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Ondol: the world’s first underfloor heating system

by KnowingKorea

Underfloor heating is now used all around the world, and is growing in popularity thanks to its convenience and efficiency.


It was re-discovered in the West during modern times by Frank L Wright, who came across the traditional Korean Ondol method. An ancient form of heating, the Ondol works by means of an outside furnace, underfloor pipes, and a special stone that retains heat for long periods and releases it gradually.


The first form of underfloor heating in the world, it was sophisticated enough to be effective, but within the means of both rich and poor to build and use. Still found in Korean homes today, it has been shown to help allergies, aid sleep and improve general quality of life.


In recent years, the use of underfloor heating has become more common around the world. In Germany, Denmark, Austria, and Switzerland, countries that have traditionally relied on convector heating systems such as radiators, nearly half of newly-built homes now have underfloor heating. The reason for the popularity of underfloor heating is the fact that it is noiseless and invisible to the eye, takes up less space, and is more energy-efficient than other forms of Heating.


Ondol gave a western architect “indescribable warmth and comfort” in winter


Frank L. Wright of the United States was the first western architect to develop a system of underfloor heating during modern times, using hot water pipes. Creator of the famous Fallingwater and Guggenheim museums, he is regarded as one of the most influential architects of the 20th century.


In his book The Natural House, Wright recalls a winter's day in 1914 when he was invited to a client’s house in Japan. In a small room, called the ‘Korean room,’ to which he was led after the meal, he encountered a new form of heating that he had never experienced before.


Wright recounted the episode as follows.


“The climate seemed to have changed. No, it wasn’t the coffee; it was spring. We were soon warm and happy again . kneeling there on the floor, an indescribable warmth. No heating was visible, nor was it felt directly as such. It was really a matter not of heating at all but an affair of climate. The Harvard graduate who interpreted for the Baron explained: the Korean room meant a room heated under the floor. The indescribable comfort of being warmed from below was a discovery...There is no other ‘ideal’ heat. Not even the heat of the sun.” (Frank Lloyd Wright, The Natural House, pp. 89-90.)


The architect was deeply intrigued by this extraordinary form of heating, whereby warmth radiated from beneath the floor to heat the four corners of the room. He soon afterwards used the concept himself to build hotel baths in Japan, and on his return to the United States, he introduced the same method of heating to his most famous buildings. The system that Wright had experienced in Japan was the Ondol, a traditional Korean method of underfloor heating.

Thanks to Ondol, even the poor can pass winter in warmth


Because Korea's history of heating technology goes back to prehistoric times, it is impossible to be precise about the Ondol's date of origin. However, early forms of Ondol have been discovered in excavated residential sites from the Old Choson period (BC 2333~BC 108), and academics believe it was used from this time onward.


The word Ondol, which means ‘warm stone’ in Korean. Heat is provided by a furnace outside the room, which sends hot air along flues beneath the floor, and the thermal energy is retained by the stone slabs placed above the flues. This stored energy is released gradually into the room, keeping it warm. Ondol was mankind's first form of underfloor heating. The Roman hypocaust was similar to the Ondol, but was used mainly at public bath-houses (thermae). It disappeared with the decline of the Roman Empire, and it was not rediscovered until the 19th century.


Thanks to the Ondol, everyone in Korea could pass the winter in warmth, whether they lived in the city or the countryside, whether they were wealthy or poor. Horace Allen, an American priest and doctor who visited Korea about a hundred years ago, left the following observations in his book Things Korean.


“However humble the hut of the peasant or coolie, it always has its tight little sleeping room, the stone and cement floor of which with its rich brown oil paper covering, is kept nicely warmed by the little fire necessary for cooking the rice twice daily. In this respect these people fare better than do their neighbors, for the Japanese houses are notoriously cold, and a fire pot for warming the fingers is the only native system of heating, while the Chinese never are warm in the raw cold of winter.”

(Horace Allen, Things Korean, p. 67)


In the case of a traditional Ondol heating system, heat from the furnace is transferred to the stone slab via a network of flues, with smoke from the fire allowed out through the chimney located at the opposite end. The floor is supported by stone piers, with the stone slabs placed on top. It is then covered with clay, and overlaid with yellow-colored oiled hanji flooring-paper.


In Korean homes today, a boiler has taken the place of the furnace, and metal side view and ground plan of the traditional Ondol system pipes carry out the function of the flues. Instead of hot stones, warm water from the boiler is circulated to heat the rooms. Though much may have changed in terms of outward appearance, so far as principle and structure are concerned, the concept has remained virtually the same.


The World's First Smokeless Heating


In Europe, until the 12th and 13th centuries, people and livestock lived under the same roof, contributing body heat to one another. At the center of the house was a fire pit for the lighting of an open fire, which was used for cooking and heating. Sometimes the fireplace was put on a raised platform made of stone or brick, but until as late as the Middle Ages, most households lacked a specific ventilation system such as a chimney. When a fire was lit, the entire house would be filled with smoke.


In certain parts of Europe, roofs were constructed in a funnel shape to let the smoke escape. These features were known as ‘Rauchschlot’ in Germany and Austria, and in mountainous regions such as the Alps, they can still be seen in houses dating from the 17th and 18th centuries.


