Ondol (ON-dol; /ˈɒn.dɒl/,^[1] Korean: 온돌; Hanja: 溫突/溫堗; Korean pronunciation: [on.dol]) or gudeul (구들; Korean pronunciation: [ku.dɯl]) in Korean traditional architecture is underfloor heating that uses direct heat transfer from wood smoke to heat the underside of a thick masonry floor. In modern usage it refers to any type of underfloor heating, or to a hotel or a sleeping room in Korean (as opposed to Western) style.

The main components of the traditional ondol are an agungi (firebox or stove) accessible from an adjoining room (typically kitchen or master bedroom), a raised masonry floor underlain by horizontal smoke passages, and a vertical, freestanding chimney on the opposite exterior wall providing a draft. The heated floor, supported by stone piers or baffles to distribute the smoke, is covered by stone slabs, clay and an impervious layer such as oiled paper.

Ondol had traditionally been used as a living space for sitting, eating, sleeping and other pastimes in most Korean homes before the 1960s. Koreans are accustomed to sitting and sleeping on the floor, and working and eating at low tables instead of raised tables with chairs.^[5] The furnace burned mainly rice paddy straws, agricultural crop waste, biomass or any kind of dried firewood. For short-term cooking, rice paddy straws or crop waste was preferred, while long hours of cooking and floor heating needed longer-burning firewood. Unlike modern-day water heaters, the fuel was either sporadically or regularly burned (two to five times a day), depending on frequency of cooking and seasonal weather conditions.

With the traditional ondol heating, the floor closer to the furnace was normally warm enough, and the warmest spots reserved for elders and honored guests. Ondol had problems such as environmental pollution and carbon monoxide poisoning resulting from burning coal briquettes.^[6] Thus, other technology heats modern Korean homes.

The famous American architect, Frank Lloyd Wright, was building a hotel in Japan, and was invited to a Japanese family's house. The homeowner had experienced the ondol in Korea, and had built an ondol room in his house. Wright reportedly was so impressed ^{[citation needed]} that he invented radiant floor heating which uses hot water as the heating medium. Wright introduced floor heating to Usonian houses in the US in the 1930s. ^[7]

Instead of ondol-hydronic radiant floor heating, modern-day houses such as high-rise apartments have a modernized version of the ondol system. Many architects know the advantages and benefits of ondol, and they are using ondol in modern houses. Since the ondol has been introduced to many countries, it is beginning to be considered as one of the systems of home heating. Modern ondol are not the same as the original version. Almost all Koreans use modern versions, so it is hard to find the traditional ondol system in Korean houses.^[8] North Korea still utilizes the basic traditional design of the Ondol that use mostly coal instead of biomass to survive the harsh winters.

• High exhaust vents jutting sideways
• Chimney for fumes
• Chimney
• Exhaust vents as sideways-oriented pipes

One of the advantages of an ondol is that it can maintain heat for an extended period. In a traditional Korean house, people usually extinguish the fire before going to sleep at night, since it can stay warm until the morning. An ondol conducts heat evenly throughout the whole room, although the part of the room closest to the agungi is much warmer. Comparing the ondol with the Western radiator: the heat from the radiator rises towards the ceiling, but an ondol keeps both the floor and the air in the room warm. The advantage of the ondol is that people do not have to worry about breakdown and repair of the ondol. ^{[citation needed]} The Ondol is part of the house, therefore, it is less likely to run into problems. Any combustible materials can be used as fuel for the ondol; there are no special fuel requirements. In contrast to heaters, such as fireplaces or charcoal-based heaters that leave ash in the room, ondol does not cause pollution in the room leaving it clean and warm. ^{[citation needed][9][10]}

The ondol has some disadvantages. Mud and stones are the main materials that make up the ondol. Such materials take quite a long time to heat up, therefore the room takes a long time to warm up. In addition, it is difficult to adjust the temperature of the room.^[9]

The dol bed, or stone bed, is a manufactured bed that has the same heating effect as ondol. The dol bed industry is estimated to be worth 100 billion South Korean won, comprising 30 to 40 percent of the entire bed industry in South Korea; dol beds are most popular with middle-aged people in their 40s and 50s.^[11][12]