Thread by @LandsknechtPike

The hypocaust heating system! Ancient Romans used this.

For a long time people thought that in the middle ages people didn't know this technology.

But the medieval people actually developed a much superior hypocaust.

The medieval monks were actually aware of this technology. But when they moved into colder climates to the north, they realized the Roman hypocausts were actually inefficient and that they have to come up with better ones. What was the problem with the Roman ones?

The Roman hypocaust stayed warm only during heating and was not practical for a cold Baltic climate. There the monks learned to pile up stones over the furnace, a technique which they might have learned from the local pagans. This is a heat storage hypocaust that was developed:

When the firing was complete, the vents in the hot plate were opened and hot air rose from the pile of stones into the room to be heated. The monks built some really elaborate hypocausts, most famously in the Teutonic Order fortress of Marienburg. A sketch of hypocaust there:

And here are the air vents on the floor of the Marienburg castle. If they opened this, the hot air from hypocaust furnace would heat up the room. The furnace did not have to be constantly lit as it could store heat and warm the place for many days with one firing!

It is now estimated that up to 1000 of such heat storage hypocausts were in use in the Baltics by the end of middle ages! Some examples. A heat storage hypocaust in Tallinn's town hall (left). And the remains of the hypocaust furnace in Arensburg castle (right).

In 1438 Spanish traveler Pedro Tafur described this curious hypocaust heating system, amazed how "people placed seats above the holes, also with holes in them. The people then sit down on those seats and unstop the holes and the heat rises between the legs to each one."

In 1822 they conducted an experiment in Marienburg castle to test the hypocaust there. A cold furnace was lit for three and a half hours. When the vents in the hot plate were opened, hot air raised the temperature of the hall from from 6 to 22.5°C in just 20 minutes!

The next day, the room's air temperature had fallen to 14°C. The air vents were opened and the temperature rose to 19°C in one hour without any additional fire being lit! The day after that the room temperature still rose from 10 to 16°C in half an hour.

Even six days later the temperature of the hall was lifted from 8 to 10°C. The amazing effectiveness of the ancient hypocaust of the Teutonic Order fortress was proven!

But no one gives them credit for this sophisticated and advanced technology.

Some people asking me about the smoke issue. The smoke ("dym" on this sketch) could escape through the chimney or a cavity in the wall. When the firing was complete and the furnace had been cleaned, the smoke flue was closed by means of a damper. Only then they opened the vents.