes warm and comfortable, it is easy to understand why it becomes part of the living space rather than simply something to walk across.
Traditionally, people sat on the floor to eat and talk, and at night they could spread out bedding and sleep in the same room. Furniture was often arranged around this floor-centered way of life. Even today, when beds, sofas, and dining tables are common in Korean homes, many people still sit or lie on the floor without thinking of it as unusual.
Traditional Korean houses also combined heated ondol rooms with maru, raised wooden floors that were not heated. The two spaces served different seasonal purposes. An ondol room was especially useful during Korea’s cold winters, while the airy wooden maru provided a cooler place during hot and humid weather.
Ondol was one of several factors that made floor-centered living practical and comfortable. Housing customs develop through many influences over long periods. But a clean, heated floor clearly made it practical to use the floor itself for sitting, sleeping, eating, and other everyday activities.
This habit did not disappear when Koreans moved into high-rise apartments. As I discuss in my article about apartment culture in Korea, apartment living transformed Korean housing on an enormous scale, but some older habits were adapted rather than abandoned. The warm floor is one of the clearest examples.
My Memories of the Araenmok at My Grandmother’s House
When I think about ondol, I do not first picture an architectural diagram. I think about my maternal grandmother’s house when I was young.
When I visited her home in winter, it was easy to tell which part of the room was the araenmok. The area near the source of heat could become noticeably hotter than the rest of the floor. I also remember the floor around that warm area looking darker after years of use and heat.
People naturally gathered around that part of the room in winter. Someone coming in from the cold could warm up there, and giving the warmer place to an older family member felt normal. In those memories, the araenmok was not simply a technical feature of a heating system. It was part of how the room was used.
One memory in particular has stayed with me. My grandmother sometimes placed bowls of cooked rice on the warm araenmok and covered them with a blanket so that the rice would stay warm.
Today, an electric rice cooker can keep rice warm for hours with the push of a button. At my grandmother’s house, the heat already stored in the floor could serve as a simple household warmer. It was an everyday use of residual heat rather than something anyone in the family would have thought of as clever engineering.
I also remember meju, blocks of boiled and mashed soybeans used to make traditional fermented sauces, hanging high indoors during the winter. The fermentation process involves far more than ondol heat alone, but the warm indoor environment was part of the household setting in which these foods were prepared and dried.
For more on what happens to meju after this stage, see my guide to Korean fermented sauces such as doenjang, ganjang, and gochujang.
These memories are one reason I prefer to think of ondol as more than an old heating device. A warm room was also a place where people slept, ate, stored food temporarily, talked with family, and carried out winter household work.
From Firewood and Briquettes to Hot-Water Pipes
The Korean heating system changed rapidly during the twentieth century.
For generations, wood and other solid fuels had been burned in traditional fireboxes. As Korea urbanized, coal briquettes, known as yeontan, became an important household fuel. They became especially common during the 1960s and remained widely used for heating and cooking well into the 1980s.
Briquettes were inexpensive and convenient compared with firewood, but they came with a serious danger. Carbon monoxide leaking from poorly ventilated or damaged heating systems caused many poisoning accidents. Older Koreans still remember newspaper reports and warnings about yeontan gas during winter.
At the same time, Korea was becoming increasingly urban and apartment-based. Sending combustion gases directly through channels beneath the floor was not an ideal system for the high-rise housing that was spreading rapidly.
Beginning in the late 1970s, hot-water floor heating expanded in Korea. Instead of sending smoke and hot gases beneath the room, the new system circulated heated water through pipes embedded in the floor. The underground channels of traditional ondol were replaced by pipes, and the heat source could be separated from the living room itself.
This change also helped separate cooking from heating. In older homes, the kitchen fire and room heating could be physically connected through the agungi. As boilers, modern kitchens, and apartments became more common, cooking no longer needed to provide the fire that heated the bedroom floor.
So traditional ondol and modern Korean apartment heating should not be described as exactly the same technology. One used combustion gases, stone, earth, and a chimney; the other generally uses heated water circulating through pipes. What connects them is the decision to heat the living space from beneath the floor.
Ondol Today: The Technology Changed, but the Warm Floor Remained
Modern Korean homes use several different sources of heat. Some apartments have individual gas boilers, while large complexes may use central or district heating. There are also electric floor-heating systems in certain types of buildings.
Whatever produces the heat, hot-water pipes beneath the floor remain extremely common. A resident controls the temperature from a wall-mounted thermostat rather than adding wood to an agungi, but the result is still a warm floor throughout the living space.
This continuity helps explain why underfloor heating survived the transition from hanok to apartment living so easily. Koreans changed the architecture of their homes dramatically during the twentieth century, but there was little reason to give up a heating style that already fit familiar indoor habits.
Ondol also had an interesting encounter with modern architecture outside Korea. American architect Frank Lloyd Wright encountered a Korean heated room in Tokyo in 1914 and later wrote about the unusual experience of feeling warmth without seeing an obvious heater. Koreana’s account of Wright and Korean ondol describes how the experience influenced his later interest in underfloor heating.
The old gudeul structure largely disappeared from modern housing, but Koreans did not abandon the warm floor when they moved into apartments. The agungi became a boiler, the smoke channels became hot-water pipes, and thermostats replaced the daily work of tending a fire.
At my grandmother’s house, a bowl of rice could sit beneath a blanket on the hot araenmok to stay warm. In a modern apartment, I adjust the heating with a control panel instead.
Technically, those are very different worlds. But both make sense within a culture in which a warm floor has remained part of what a comfortable Korean home feels like.
About the Author 🇰🇷
I live in South Korea and write about the small details of Korean life and culture that I experience and observe firsthand.
English is my second language, so I sometimes use tools such as Gemini and Google Translate to help with translation and editing. The experiences, observations, opinions, and cultural perspectives in my articles are my own, and I personally review and edit every article before publishing it.
My goal with Korea Core Info is simple: to share everyday Korea from a local perspective and help international readers discover things they might not always find in a typical travel guide.
I hope my stories and guides help you understand Korea a little better — not just as a place to visit, but as a place where people live their everyday lives.