Calculator Inputs
Velocity Plot
The graph compares candidate round diameters against estimated flue velocity. It helps you see when a selected size becomes too fast or too slow.
Example Data Table
| Case | Fuel | Input | Height | Horizontal | Elbows | Suggested nominal |
|---|---|---|---|---|---|---|
| Small utility boiler | Natural gas | 120 kW | 7 m | 2 m | 1 | 150 mm |
| Medium plant room | Natural gas | 300 kW | 9 m | 3 m | 2 | 200 mm |
| Large commercial unit | Propane | 550 kW | 12 m | 4 m | 2 | 250 mm |
| Oil-fired backup boiler | Heating oil | 400 kW | 10 m | 2.5 m | 3 | 250 mm |
Formula Used
1. Heat input conversion: Input BTU/h = kW × 3412.142
2. Fuel use: Fuel use per hour = heat input ÷ fuel heating value
3. Standard flue gas flow: Theoretical flue gas = fuel use × fuel flue-gas factor
4. Excess air correction: Actual standard flow = theoretical flow × (1 + excess air)
5. Hot gas correction: Hot flow = standard flow × (Tflue + 273.15) ÷ (20 + 273.15)
6. Base area: Area = hot volumetric flow ÷ target velocity
7. Layout adjustment: Adjusted area = base area × layout multiplier × safety factor
8. Round size: Diameter = √(4 × area ÷ π)
9. Natural draft estimate: ΔP = g × h × (ρambient − ρflue)
This page gives an engineering estimate for early design comparison. Final vent selection must match the appliance listing, local code, and manufacturer vent tables.
How to Use This Calculator
- Enter the boiler heat input and choose kW or BTU/h.
- Select the fuel and appliance category.
- Enter stack temperature, ambient temperature, and total vertical height.
- Add the lateral run, number of elbows, and elbow equivalent length.
- Set target velocity, excess air, and a safety factor.
- Enter the appliance flue collar size so the result cannot shrink below it.
- Submit the form and review diameter, velocity, draft, and warnings.
- Export the result as CSV or PDF for project notes.
FAQs
1. What does this calculator estimate?
It estimates flue gas flow, required vent area, a round flue diameter, a rectangular opening option, draft potential, and likely warnings for weak layouts.
2. Is this enough for final installation?
No. It is best for concept design, budgeting, and quick comparisons. Final venting should follow the appliance listing, local regulations, and manufacturer instructions.
3. Why does the calculator ask for appliance category?
Different boiler categories vent very differently. Natural-draft systems behave unlike positive-pressure or condensing systems, so the page adds a caution factor and special warnings.
4. Why is horizontal length important?
Long lateral runs and elbows increase resistance, cool the gases, and weaken draft. That can require a larger flue or a shorter, straighter routing.
5. Why can oversizing be a problem?
Very large flues can slow the gas stream, reduce draft strength, and encourage condensation. A vent should be big enough, but not unnecessarily oversized.
6. What is a good target flue velocity?
That depends on appliance type and vent system. Many designers use a moderate velocity target, then verify the final result against equipment data and vent tables.
7. Why does low stack temperature raise risk?
Cooler flue gas is more likely to condense moisture inside the vent. That can damage unlisted materials and change draft behavior, especially in long runs.
8. Can I use rectangular flues with this page?
Yes. The calculator provides an equivalent rectangular opening suggestion. Round vents usually draft more efficiently, but rectangular dimensions help during space planning.