Inputs
Qstack ≈ FuelRate × FlueYieldSTP × (1+EA) × (Tstack/Tamb),
with temperatures in Kelvin and EA as fraction.
Draft, pressure-drop, dew-point, wind & insulation
Liner insulation & wall temperature
Results
Results table
| # | Mode | Q (m³/s) | Target v (m/s) | D req (mm) | D selected (mm) | v at D (m/s) | ΔP_buoy (Pa) | ΔP_wind (Pa) | ΔP_total (Pa) | ΔP_loss (Pa) | Margin (Pa) | Dew bulk OK | T_wall_in (°C) | Dew wall OK | Timestamp |
|---|
Example data
These illustrative scenarios populate automatically using the same sizing formula. Use them to validate behavior or compare configurations.
| # | Scenario | Inputs | Q (m³/s) | Target v (m/s) | D req (mm) | D selected (mm) |
|---|
Standard diameter quick-reference (flow bands)
Computed for velocity bands 4–7 m/s at each inner diameter.
| Diameter (mm) | Area (m²) | Q @ 4 m/s (m³/s) | Q @ 4 m/s (m³/h) | Q @ 7 m/s (m³/s) | Q @ 7 m/s (m³/h) |
|---|
Common liner materials, roughness ε, and service notes
| Material / Liner | Typical ε (mm) | Approx. max temp (°C) | Notes |
|---|---|---|---|
| Stainless steel 304/316 | 0.045–0.10 | 550–650 | Good corrosion resistance; common for condensing and gas appliances when insulated. |
| Carbon steel (black) | 0.10–0.15 | 500–600 | Cost-effective; protect from acidic condensate; consider coatings or liners. |
| Masonry with smooth liner | 0.30–1.00 | >800 | Higher roughness; check draft and sweeping access; often needs insulation upgrades. |
| Ceramic fiber/insulated systems | ≈0.05 | 700–1000 | Low heat loss; keep wall temperatures high; verify system certification limits. |
Values are indicative for preliminary design. Confirm with supplier data, standards, and local codes.
Formula used
The calculator sizes a circular chimney using the target gas velocity method. The core relationship is:
D = √(4 Q / (π v)) where D is internal diameter (m), Q is volumetric flow at stack conditions (m³/s), and v is the design gas velocity (m/s).
If the volumetric flow is known directly (e.g., from fan curves, combustion calculations, or measurements), use the Known flow mode.
When estimating flow from fuel burn, the tool applies an ideal-gas expansion to a user-provided dry flue volume at STP and the selected excess air percentage:
Qstack ≈ FuelRate × FlueYieldSTP × (1 + EA) × (Tstack/Tamb), converting to m³/s thereafter. Temperatures are absolute (Kelvin), and EA is a fraction (e.g., 0.15).
Draft and pressure loss
Buoyancy draft: ΔPbuoy ≈ g·H·(ρamb − ρgas), with ideal‑gas densities ρ = P/(R T).
Wind-induced assist at the top: ΔPwind = −½ ρamb Cp U², where negative Cp gives suction and positive draft contribution.
Pressure loss: ΔPloss = (f·L/D + ΣK)·(ρ·v²/2), with f from Swamee–Jain or fixed.
Wall temperature with insulation
1‑D steady heat transfer per unit area: q = (Tgas − Ta)/(1/hi + Rins + 1/ho). Inner wall temperature: Twall,in = Tgas − q/ hi, where Rins=t/k or a direct R‑value.
Dew-point checks
Bulk‑gas check compares stack gas temperature to the dew point plus margin. Wall check uses Twall,in with the same margin.
Important: Real systems may require detailed plume, radiation, transient, and code‑specific checks. This tool focuses on preliminary sizing with simplified methods.
How to use this calculator
- Select Known flow if you have volumetric flow at stack conditions.
- Otherwise, open Fuel-based estimator and enter burn rate, STP flue yield, excess air, and temperatures.
- Enter a target gas velocity (typ. 4–7 m/s).
- Adjust rounding and the list of standard diameters if needed.
- In checks, set height, length, friction, wind, insulation, and dew-point options.
- Click Compute diameter to update outputs, then Add result to table.
- Use Download CSV or Download PDF to export your table.