Solar Chimney Ventilation Calculator

Understanding solar chimney ventilation

A solar chimney is a passive ventilation strategy that uses solar heat to warm a vertical shaft so that the air inside becomes warmer and less dense than the surrounding outdoor air. Because warm air is buoyant, it rises up the shaft and exits at a high outlet. That upward movement creates a small pressure difference between the base and the top of the chimney. When the building has a suitable low-level inlet (or a planned air path), the chimney can exhaust indoor air and draw in replacement air without fans.

This calculator estimates the resulting airflow using a widely used stack effect approximation. It is best suited to early-stage design and quick comparisons: “What happens if I make the shaft taller?”, “How much does area matter?”, or “How sensitive is the concept to temperature difference?” The output is an estimate, not a guarantee. Real buildings include additional losses and driving forces (wind pressure, internal partitions, leakage paths, and time-varying solar gain) that can change the actual flow.

If you are using this tool for a retrofit or a concept sketch, treat the result as a starting point for sizing and sanity checks. For final design, you may need measured loss coefficients, multi-zone airflow modeling, or computational fluid dynamics (CFD), especially when the building has multiple openings, long horizontal runs, or strong wind exposure.

How to use the calculator

1. Chimney height (m): enter the vertical distance from the inlet level to the outlet level. Height is the main source of stack pressure.
2. Chimney area (m²): enter the internal free-flow cross-sectional area of the shaft. Area scales flow approximately linearly.
3. Indoor temperature (°C): the air temperature at the base/inlet side (often near the occupied zone or the plenum feeding the shaft).
4. Outdoor temperature (°C): the air temperature at the outlet/top side (often near roof level, in shade if possible).
5. Discharge coefficient: a dimensionless factor (typical range 0.55–0.70) that lumps entrance/exit losses and friction into one value.
6. Press Calculate to compute airflow in m³/s and m³/h. Use Copy Result to copy the summary line.

The model requires a positive temperature difference (ΔT = T_in − T_out > 0). If outdoor air is warmer than indoor air, buoyancy may weaken, stall, or reverse. In practice, designers often add backdraft dampers or operable vents to prevent unwanted reverse flow.

Formula and assumptions

The calculator uses a simplified buoyancy-driven flow equation derived from Bernoulli’s principle and the ideal gas relationship. Temperatures are converted from °C to Kelvin internally. The mean absolute temperature term accounts for the fact that air density changes with absolute temperature, not with Celsius temperature.

• Q = volumetric airflow rate (m³/s)
• C_d = discharge coefficient (dimensionless)
• A = chimney cross-sectional area (m²)
• g = gravitational acceleration (9.81 m/s²)
• H = chimney height (m)
• ΔT = T_in − T_out (K; numerically the same difference as °C)
• T_m = mean absolute temperature = (T_in + T_out)/2 (K)

Assumptions: steady flow, a single dominant vertical shaft, modest temperature differences, and losses represented by a single coefficient. The equation does not explicitly model wind pressure, internal building resistance, or heat transfer along the shaft. In real installations, the effective discharge coefficient can change with Reynolds number, screens, louvers, bends, and the quality of the inlet/outlet details.

Worked example (step-by-step)

Consider a small building with a 3 m tall solar chimney. The shaft has an internal area of 0.5 m². Indoor air at the base is 30 °C and outdoor air at the outlet is 20 °C. Assume C_d = 0.65.

1. Convert to Kelvin: T_in = 30 + 273.15 = 303.15 K; T_out = 20 + 273.15 = 293.15 K.
2. Compute ΔT: 303.15 − 293.15 = 10 K.
3. Compute mean temperature: T_m = (303.15 + 293.15)/2 = 298.15 K.
4. Compute flow: Q = 0.65 × 0.5 × √(2 × 9.81 × 3 × (10/298.15)).
5. Convert to hourly flow: m³/h = Q × 3600.

