A rocket stove’s riser pulls air through the fire because the hot gas inside it is lighter than the cool air outside, and the taller the riser, the stronger the pull. The overview gives a minimum of 25 to 30 cm for reliable operation and says gains diminish beyond about 60 cm. The pull is only a few pascals, about the same as a light breeze, so wind can overpower it.
Part of Rocket Stove Design
Hot gas in a vertical riser is lighter than the cool air outside, so it rises and draws fresh air in behind it. The pull is small and rises with riser height.
Hot Gas Rises And Draws Air In Behind It
Hot gases rise through the insulated burn tunnel and draw fresh air in through the fuel magazine. That self-sustaining airflow is what makes the stove work. The overview says a minimum riser height of 25 to 30 cm is needed for reliable operation, and taller is better up to about 60 cm, after which the gains diminish.
The Draft Is Small In Pascals
The pressure difference is Δp = (ρ_out − ρ_gas) × g × h, where ρ is density, g = 9.81 m/s² and h the riser height. Air density follows ρ = ρ₀ × 293 ÷ T with T in kelvin, where ρ₀ = 1.204 kg/m³ at 20 °C (standard values).
Worked example. Take the overview’s chamber temperature of 600 °C = 873 K as the gas temperature. This overstates the average, because the gas cools as it rises, so the result is an upper estimate.
- Gas density: 1.204 × 293 ÷ 873 ≈ 0.404 kg/m³.
- Difference: 1.204 − 0.404 = 0.80 kg/m³.
- Riser 0.30 m: 0.80 × 9.81 × 0.30 ≈ 2.4 Pa.
- Riser 0.60 m: 0.80 × 9.81 × 0.60 ≈ 4.7 Pa.
Doubling the height doubles the ideal draft, but the real gain falls off because the gas cools as it climbs and friction grows, which fits the overview’s “gains diminish”.
For comparison, wind of 2 m/s has a dynamic pressure of ½ × 1.204 × 2² ≈ 2.4 Pa, the same as the 30 cm riser’s pull. That is why a stove can reverse or smoke in a gust, and why exhaust leaks and open doors indoors matter.
Common Mistakes
- A riser below 25 cm. The draft is too weak for reliable operation.
- Cold riser at start. Cold gas has no buoyancy; see Starting & Feeding.
- Ignoring wind. Shelter the stove or add a short chimney pipe.
Safety
A weak or reversed draft puts smoke into the room
When the draft fails, smoke and carbon monoxide come out of the feed opening instead of the top. Carbon monoxide has no smell. Do not use a stove indoors without an exhaust pipe to the outside, and treat smoke coming back at you as a reason to leave and ventilate. See Carbon Monoxide Risks.
What’s Next
- Learn next: Insulated Burn Chamber — what keeps the gas hot
- Combine with: Chimney Draft — the same principle on a house chimney