Though houses with Rauchschlot solved the problem of excessive smoke, the quality of heating suffered, as the heat escaped from the house along with the smoke. To live in the cold without smoke, or to live in warmth and endure the smoke, was a perpetual dilemma.


By means of the Ondol system, Korean homes were able to enjoy the best of both worlds. Traditional examples of Ondol are relatively well preserved today in the Royal Palaces of Korea. As it was believed that that no smoke should be seen in the King’s place of residence, the furnace was hidden beneath the building. To minimize the amount of fire and smoke, charcoal was used instead of firewood.

Chimneys were also placed far away from the King's sight, and the distance between the furnace and chimney became very great as a result, with a minimum distance of 28 meters.


The World's First Heat Storage System


An open fire or fireplace may be warm while it is lit, but as soon as the fire goes out, the heat quickly disappears. On the other hand, because the Ondol is a form of heating which retains thermal energy in stone slabs beneath the floor, the room can be kept warm for long periods without the need to create further heat.


In the grounds of the Chilbulsa Temple in Kyongsang Province, there is a famous building called Ajabang which is said to have kept warm for a month and half (100 days in other accounts) after just one heating. Tradition has it that in the reign of the Silla King Hyo-gong (r. 897-912), a Buddhist master called Damgong built a specially designed Ondol so that even in the depth of winter, practitioners could keep warm and devote themselves single-mindedly to inner cultivation. Master Tamgong’s Ondol was unfortunately destroyed by fire during the Korean War. It was restored in the 1980’s, but modern technicians were not able to replicate the millennium-old structure completely. The warmth from one heating now lasts for around a week.


The secret of the lasting warmth of the Ondol lies in the stone itself - mica. Mica is a mineral commonly found in indigneous and metamorphic rocks. The ‘white’ variety of mica can withstand temperatures of 400~500°C, and because its thermal conductivity is low, it retains heat for a long time and releases it gradually. Also, being an alkaline rock rich in minerals such as magnesium oxide and calcium, when heat is applied, it emits a large amount of far-infrared radiation. Far-infrared radiation has a longer wavelength than visible light, and due to its high thermal conductivity, it is widely used in natural health treatments such as thermal therapies.


Ondol, Warming the Body and Mind


When a fire is lit, the flames naturally point upwards. They may be blown to the side by a gust of wind, but they always return when the wind calms.


Similarly, smoke and heat both rise upwards. The Ondol is a system of heating which aims to mirror this principle of nature.


Fireplaces, iron stoves or radiators are all designed to supply heat horizontally, and are therefore less able to provide heat efficiently. Moreover, since the heated air rises upwards, one’s head can become uncomfortably hot, sometimes causing a headache, while our lower body does not receive the heat and remains cold. In underfloor heating, because the heat rises from the floor, which is the lowest point in the room, every part of the room is evenly heated. Also, as our feet touch the warm floor, the blood vessels are stimulated, helping circulation.


Another advantage of underfloor heating is hygiene. With ordinary floors, carpets are used, and outdoor shoes are worn inside. The air inside becomes easily polluted from the dirt, dust and germs picked up from outside. With the Ondol system, neither carpets nor shoes are required, and one can live in comfort and health upon the traditional hanji (Korean handmade paper) floor.


Most importantly, the Ondol fulfils the primary purpose of heating, namely keeping the interior warm. However long a fire is lit, one can only be warm when close to it, and as soon as the fire is out, the room grows cold again. Even when standing next to the fireplace, one has to turn constantly to warm the back as well as the front. Hence the saying, “However large the fireplace, it is never too large.”


A German princess Liselotte von der Pfalz, notes in her letter home in 1695 that ‘the water and wine have frozen at the King’s table,’ later writing in 1701 that ‘All that keeps me warm at night are the six small dogs I take them with me to bed.’


Thanks to the Ondol, even a poor peasant in Korea could live in far greater warmth than any king in Europe. Reverend J.S. Gale, who lived in Korea as a missionary from 1889 to 1896, describes his experiences in his book, Korean Sketch, as follows.


“Sleeping in a small Korean hut I found, at first, to be one of my hardest trials. In a tight room, without one particle of ventilation, the floor heated nearly to the frying point, you spread your blanket. The inexperienced traveler, pursued by fiery dreams, baked almost brown, gasps for breath and wishes for the morning. But after a year or two of practice, one gets to like the hot floor, for as the natives say, it lets you out after a cold day's journey.”

(James Gale, Korean Sketches, p. 134.)


In several places throughout the book, he likens the Ondol to a ‘frying fan floor,’ and speaks of his ordeals trying to sleep inside the rooms heated to a point far beyond comfortable warmth. This would have perhaps been due to the Korean custom of lighting a larger fire than usual upon receiving a guest.


Underfloor heating has spread to many places in the world today, from China to Japan, across the Middle East to Europe, warming people's bodies and minds. Aseda Kazumi, a researcher at Tokyo Gas, which provides underfloor heating services in Japan, says that clients have often told him stories of how allergies have disappeared, how they sleep better at night, and even how their families have started to come back home early for the pleasure and comfort of underfloor heating.


When the body is warm and comfortable, the mind naturally becomes warm and happy too. There may be many advantages to underfloor heating, but the warmth and happiness of the mind is its greatest benefit.