Ancient Greeks had their hearths and ancient Romans had their hypocausts, but ancient Koreans appear to be the first to have enjoyed underfloor heating systems. The neat invention was called ondol, which means “warm stone” in Korean (on means “warm,” and dol means “stone”); ondol systems functioned by means of underfloor stone slabs heated by the furnace. Why was that so special? Well, just keep scrolling to find out!

A Smokeless System

Before chimneys became a thing, Europeans during the Middle Ages had trouble keeping the smoke out of their homes while keeping the heat in. Funnel-shaped roofs (called ‘Rauschschlot’) in German homes were able to take care of the smoke, but preventing the heat from escaping along with the smoke was still a problem. The ondol system kept the smoke outside via flues (ducts), while keeping indoor spaces warm. Similar to ondols, Roman hypocausts had flues, but were primarily used for heating up baths.

The flue of an ondol system.

An Affordable System

Apparently the easy-to-install nature of ondols had a democratizing effect on their users. In the West, even the upper echelon had trouble staying cozy, as German Princess Elizabeth von der Pfalz, writes during the 1690s: ‘All that keeps me warm at night are the six small dogs I take [with me to bed].’ In contrast, ondols kept both the rich and the poor warm. Horace Allen, the first American Protestant missionary in Korea and the founder of Severance Hospital, recounts the use of ondols during the Joseon era:

However humble the hut of the peasant or coolie, it always has its tight little sleeping room, the stone and cement floor of which with its rich brown oil paper covering, is kept nicely warmed by the little fire necessary for cooking the rice twice daily. In this respect these people fare better than do their neighbors, for the Japanese houses are notoriously cold, and a fire pot for warming the fingers is the only native system of heating, while the Chinese never are warm in the raw cold of winter.

(Things Korean, p. 67)
Firewood for an ondol system.


As with all other inventions, the ondol system too had its downsides. Reverend James S. Gale, another missionary, hilariously attests in an account that the ondol floor “heated nearly to the frying point,” and that “the inexperienced traveler, pursued by fiery dreams, baked almost brown, gasps for breath and wishes for the morning” (Korean Sketches, p. 134). Ultimately, however, Gale concluded, “But after a year or two of practice, one gets to like the hot floor,” reminiscent of someone who would fit right in at the modern Korean sauna. Aside from the steep learning curve, ondol systems can also prove to be very dangerous when combined with coal briquettes. In fact, during the 1960s, the leakage from this combination often bred carbon monoxide poisoning, making many Koreans perish during their sleep.


Ondols had their glory, but we have something more glorious. The modern furnace is better than any ondol or hypocaust. There’s certainly no need to tape yourself to six dogs every night, and no need to check every night for any forgotten coal briquettes. Simply call Don’s Heating and Cooling and fix all your indoor temperature problems! Heating systems can be smokeless, affordable, and modern!