A rocket mass heater sends the stove’s hot gases 3 to 5 metres through ducting buried in an earthen bench, which absorbs the heat during a burn of 1 to 2 hours and releases it slowly over 12 to 24 hours. Because the exhaust travels such a long path, the duct and chimney must be sealed and lead to the outside, and a room with any smoke or leaking joint is a carbon monoxide hazard.
Part of Rocket Stove Design
A rocket stove used for space heating moves the exhaust through a thermal mass before it leaves, so the mass, not the flame, warms the room. This is a Phase 2 project worth understanding early.
The Mass Stores What The Fire Makes In An Hour Or Two
The overview describes the design: extend the exhaust path through a thermal mass, typically a long earthen bench filled with exhaust ducting. The hot gases travel 3 to 5 metres through the bench before exiting a short chimney. The mass absorbs heat during burning and releases it over 12 to 24 hours; a 1 to 2 hour burn of a small armload of sticks can keep a room warm overnight. See also Heat Storage Systems.
Worked Example: How Much Heat Can A Bench Hold?
Take a bench of 1 m³ of earthen mass. The inputs are illustrative assumptions: density about 1,700 kg/m³ and specific heat about 0.85 kJ/kg·K (typical for earth and cob), warmed by 20 K.
- Mass: 1 m³ × 1,700 kg/m³ = 1,700 kg.
- Heat stored: 1,700 kg × 0.85 kJ/kg·K × 20 K = 28,900 kJ, which is 8.0 kWh (28,900 ÷ 3,600).
- Released over 24 h: 28,900 kJ ÷ 86,400 s ≈ 0.33 kW; over 12 h: 28,900 ÷ 43,200 ≈ 0.67 kW.
So that bench gives roughly 330 to 670 W of gentle heat for the day. A bigger or hotter bench stores more, and a long cool-down is only possible if the gas leaves cooler than it entered, which also means a weaker draft.
Method: What Makes It Safe
- Seal every joint in the duct and chimney with clay, and seal cracks as the bench dries.
- Give the chimney a clear route outside, rising above the roof.
- Warm the chimney first with a small hot fire before the main burn, so the draft does not reverse.
- Never close the exhaust while embers glow. Embers still make carbon monoxide.
- Watch the first burns for smoke seeping from the bench or joints. Any smoke in the room means stop feeding and ventilate.
Common Mistakes
- Too long or too narrow a duct. The draft dies. Keep the duct as wide as the riser.
- Cracks after drying. They leak. Re-seal.
- Sleeping beside a burning or glowing heater without ventilation.
Safety
The longest exhaust path of any rocket stove
Carbon monoxide has no smell and no colour. Headache, nausea and drowsiness are what a cold, tired person already expects to feel, which is why it kills indoors. Feeling sleepy near a burning heater is a reason to get out and ventilate, not to lie down. Ventilate with an inlet and an outlet. See Carbon Monoxide Risks and Indoor Use & Ventilation.
What’s Next
- Learn next: Indoor Use & Ventilation — the rules for indoor stoves
- Combine with: Heat Storage Systems — the Phase 2 follow-up