Inputs (all fields editable)
Sea level ≈ 101325 Pa
41220
Important: The sizing logic uses ideal-gas and stoichiometric approximations with user-set excess air and velocity targets. Always verify against manufacturer instructions and local codes.
Results
Calculated internal diameter
—
Nearest standard size (rounded up)
—
Volumetric flow (m³/h)
—
Gas density (kg/m³ @ Tflue)
—
Velocity at chosen standard (m/s)
—
—
Example Data Table
| Input (kW) | Fuel | Excess air (%) | Tflue (°C) | Velocity (m/s) | D calc (mm) | D std (mm) |
|---|
Formula Used
-
Stoichiometry for hydrocarbon fuel
CxHywith excess air fractionE = EA/100:Stoichiometric oxygen molesνO2,st = x + y/4.
Delivered oxygen molesνO2,del = (1+E)νO2,st; accompanying nitrogenνN2 = 3.76(1+E)νO2,st.
Products per mole fuel:CO₂: x,H₂O: y/2,N₂: 3.76(1+E)νO2,st,O₂: EνO2,st. -
Fuel molar rate from rated input power
P(kW):\u1d62\u0307fuel = 1000P / LHVmol(mol/s). -
Product molar rate:
\u1d62\u0307prod = \u1d62\u0307fuel \u00D7 \u2211\nuproducts(mol/s). -
Volumetric flow (ideal gas):
\u1d52\u0307 = \u1d62\u0307prod R Tflue / p(m³/s). - Target velocity band depends on vent category: Cat I ≈ 5–10 m/s, Cat III ≈ 8–15 m/s, Cat IV ≈ 10–18 m/s.
-
Diameter from velocity:
D = \u221A\left(\frac{4\u1d52\u0307}{\u03C0 v_{eff}}\right). Herev_{eff}applies a derating for elbows and runs:v_{eff} = v \times \left(1 - \min(0.3,\, 0.02N_{elbow} + 0.002L_{horiz} + 0.01H)\right).
Note: This model is for educational screening. Field-validated sizing tables and manufacturer documentation govern final selection.
How to Use
- Enter boiler rated input. Keep units in kilowatts.
- Select fuel. For uncommon fuels, use the “Custom” option.
- Set excess air and flue gas temperature per burner setup.
- Choose vent category and velocity target within the suggested band.
- Enter approximate layout: height, horizontal run, and elbow count.
- Press Calculate, then export results to CSV or PDF.
FAQs
For natural-draft Category I, start around 6–9 m/s. Pressurized Category III/IV vents often use 10–15 m/s. Stay within manufacturer limits for noise and pressure drop.
Higher efficiency usually lowers flue gas temperature and flow, often reducing required diameter. Always check condensing venting materials and drainage requirements for Category IV equipment.
More excess air increases exhaust volume and oxygen content, raising velocity at a given diameter. The calculator models this directly from the stoichiometry.
Headered or common vent systems need combined flow analysis and backflow safeguards. Treat each appliance per code and manufacturer instructions; this tool handles single-appliance screening.
The tool rounds up to the next value among: 60, 80, 100, 110, 125, 130, 150, 160, 180, 200, 230, 250, 300 mm. Edit the list in the script if needed.
No. Use this as an engineering pre-check. Final venting must follow local codes, listing standards, and the appliance manufacturer’s certified instructions.
Reference Data: Fuel Stoichiometry & LHV
| Fuel | Formula | LHV (kJ/mol) | O₂ Stoich (mol/mol fuel) | CO₂ (mol) | H₂O (mol) | N₂ (mol) |
|---|---|---|---|---|---|---|
| Natural Gas | CH₄ | 802 | 2.00 | 1.00 | 2.00 | 7.52 |
| Propane | C₃H₈ | 2043 | 5.00 | 3.00 | 4.00 | 18.80 |
| No.2 Oil | ≈ C₁₂H₂₃ | 7350 | 17.75 | 12.00 | 11.50 | 66.74 |
Values are at stoichiometric conditions (0% excess air). N₂ calculated as 3.76 × O₂stoich from air composition.
Reference Data: Standard Flue Sizes & Areas
| Diameter (mm) | Area (cm²) | Area (m²) |
|---|
Areas assume clear internal diameter; real systems vary with liner thickness and joints.