Hypocaust Underfloor Heating

Yesterday’s blog post about a built-in mass heater was popular. Caleb, one of our readers, reminded us of hypocausts – underfloor heating systems — that have been used for centuries. Even if you’ve read about this before, it pays to go back and reread this type of information from time to time. For instance, I didn’t know hypocausts were used in Alaska about 5,000 years ago. They were used in Korea and China 7,000 years ago. Now imagine combining a hypocaust with a masonry heater for cooking, space heating and water heating. With adequate fuel, you’d have a warm, cozy home even in the most brutally cold environment. Anyone planning a move to Alaska, Canada or Siberia?

From Wiki:
Roman operation
A hypocaust was an ancient Roman system of underfloor heating, used to heat houses with hot air. Hypocausts were used for heating hot baths (thermae), houses and other buildings, whether public or private. The floor was raised above the ground by pillars with a layer of tiles then a layer of concrete then another of tiles on top; and spaces were left inside the walls so that hot air and smoke from the furnace would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room. Ceramic box tiles were placed inside the walls to both remove the hot burned air, and also to heat the walls. Rooms requiring the most heat were placed closest to the furnace, whose heat could be increased by adding more wood to the fire. It was labour-intensive to run a hypocaust as it required constant attention to tend the fire, and expensive in fuel, so it was a feature of the villa and public baths.

Non-Roman examples
Excavations at Mohenjo-daro in what is now Pakistan have unearthed what is believed to be a hypocaust lined with bitumen-coated bricks. If so, the structure would pre-date the earliest Roman hypocaust by as much as 2000 years.

In Spain the Roman system was adopted for the heating of Hispano-Islamic (Al Andalus) baths (hammams). A derivation of hypocaust, the gloria, was in use in Castile until the arrival of modern heating. After the fuel (mainly wood) was reduced to ashes, the air intake was closed to keep hot air inside and to slow combustion.

Korean traditional houses sometimes use an ondol, which is similar to a hypocaust, drawing smoke from a wood fire typically used for cooking.

More information from Wiki

2 thoughts on “Hypocaust Underfloor Heating”

  1. In our region, here in British Columbia there are several examples of exterior furnaces ideally located at a lower elevation than the house. Similar to a root cellar, the large furnace is inside and there is a smoke stack comes out of the shelter, and there are large tubes or ducts travelling either underground or the soil has been backfilled to insulate the ductwork. The ducts go into the crawl space of the house, carrying the hot air.in, with or without fans, Sometimes they can be used to heat more than one building. The layout of the cellar and covered ductwork if not totalling underground sometimes looks similar to an igloo. This is not the new exeritor furnaces that are on the market, they are much older than that. I thought that they were called “Hostas” but I have searched the net to try to find specs to build our own and can’t find any. I was wondering if anyone has come across anything.


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