The result is on the order of 0.8 m³/s (roughly 2,900 m³/h), depending on rounding. Interpretation: because the relationship contains a square root, doubling height or doubling ΔT increases flow by about √2 (around 41%), while doubling area doubles flow (all else equal).

Design notes (what to check in practice)

A solar chimney cannot move air unless the building can supply replacement air. Provide a low-level inlet (or multiple inlets) and a clear air path from inlet to occupied zones to the chimney base. If the inlet is too small, it becomes the bottleneck and the actual flow will be lower than the estimate. In many designs, the inlet free area is sized to be at least comparable to the chimney outlet free area after accounting for screens and louvers.

Solar gain is the “engine” of the system. Dark absorptive surfaces, glazing, and insulation can increase shaft air temperature, while shading and reflective finishes reduce it. Thermal mass can smooth performance over time, but it can also delay peak flow. Wind can either assist or oppose stack effect depending on direction and outlet geometry; a well-designed cowl or outlet can reduce adverse wind effects.

For comfort and indoor air quality, it helps to translate airflow into a building metric such as air changes per hour (ACH). If you know the ventilated volume V (m³), then ACH ≈ (m³/h) / V. For example, 900 m³/h through a 300 m³ space is about 3 ACH. This is a useful check when comparing against typical ventilation targets, but always confirm local codes and the needs of the occupancy.

Limitations and when to use a different method

This calculator is intentionally simple. It can overestimate performance when friction losses are high (long shafts, rough surfaces, tight bends), when inlet/outlet areas are restrictive, or when the temperature difference is not sustained by solar heating. It can also underestimate performance when wind assists the outlet or when the shaft is significantly hotter than the indoor air due to strong solar gain.

• Low-rise buildings: short height reduces stack pressure; meaningful flow may require large areas or multiple shafts.
• Hot/humid climates: small ΔT reduces buoyancy; wind-driven ventilation, night flushing, or hybrid fans may be needed.
• Transient conditions: clouds, shading, and thermal mass cause time-varying flow; night-time reversal is possible.
• Complex airflow paths: multiple openings, internal partitions, and wind pressures require multi-zone or CFD analysis.

If you need higher confidence, consider: (1) estimating inlet and outlet pressure losses separately, (2) using wind pressure coefficients for the facade and roof, and (3) validating assumptions with on-site measurements or a calibrated model.

Frequently asked questions

What temperature should I enter for “indoor” and “outdoor”?

Use the air temperature at the chimney inlet for indoor temperature and the air temperature near the chimney outlet for outdoor temperature. In many cases the outlet is near the roof where air can be warmer than at ground level. If the outlet is sun-exposed, consider using a shaded ambient value for outdoor temperature and treat additional heating as part of the effective ΔT you expect the chimney to create.

Does a bigger chimney always mean better ventilation?

Larger area generally increases flow, but only if the rest of the system can support it. The inlet, internal air path, and outlet details can become the limiting resistance. Also, very large shafts may be harder to heat uniformly, which can reduce the temperature rise that drives buoyancy.

Introduction: Why does the calculator require indoor air warmer than outdoor air?

The simplified stack equation used here assumes buoyancy drives upward flow because the air column in the chimney is warmer (less dense) than the outside air. If ΔT is zero or negative, the buoyancy term becomes zero or imaginary in the square root, and the model is not applicable. In real buildings, wind can still drive flow even with ΔT ≤ 0, but that is a different mechanism.

How do I choose a discharge coefficient?

Start with 0.65 for a reasonably smooth shaft with decent inlet/outlet detailing. Use a lower value (0.55–0.60) if you expect screens, louvers, rough surfaces, or sharp transitions. Use a higher value (up to about 0.70) for smooth, well-rounded entries and exits. If you have measured data or manufacturer loss information, that should take precedence.

Can I use this for night flushing?

Yes, as a rough estimate, if indoor air remains warmer than outdoor air at night (for example, after a hot day). However, night-time radiative cooling can also cool the chimney and change the direction of flow. If reverse flow is a concern, include dampers or operable vents and consider a time-based model.