Flue Size Calculator
Calculate the minimum required flue cross-sectional area and diameter for chimneys and venting systems based on appliance heat input, flue height, and fuel type.
Results will appear here.
Formulas Used
1. Heat released to flue:
Qflue = Qinput × (1 − η)
2. Reference flue gas volume flow:
Qref = Qflue × kfuel (m³/s at 0 °C)
3. Actual volume flow (ideal gas correction):
Qactual = Qref × (Tflue / 273.15)
4. Height correction factor:
fh = clamp(√(5 / H), 0.8, 1.2)
5. Required cross-sectional area:
A = Qactual / (vmin × fh)
6. Circular diameter: d = √(4A / π)
Square side: s = √A
Rectangular (1:1.5): a = √(A / 1.5), b = 1.5a
Assumptions & References
- Flue gas volume flow coefficients (kfuel) are derived from BS EN 13384-1 and CIBSE Guide B combustion data: 0.00035 m³/s·kW (gas), 0.00040 (oil), 0.00055 (solid fuel).
- Minimum flue gas temperatures are conservative design values to prevent condensation: 120 °C (gas), 160 °C (oil), 200 °C (solid fuel).
- Minimum draught velocities: 2.0 m/s (gas), 2.5 m/s (oil), 3.0 m/s (solid fuel) per CIBSE and HETAS guidance.
- Ideal gas law is used to correct volume flow from 0 °C reference to operating temperature.
- Height correction factor accounts for increased natural draught in taller flues (simplified; full calculation per BS EN 13384-1 is more complex).
- Standard circular liner sizes follow UK/EU market availability (100–400 mm).
- This calculator provides indicative sizing only. Final design must comply with BS EN 13384-1, Building Regulations Approved Document J (UK), or applicable local codes, and be verified by a qualified engineer or HETAS/Gas Safe registered installer.