I first read the play when I was a kid and never understood what the hell that meant. Now I do.

Russian stoves are massive masonry structures designed to suck every last bit of heat from burning fuel that it possible can, store that energy in its masonry, and release it into the room over time. The heat is sent through a flue system that doesn’t just go straight up, but travels, and that flue system is covered with brick or stone. Koreans have a similar traditional heating system called an ondol. The Romans used something similar in the caldarium (though the floors would be too hot to walk on barefoot) to heat their bathhouses. The heat and smoke from a fire is sent through a series of chambers under a masonry floor to heat the house.