Ondol​—A Unique Home Heating System


SHIVERING in Korea’s cold winter weather, we are invited into the home of a smiling householder. The welcoming indoor air warms our cold bodies, though no heater or radiator is in sight. After removing our shoes at the entrance, we step up onto the floor and find that it is warm. Sitting on the floor and resting our hands on it, we feel our frozen hands begin to thaw.

In Korea almost every house has this kind of floor heating. It is called ondol. How does this unique home heating system work? And what impact has it had on the Korean way of life? Before answering those questions, let us consider the history of the traditional ondol.

History of Floor Heating

The history of floor heating goes back to before Jesus Christ came to the earth. According to archaeological findings and historical records, the ancient Romans may have been the first to use a floor heating system. * By the fourth or fifth century C.E., a floor heating system was popular throughout the Korea Peninsula, and it came to be called ondol. The name is derived from Chinese characters meaning “warm hollows.” The Chinese historical record the Books of Old Tang makes reference to ondol, saying: “In winter [Korean] people made themselves warm by making long hollows and supplied fire in them.”

How Traditional Ondol Worked

Traditionally, the source of heat for the ondol was a fireplace. This might be located in the kitchen or on the outside wall of the living room. A kitchen with two or three fireplaces could be surrounded by a matching number of ondol-heated rooms. In an old Korean kitchen, you might find one or two big  iron cauldrons on the fireplace. Thus, the fire used for cooking rice or soup was also used to heat the room next to the kitchen! Efficient, was it not?

In general, the kitchen was built two and a half to three feet [about a meter] lower than the room that was being heated. The difference in level made it easy for the smoke and hot air to run under the floor of the elevated room. Smoke running under the floor? Yes, that is the secret of ondol.

Horizontal flues​—passageways for heat and smoke—​ran beneath the room’s floor, connecting the fireplace and the chimney. Hot air from the fire passed through the flues  and heated the stone and mud floor. This was not as easy as it sounds. Two conflicting requirements had to be met. For the fuel to burn well, its smoke had to pass quickly through the flues and go out the chimney unhindered. Flues that were straight and short were best for that purpose. For the heat from the fire to warm the floor, however, the hot air and smoke had to stay in the flues as long as possible. To accomplish this, the flues were made to cover the area under the whole floor, thus preventing the hot air from going out through the chimney too quickly. When a happy medium between fast and slow was reached, a room could be kept warm all night with a fire that lasted only a couple of hours.

It is said that there was once an ondol room​—hundreds of years old—​that had incredible thermal efficiency. Because of the design of the room’s flue structure, its floor would remain hot for 45 days with just one heating! Warmth could supposedly be felt for 100 days. Unfortunately, that room was destroyed during the Korean War in the early 1950’s. In 1982, engineers restored the structure, and tourists can visit its ondol room. The present thermal efficiency is not nearly as good as the original. Still, after one heating, the floor remains warm for ten days in spring and fall, and for three days in winter, even when the temperature is below 14 degrees Fahrenheit [-10°C].

Another secret of the ondol heating system is in the design of the floor itself. Before the floor was laid, flues for hot air were made. The flues were then covered with flat, thin stones that were two or three inches thick. As the floor near the fireplace is naturally warmer, thicker stones were used there to prevent heat loss. Next, yellow earth was laid on top of the stones, and the floor was leveled. Finally, several layers of yellow paper sheets were pasted onto the base.

In a room heated by the traditional ondol system, the floor at the far end of the room tended to be cool. Elderly people, such as grandparents or parents, as well as guests were thus invited to sit in the warmer area. This was an expression of respect.

The traditional ondol rooms found in the northern part of the Korea Peninsula differed somewhat from those in the south. In the north the ondol-heated room and the kitchen were not separated by a wall. Heat from both the fireplace and the ondol floor kept the room warm. In the south, a wall separated the kitchen from the living room, preventing the smoke from disturbing people sitting there.

Koreans traditionally used wood to fuel this type of fireplace. So before starting the ondol, they stacked dried firewood next to the fireplace. Then they used paper and straw to start the fire. Charcoal was also used as fuel. In the 20th century, however, Koreans began using charcoal briquettes. It is essential, of course, that an ondol room be well maintained. If cracks develop in a flue,  carbon monoxide can leak into the living room through the floor, possibly resulting in fatalities.

Modern-Day Ondol

Nowadays, the traditional ondol system is rarely seen in Korean homes. Instead, modern-day houses, including high-rise ondol apartments, use an updated version of ondol​—hydronic radiant floor heating. This uses hot water instead of hot air to warm the floor. Interestingly, the Koreans were not the ones who developed this system.

In the early 1900’s, when the famous American architect Frank Lloyd Wright was building a hotel in Japan, he was invited to the home of a Japanese nobleman. There Wright found a room that was different from typical Japanese rooms. The floor was covered with yellow paper and was warm. It was a Korean ondol room! The Japanese gentleman had experienced ondol in Korea and could not forget it. So back in Japan he had an ondol room built in his house. “The indescribable comfort of being warmed from below” impressed Wright. He decided then and there that ondol was the ideal heating system and began incorporating it in his buildings. Wright invented radiant floor heating, using hot water running through pipes instead of hot air through flues.

The radiant floor heating system was a good match with the life-style of Koreans at that time. Once it was imported, the simplified system caught on quickly. Today almost all Korean homes use it.

Ondol and Life-Style

Ondol has had a great impact on the Korean life-style. For one thing, because the floor is much warmer than the indoor air, people naturally sit on the warm floor rather than on colder chairs. Koreans thus sit, eat, associate, and sleep on the floor. To keep the floor even warmer, they sometimes cover it with a thick bed quilt called ibul. When family members come in from outside, they put their cold legs under the bed quilt to enjoy the comfortable warmth together​—a real bonding experience!

As the Korean life-style has become increasingly Westernized, the younger generation often prefer to sit on chairs at a table and to sleep in beds. Still, most Koreans prefer the snugness of an ondol floor, using a hot-water floor heating system. If you visit Korea, you will no doubt enjoy this unique home heating system​—ondol.


^ par. 6 The central heating system developed by the Romans was called hypocaust. It was made up of an underground furnace and tile flues that distributed the heat.

[Diagram/Pictures on page 23]

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Top view of a flue layout

1 Fireplace

2 Horizontal flues

3 Chimney

→ → 2 → →

→ → 2 → →

→ → 2 → →

● 1 → → 2 → → ● 3

→ → 2 → →

→ → 2 → →

→ → 2 → →


A fireplace was used both for cooking food and for heating the adjoining room

A good chimney-and-flue system was essential for an efficient “ondol”

[Credit Line]

Location: Korean Folk Village

[Picture on page 24, 25]

In an “ondol” room, the warmest area is reserved for older ones

[Credit Line]

Location: Korean Folk Village