Ancient royal floor heating in the Forbidden City
(This article was published in the 8th edition of Science and Technology Daily on December 10, 2021, with amendments)
The 600-year-old Forbidden City, also known as the Forbidden City, not only has a splendid ancient palace complex, but also contains a lot of wisdom of ancient court life, and floor heating is a typical representative of it. The Ming Dynasty eunuch Liu Ruoyu wrote the "Zhi Zhongzhi" volume 17 "Great Internal Regulation Chronicle" has: "Right to the east is known as mao qin temple, the former emperor created the earth kang here, Heng Lin Yuzhi"; volume 20 "Good Diet Shang JiLuo" has: "October ... It is that the night has gradually grown, and the inner ministers have begun to burn the ground kang"; the above can show that the Forbidden City already had floor heating during the Ming Dynasty. Despite the cold outside in winter, the emperors who lived in the palace lived very comfortably and warmly. The Qianlong Emperor wrote in the poem "Winter Nights Are Occasionally Formed": "People suffer and winter days are short, and I love long winter nights." The Daoguang Emperor wrote in the "Complete Collection of Yangzheng Bookstore" that "the fine cloth of the flower bricks is good at strange work, and the dark heat pine branches are baked underground". Here, the "bird road" and "sheep intestine" are the passages for underground heating. Different from the modern people's use of water floor heating (buried under the floor pipe, with hot water as the heat medium, circulating in the water pipe, heating the floor) or electric floor heating (burying cables or electric heating film under the floor to heat the floor), the ancient floor heating in the Forbidden City is an outdoor fire, heating indoor floor tile heating method, commonly known as "fire" or "warm ground".
The fire ground consists of an underground operating port located outside the window, an underground furnace chamber inside the window, a fire tunnel (a passage for heat transfer) under the indoor floor tiles, and a smoke outlet on the side of the outdoor base (Figures 1-2). The basic principle of heating is: the service personnel stand in the operation port, put the charcoal into the underground furnace chamber to burn, heat the air; the heated air spreads along the path of the fire channel to the surroundings, during which the ground brick is heated, using the heat storage of the floor tile itself and the law of heat radiation upwards, so that the heat is conducted from bottom to top, thereby maintaining the warmth of the room; the air temperature after heat dissipation is reduced, and the smoke and dust (a small amount) in the charcoal fire is discharged from the smoke outlet.
Figure 1 Schematic diagram of the fueze plane
(1- outdoor operating port; 2- underground furnace chamber; 3, 4, 5- fire channel, dotted arrow is the direction of heat transfer underground; 6-window; 7- smoke outlet; 8- rammed earth schematic map)
Fig. 2 Cross-sectional view of the fire ground in 1-4 directions
The operator's opening is located below the ground outside the window, and the size is generally 0.8×0.8×1m (length× width× height). In the Ming Dynasty, the personnel responsible for burning charcoal in the Forbidden City were arranged by the Department of Salary, and in the Qing Dynasty, the personnel of the Department of Internal Affairs were created. The service personnel enter the operation port and can burn charcoal into the furnace chamber under the room. The opening is covered with a thick plank when not in use, preventing small animals from burrowing in and facilitating the walking of the palace personnel outdoors (Figure 3). The operation port is outdoors, and the service personnel stand in the operation port, which can see the room through the window, which is convenient for communicating with the indoor users, and timely increase or decrease the charcoal fire to ensure the appropriate indoor temperature. The other operation port is located outdoors, which is also conducive to avoiding the spread of smoke generated by indoor fire sources or other safety hazards indoors.
Fig. 3 Fire operation port and smoke outlet (dotted box part)
After the service staff burns the charcoal, the heat spreads under the floor tiles along the direction of the width of the building, and the fire path in this direction is called the main flue; there are several flues of smaller width on both sides of the main flue, which are perpendicular to it, called the flue. The distribution of the above main and branch flues is like a centipede, so it is also commonly known as the "centipede road". It should be noted that because the heat is transferred from the bottom up, the fire source position is located at the lowest point in the room, and when the main flue extends from the fire source position to the room, its bottom height gradually increases, and the flue profile rises in a slope. In this way, the heat can be spread from the bottom of the window to the room relatively quickly. In general, the main flue section is larger in size and its upper cover is made of brick masonry; the branch flue section is small in size and the cover is a snap tile (Fig. 4). In addition, the main and secondary flue covers are then overhead ground bricks, and the overhead method is: a number of brick piers are erected on the rammed soil layer between the flues, the ground bricks are placed on the piers, and the seams between the face bricks are smeared with mortar. The scientific nature of this practice is that: on the one hand, the floor tile layer can be evenly heated, and the temperature is not too high; on the other hand, if there is smoke seeping out of the flue, it will still be limited to the ground below the ground and discharged from the smoke outlet.
Figure 4 Main flue and branch flue
Underground heat is diffused through the main and branch flues, then rotated and discharged outdoors through the smoke outlets on the side walls of the outdoor pedestal (Figure 3). It should be noted that the smoke outlet is equivalent to the "chimney" of the ancient building of the Forbidden City, which is different from the chimney of the ordinary building standing on the roof, it is located below the indoor ground, which does not affect the overall appearance of the building, and can play a good smoke exhaust effect. This practice can explain the saying that "the Forbidden City burns charcoal for heating in winter, but there is not a chimney in sight". In addition, in order to avoid small animals from drilling into the indoor underground from the outlet, the outlet is often paved with hollow brick carvings with copper coin patterns to achieve a practical and beautiful effect. Originating from present-day Yi County, Hebei Province, the charcoal used for fire was made of a kind of hardwood and was called "red charcoal". Each piece of charcoal is processed into chunks about 32 cm long and about 10 cm in diameter. This charcoal produces high heat and low smoke. Volume 16 of the ZhizhongZhi also records that this charcoal has the advantages of "warm and durable, gray and not explosive". The charcoal ash produced by the burning of charcoal is also collected and used as a liner in the toilet and bedpan in the palace.
There is a view that "the walls in the palace are actually hollowed out 'sandwich walls', commonly known as 'fire walls', and the heat warms the entire hall along the sandwich wall", which is wrong. From the perspective of engineering practice, it has not yet been found that there is a "sandwich wall" for heat transfer in the ancient walls of the Forbidden City.
The ancient royal floor heating in the Forbidden City, by burning the fire at the outdoor operation mouth, the heat generated by the fire source spreads under the indoor underground, and gradually spreads to the indoor space, achieving the effect of indoor insulation. This scientific and simple way of heating reflects the wisdom of ancient craftsmen.
(Author's information: Zhou Qian, Institute of Forbidden Studies, Palace Museum)