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Heated Floors (Ondol / Hypocaust)

4 min read

A heated floor sends a fire’s gases or hot air through a space beneath the floor, as the Korean ondol does under a stone slab and the Roman hypocaust does under a raised floor on pillars, so that the floor becomes a gentle radiant heater. A 20 m² floor of 15 cm of stone and earth holds 50-63 MJ for each 10 K it cools and gives that up over roughly 9-12 hours, but every crack in the flue floor is a carbon monoxide leak into the room where people sleep.

Part of Retained-Heat Structures

A heated floor is a floor with flue passages or an air space beneath it, so the whole floor surface radiates the heat of a fire.

Two Ancient Forms

  • Korean ondol: a fire at one end of the building; flue gases travel under a stone floor and leave through a chimney at the other end; the whole floor becomes a radiant heating surface (parent page).
  • Roman hypocaust: a raised floor on pillars, with hot air circulating underneath and up through wall cavities (parent page).

Both give even, comfortable heat and both need careful construction to avoid smoke leaks. The parent page says the floor slab stores heat for 8-16 hours.

How Long Does The Floor Hold Heat?

Release time = stored heat ÷ output. Inputs, assumed: floor area 20 m²; slab plus earth fill 15 cm thick (3 m³); volumetric capacity 1.66 MJ/m³·K for earth and 2.10 MJ/m³·K for stone (parent page); the floor cools by 10 K during the period; output 1.5 kW.

  1. Volume = 20 × 0.15 = 3 m³.
  2. Heat released for 10 K of cooling: earth 3 × 1.66 × 10 = 49.8 MJ, about 50 MJ; stone 3 × 2.10 × 10 = 63 MJ.
  3. At 1.5 kW: earth 49,800,000 J ÷ 1,500 W = 33,200 s = 9.2 hours; stone 42,000 s = 11.7 hours.

That puts the release in the 9-12 hour range, inside the parent page’s 8-16. A thin slab (say 5 cm) would give a third of that time. The output of 1.5 kW is also an assumption; a floor 7 K above a 20°C room at roughly 11 W/m² per kelvin (a typical combined radiation and convection figure, recalled) gives 11 × 7 × 20 = 1.5 kW.

What You Need

  • A fire and a firebox at one end, with a flue path of channels under the floor and a chimney at the other
  • Stone slabs or heavy brick for the floor, sealed at every joint with clay
  • A bed of earth or sand beneath them for mass
  • Pillars or channel walls for a hypocaust
  • A tight chimney
  • Dry fuel

Method: Lay The Floor

Step 1: Excavate and lay the flue channels so gas flows without sharp bends.

Step 2: Cover the channels with slabs, bedding every joint in clay and seal over with a layer of clay or lime plaster.

Step 3: Cure slowly, and test by lighting a small smoky fire and looking for smoke through any crack or joint.

Step 4: Fire on a regular rhythm, as with other mass heaters.

Common Mistakes

  • Thin slabs. They are hot and cold within hours.
  • Cracked or open joints. Gas leaks into the room.
  • A weak draft. The fire smoulders and the room fills with smoke.

Safety

Carbon monoxide through the floor, fire in timber, and long skin contact

Flue gas under a floor passes through every crack: carbon monoxide is colourless, odourless and fatal, and it has poisoned sleepers in ondol houses. Seal joints, smoke-test, keep the fire and chimney sound, and never close a damper on glowing coals. Keep timber joists and bedding away from hot flues. Skin in long contact with a warm floor can burn at temperatures only a few degrees above body heat, so cushion where children, the elderly and anyone who cannot feel heat will lie.

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