A retained-heat structure burns one short, hot fire and passes its smoke through channels in a large mass of brick, stone or cob, which then releases the heat into the room for half a day or more. One to three tonnes of masonry warmed by 100 K holds 84-252 MJ, which is 23-70 kWh, and spreads its output over 12-24 hours. Three forms do it: the masonry heater, the rocket-heater bench and the heated floor.
Part of Heat Storage Systems
Retained-heat structures are buildings or building parts whose flue passages run through a heavy mass that stores a fire’s heat and gives it off slowly.
Fire Hot And Fast, Heat Slow
A fire is intense for an hour; a person needs warmth for twelve. The structure turns one into the other: a fast, hot, clean fire (fuel burns best at high temperature) puts heat into the mass through channels, and the mass releases it over a long period. This is the arithmetic of a store (Thermal Mass Fundamentals).
Inputs: masonry specific heat 0.84 kJ/kg·K (brick, parent page); masses 1,000 kg and 3,000 kg (parent page: 1-3 tonnes); temperature rise of the mass above the room, assumed 100 K on average.
- 1,000 kg: 1,000 × 0.84 × 100 = 84,000 kJ = 84 MJ = 84 ÷ 3.6 = 23 kWh.
- 3,000 kg: 252 MJ = 70 kWh.
- Released over 12-24 hours, the average output ranges from 23 kWh ÷ 24 h = 1.0 kW to 70 kWh ÷ 12 h = 5.8 kW.
A larger swing or mass raises the capacity; a smaller swing lowers it.
Three Forms
- Masonry heater (Kachelofen): a hot fire with long internal flues in 1-3 tonnes of brick or stone (Masonry Heaters (Kachelofen)).
- Rocket mass heater bench: a rocket stove whose exhaust runs through a long cob or brick bench (Rocket Mass Heater Benches).
- Heated floor: flue gases or hot air pass under a floor (ondol, hypocaust) ( Hypocaust)).
All three need good draft and a sealed, sound exhaust, because every one passes combustion gas through a long path (The Chimney Effect).
What You Need
- Brick, stone or cob in quantity (Brick Making, Cob Building)
- Mortar or clay (Lime & Cement)
- A chimney sized to the flue (Chimney Draft)
- Dry firewood, split small, for the hot fast fire
- A carbon monoxide plan: sealed joints, a tight chimney, and ventilation
Method: Choose One
Step 1: Decide the room’s need: kilowatt-hours per day.
Step 2: Match mass: about 23 kWh per tonne at a 100 K swing (derived above); halve it for a 50 K swing.
Step 3: Pick the form by room and skill: a bench is simplest, a masonry heater most efficient, a floor the most even.
Step 4: Build slowly; dry masonry fully before the first fire.
Common Mistakes
- A slow, smouldering fire. It makes smoke and creosote, not stored heat.
- Closing the damper too soon. Carbon monoxide stays in the room.
- Leaky joints.
Safety
Carbon monoxide, chimney fire and hot surfaces
Every retained-heat structure routes combustion gas through long channels; a crack leaks carbon monoxide, which is colourless, odourless and fatal. Seal joints, check draft, and never close a damper while embers still glow. Keep wood and cloth clear of hot surfaces. Surfaces above about 60°C burn in seconds, and children and the elderly burn at lower temperatures and shorter exposures than adults (Chimney & Stove Safety).
What’s Next
- Learn next: Masonry Heaters (Kachelofen) — the most developed form
- Combine with: Rocket Mass Heater — the simplest form