I think it’s called an ondol, but I could be spelling that wrong (I always seem to hear a vowel wrong these days). It’s a system of heating a room from below, and it uses hot water to do so. There’s usually one under the floor of apartments, and when you sleep on the floor on a yo (a floor-pad) like I do, the warmth radiates up out of the floor and permeates everything… the room, your body. My back feels so good the morning after I sleep on the warm floor, even when it’s on the lowest heat possible.
Sometimes in restaurants the ondol is on too high and the result is a stuffy, oppressively hot room, a place I want to get out of right away. I’ve trained myself to relax, sit on the floor, and just take off my jacket and sweater and adjust to the heat. But it sometimes is just too hot, and the restaurant owner notices this poor white guy sweating and turns the heat down. The problem is, the heat still radiates a long time after you turn it down or off, so usually it’s only cooler by the time I finish eating.
But even so, I love the ondol and I think it’s the best way to heat a space if you’re sleeping on the floor.