of the Ondol. 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. The Ondol has some disadvantages. Mud and stones are one of 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. Dol bed 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 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. See also Underfloor heating Korean architecture Culture of Korea Hypocaust Rocket mass heater Gloria (heating system) Kang bed-stove Masonry heater Frank Lloyd Wright References External links The Herbert Offen Research Collection of the Phillips Library at the Peabody Essex Museum Architecture in Korea Heating Energy in South Korea Korean culture Korean inventions Residential heating appliances