The kang (; Manchu: nahan, ) is a traditional heated platform, 2 metres or more long, used for general living, working, entertaining and sleeping in the northern part of China, where the winter climate is cold. It is made of bricks or other forms of fired clay and more recently of concrete in some locations. The word kang means "to dry". Its interior cavity, leading to an often-convoluted flue system, channels the hot exhaust from a firewood/coal fireplace, usually the cooking fire from an adjacent room that serves as a kitchen, sometimes from a stove set below floor level. This allows a longer contact time between the exhaust (which still contains much heat from the combustion source) and (indirectly) the inside of the room, hence more heat transfer/recycling back into the room, effectively making it a ducted heating system similar to the Roman hypocaust. A separate stove may be used to control the amount of smoke circulating through the kang, maintaining comfort in warmer weather. Typically, a kang occupies one-third to one half of the floor space, and is used for sleeping at night and for other activities during the day. A kang which covers the entire floor is called a dikang (). Like the European cocklestove, a massive block of masonry is used to retain heat. While it might take several hours of heating to reach the desired surface temperature, a properly designed bed raised to sufficient temperature should remain warm throughout the night without the need to maintain a fire. History The kang is said to be derived from the heated bed floor called a huoqiang found in Neolithic China, according to archeological excavations of building remains in Banpo Xi'an. Sites in Shenyang, Liaoning, show humans using the heated bed floor as early as 7,200 years ago. The bed at this excavation is made of 10 cm pounded clay on the floor. The bed was heated by zhidi, placing an open fire on the bed floor and clearing the ashes before sleeping. In the excavated example the repeated burning is believed to have turned the bed surface hard and moisture resistant. The first known heated platform appeared in what is now Northeastern China and used a single flue system like the Roman hypocaust and the Korean ondol. An example was unearthed among 1st-century building remains in Heilongjiang Province. Its flue is 'L' shaped, built from adobe and cobblestones