1. What is ONDOL
Ondol is a traditional Korean house heating method. Ondol(溫突) means ‘Warmed Stone’.
It is make a fire at the fuel hole and heated Gudeuljang(room floor), then heating is done when hot smoke contained heat which is generated at the fuel hole passes through under room floor.
Ondol had traditionally been used with a living space for sitting, eating, sleeping and pastimes, it took the system of low temperature radiant heat and it is known as the unique method, nothing’s compared to the scientific and superior heating method of Ondol in the world.
Du.Han.Jok.Yeol 두한족열<頭寒足熱> - under-floor heating in the winter season, Ondol, is made warmer the feet than the head part, in Oriental medicine, it is helpful to improve the metabolism, so Ondol method, which is heated on the floor, is more comfortable than convection type heating
2. Ideal Heating Method
How to make comfortable heating along with energy saving is that make the feet warmer and the head cooler. 
This heating method can protect from dryness of the respiratory organs and muscle because this heating method can drop the room temperature and raise the relative humidity, so it is known as an ideal heating for health.