They used to use these (Ondol) extensively in Korea, up until the late '80s or early 90s. Dozens of people died each winter from carbon monoxide poisoning, too. They used the charcoal to heat the floor; hot air circulated through channels in the floor, and if the floor was properly sealed, you were safe. If it developed cracks, you died. I suffered from CO poisoning twice during the time I was stationed here in the Army.
Many homes and buildings were made of compressed ondol back in the day. It was not very durable, but it was certainly inexpensive.
For years there was a very large standing reward for anyone who could devise a safe method to heat homes that was as economical as ondol. I don't know if anyone ever collected on it or not. Now most homes and apartments here in Korea have hot water heating that runs under the floors.
I tried to find a pic of ondol online, but couldn't; lots of pix of buildings heated by ondol - none of the charcoal itself.