Rocket stoves and rocket mass heaters rely on natural draft to pull combustion air into the burn chamber and exhaust hot gases safely. The draft arises from the density difference between hot flue gases and the cooler outside air. Understanding how chimney height and temperature affect this pressure differential is essential for designing efficient, smoke-free systems. This calculator helps builders estimate the draft pressure available in a chimney and the resulting theoretical airflow rate through the flue.
The governing principle is the stack effect. When the gas inside a chimney is hotter than the ambient air, its density is lower, creating a buoyant force that drives flow upward. The pressure difference \(\Delta P\) between the base and the exit of the chimney can be approximated by:
where \(g\) is gravitational acceleration, \(h\) is chimney height, \(\rho_o\) is outside air density, \(T_o\) and \(T_i\) are absolute temperatures of the outside air and flue gas respectively. This pressure difference, typically measured in Pascals, is small—often only a few units—but sufficient to sustain combustion when the system is well tuned.
Once the pressure difference is known, the volumetric airflow rate \(Q\) through a round chimney of diameter \(d\) can be approximated by the Bernoulli equation: \(Q = C A \sqrt{2 \Delta P / \rho_i}\), where \(A\) is cross-sectional area, \(\rho_i\) is flue gas density, and \(C\) is a discharge coefficient accounting for friction and turbulence. For simple estimates we may take \(C\approx0.65\). Although real systems are more complex due to bends, surface roughness, and heat exchange with the chimney walls, this provides a baseline for design.
Consider a small rocket heater with a 2.5-meter vertical chimney. If the flue gas temperature averages 200°C (473 K) and the outside air is 20°C (293 K), using standard outside air density of 1.2 kg/m³, the draft pressure becomes \(9.81 \times 2.5 \times 1.2 (1/293 - 1/473) ≈ 4.4\) Pa. With a chimney diameter of 10 cm, the cross-sectional area is \(0.00785\) m². Assuming flue gas density near 0.8 kg/m³ at 200°C, the airflow rate estimates to \(0.65 \times 0.00785 \sqrt{2\times4.4/0.8} ≈ 0.029\) m³/s, equal to about 105 m³/h.
The following table shows draft pressures for various chimney heights at a fixed temperature difference of 180°C between flue and ambient air:
| Height (m) | Draft Pressure (Pa) |
|---|---|
| 1.5 | 2.6 |
| 2.5 | 4.4 |
| 3.5 | 6.1 |
| 5.0 | 8.7 |
These values illustrate how even modest increases in height significantly improve draft. Builders often extend chimneys to overcome cold starts or sluggish combustion, especially in tall structures or where the flue must pass through roofs.
Draft is also affected by ambient conditions. On hot summer days when the outside air temperature approaches that of the flue gas, density differences shrink, reducing draft. Conversely, cold winter air enhances draft. Wind can create positive or negative pressure zones at the chimney termination, either assisting or opposing the stack effect. Installing a proper cap and ensuring adequate height above roof ridges mitigates wind interference.
Beyond height and temperature, surface roughness and bends introduce frictional losses not accounted for in the simple stack formula. For rocket stoves, smooth internal surfaces and gentle curves maintain laminar flow, while sharp bends or constrictions increase resistance. Some designers incorporate cleanout ports and removable sections for maintenance, recognizing that soot buildup narrows the flue and diminishes draft over time.
Understanding draft dynamics aids in tuning rocket mass heaters. Users can adjust feed rates, add secondary air inlets, or modify chimney dimensions to achieve complete combustion with minimal smoke. Monitoring draft with a simple manometer, such as a U-tube filled with water, provides feedback. A reading of 5 Pa corresponds to about 0.5 mm of water column. If draft falls below 2 Pa, smoke may spill from the feed tube, signaling the need for system adjustments.
While this calculator offers a simplified model, it provides a valuable starting point. Designers can experiment with different chimney heights or diameters to see how draft scales, ensuring the stove operates within safe and efficient parameters. Incorporating thermal mass around the chimney, a hallmark of rocket mass heaters, can influence draft as stored heat continues to drive flow after the fire dies down. For off-grid builders seeking to maximize fuel efficiency and comfort, these insights inform better decision-making.
The study of stack effect has applications beyond rocket stoves, including passive ventilation in buildings, smoke control in skyscrapers, and industrial furnace design. By exploring the relationships between temperature, density, and height, the calculator also serves educational purposes for students of thermodynamics and fluid dynamics. Whether you are constructing a backyard experimental stove or designing a full-scale heating system, understanding chimney draft is a critical component of success.