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Nancy E Sutton


Brilliant! Thank you for this exciting history : )

Jan Steinman


Glad you mentioned ondol, the Korean equivalent. When I lived in Korea in the early 1980s, ondol was still widely used, especially in the countryside.

The fuel is cylinders of charcoal with holes through its length. It was put into horizontal flues that run under the floor, then to an elbow in the wall, and up to a chimney.

However, it was considered a public safety menace. Unless the underfloor flues were properly maintained, carbon monoxide would seep into the room.

Once or twice a year, you'd hear of an entire family who died, occasionally killing the odd American GI, too. The threat was to the point that the local base commander forbid living off-site.

I don't know if ondol is still used. When it worked right, it was lovely! Warm floors are so nice!

But when it failed, it failed catastrophically, at least for the unfortunate residents.

I wonder if in your research you uncovered any tales or evidence of the CO poisoning potential of these ancient hypocausts?

kris de decker


@ Jan Steinman

I don't know the answer for sure but it seems to me that CO poisoning was not a major problem with the heat storage hypocaust. It was fired intermittently, and once the fire was extinguished, only hot air could come through the vents for the following hours and days. The Korean ondol is fired more regularly, which increases the chance that people sleep while the fire is burning.

M. Ehrlich


@ kris de decker

You are correct, the CO poisoning would not happen as the heat plate perforations/ air vents were not opened until the fire had burnt.

Like with wood burning stoves/furnaces still present in older houses in Estonia and Latvia, you cannot close the damper when there are still flames, the opportune moment for maximum effectiveness (as not to lose too much valuable heat) to close the damper is when the flames are gone, leaving just live coals/ embers that do not emit CO.

Necessity is mother of invention. Like Tvauri explains, these heat storage hypocausts rather common in 14th-15th century Livonia -- where the climate was much colder compared to the Central and Western Europe -- were born combining the ancient method of heat storage (stones) used by the indigenous inhabitants of present day Finland, Estonia, and Latvia in their saunas for hundreds of years with the hypocaust technology originating from Rome used by monks and religious orders.



'Warm' is a vague and ambiguous term..... doubtful the upper floors of a structure of any material were comfortable in today's context. Second floor above the warming mechanism would be marginal, third floor would be for the cure of hot flashes.

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