A rocket mass heater bench routes a rocket stove’s exhaust through a long duct in a cob or brick bench, typically 2-4 m long, 60 cm wide and 45 cm tall, which stores the fire’s heat and stays warm for 12 hours or more. A 3 m bench of that size holds about 11 kWh (40 MJ) for a 30 K rise, and because people sit and sleep on it, the bench must be sealed against carbon monoxide and its top kept below the temperature that burns slowly through skin.
Part of Retained-Heat Structures
A rocket mass heater bench is a seat or bed of cob or brick with the exhaust duct of a rocket stove running through it.
How Much Heat The Bench Holds
The parent page’s bench is typically 2-4 m long, 60 cm wide and 45 cm tall. Inputs, assumed: length 3 m; clay-earth volumetric heat capacity 1.66 MJ/m³·K (parent page: 0.92 kJ/kg·K × 1,800 kg/m³); the duct volume is ignored; average rise 30 K above the room.
- Volume = 3 × 0.6 × 0.45 = 0.81 m³.
- Heat = 0.81 × 1.66 × 30 = 40 MJ = 40 ÷ 3.6 = 11.2 kWh, about 11 kWh.
- Spread over 12 hours: 11.2 ÷ 12 = 0.93 kW.
That fits the parent page’s 12+ hours at a modest output. The bench also weighs: 0.81 × 1,800 = 1,458 kg; its force is 1,458 × 9.81 = 14,300 N, spread over a footprint of 3 × 0.6 = 1.8 m², or 14,300 ÷ 1.8 = 7.9 kPa. Check the floor carries it (Foundation Types).
The Bench Is Where You Sleep
Skin burns slowly at modest temperatures: in the Moritz and Henriques tests (recalled), a contact of around 44-47°C sustained for hours can injure skin that has no time to cool, which is why heating pads and hot-water bottles cause burns on sleepers. The parent page says you sleep on stored heat. Cushion the bench top with a thick pad or mattress, keep the surface near body temperature where people lie, and never let a child, an older person or someone who cannot feel heat sleep directly on a hot bench.
What You Need
- A rocket stove combustion unit (Rocket Stove Design, Rocket Mass Heater)
- A duct of fire-resistant material buried in the bench: brick, tile or metal
- Cob or brick for the bench (Cob & Earthen Mass Stoves)
- A short chimney at the end of the bench, through the roof or wall
- Sealing clay for every joint
- A cleanout port at the end
Method: Build The Bench
Step 1: Build the combustion unit (parent page: rocket stove) and run the exhaust into the duct.
Step 2: Lay the duct through the bench base, with joints sealed in clay.
Step 3: Cover it in cob or brick to the bench height; leave the cleanout port.
Step 4: Exhaust leaves through a short chimney at the end (parent page).
Step 5: Cure slowly; test with a smoke test (a smouldering rag at the stove) and look for smoke at every joint before the first fire.
Common Mistakes
- Leaks in the duct. They fill the room with smoke.
- Bench too short for the duct. The exhaust leaves hot and the heat is wasted.
- No cushion on top.
Safety
Carbon monoxide and slow burns from a heated sleeping surface
A long duct in an earthen bench can crack as it dries; exhaust with carbon monoxide, which is colourless, odourless and fatal, then enters the room. Seal every joint, smoke-test before use, vent the exhaust to the outside, and never sleep in a room where you cannot be sure it is tight. Sleeping for hours on a surface well above body temperature causes burns that are not felt in time, especially in children, the elderly and anyone with numb skin. See Rocket Mass Heater for why the exhaust must be sealed and led out.
What’s Next
- Learn next: Hypocaust) — spreading the same heat across a floor
- Combine with: The Chimney Effect — draft through a long duct