When humans build villages, there has been requirements for settlement of winter, there seems many ways to heat the cold house. Ondol(온돌, 溫突) is one of the installation in Korean traditional architecture, the underfloor heating system that uses heat transfer from the smoke.
The structure of typical ondol is as the below:
The main conceptions of traditional ondol are agungi accessible from an adjoint passages which was furnace underlain by horizontal smoke. Agungi began to flame a heat from fire, the furnace cook a meal, the flame would extend horizontally along with the passages. The smoke containing heat was transferred through the passages, it warmed stones underfloor heating the floor of room. The warm stone was persistent heat, heating the floor was continued many period of time.
The ondol system heated floor, not directly heated atmosphere of the room. It allowed households feel refreshing perception compared to modern western houses, because heat is not directly to the air. The ondol system directly heated floor only, so Korean people stayed sedentary life. It is often said to be distinguished from standing life style at the west.
The ondol system also had many positive effect on persistence of the hanok. The old hanok had insecticide effect on wooden piller crack occurrences, the positive effect of the ondol system is that smoke keeping insects away from the house, so it was able to Hanok to be more endurant. The one more notable positive effect was that it provided the heating for a home and a cooking simultaneously, so household was able to reduce a work hard.
The disadvantage was that heating required massive fuel so household should collect it regularly. One more disadvantage was that distant location from agungi, temperature was not uniformity. The floor closer to the furnace was normally warm enough, so the warmest spot was reserved for elderly.
In modern day Korea, from 1950s and 1960s, official construction led by government and architects had experimented an installation of applicative ondol system to be modernized. The apartment houses used the modernized form of ondol, which had replaced the traditional gudeul passages to the pipe of transduction of warm waters, which was application of a heat transfer structure from traditional ondol system.
That is one of the Korean traditional science behind the architecture, it is research topic so worth to learn. It is heritage of Korean people.