Glorrrr-i-ya! G-L-O-R-I-A! That's not just a great classic rock song, it's also a medieval heating device used by Spaniards to this day.
Castillans developed this subterranean heating system back during the early middle ages as a descendant of the Roman hypocaust, one of the planet's first central air and heating concepts. In Spain, they call this "gloria." The underlying principle is that it provides a slow rate of combustion which allows people to use smaller fuels like hay, rather than large chunks of wood.
This technology is still very much appreciated in Spain's smaller, remote villages, saving families a lot of money each year on resources.
The gloria doesn't cover very a wide area for heating, but for a small rustic home its scope is just wide enough. It can typically reach one or two rooms, and needs to have hay and twigs added to its box regularly—located outside the home in an open, courtyard area.
Heat is channeled through one or more ducts that run underneath the floor of the heated room, and the chimney is located on the other side of said room. During the summer months, natural air flows into the gloria to cool the space.
One unfortunate aspect to using a gloria is having to go outside in the cold to get it operating. This is a definite drag during a chilly winter morning, especially if it's raining. But if you have the resources and you enjoy saving money, it's not a terrible sacrifice to make.
The gloria's closest relative would be the American crimean oven or the Korean ondol. Similarly, these are underfloor heating systems reliant on heat transfer from a "stove" area across one or two rooms. The ondol has been found at archaeological sites in present-day North Korea, dating back to circa 1000 B.C. They were also used as heat for cooking, by having a flue entry located beside the furnace. Because Koreans have traditionally sat and slept on the floor, the underfloor heating system has been extra effective.
Famous American architect Frank Lloyd Wright was so enamored by the ondol that he invented radiant floor heating which uses hot water to heat up the floor, a system applied to all of Wright's buildings.
Just because it's old doesn't mean it's broke, is a mantra to live by. Some inventions are so brilliant they don't have an expiration date. The gloria and ondol fall into this category.
Plus, they have one inherent benefit: