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Chimney Sizing Calculator

Work out chimney sizing instantly with clear inputs, formula shown and shareable results.

About Chimney Sizing Calculator

Natural draft comes from the density difference between hot flue gas and outside air: 0.52 x height x pressure x (1/Tambient - 1/Tflue) with absolute temperatures. Flue area is set by volume flow, taken as roughly 11.5 cubic feet of products per cubic foot of natural gas and expanded for temperature, sized at about 900 fpm.

Formulas

Theoretical draft

D = 0.52 x H x P x (1/Ta - 1/Tg)

H
Height, ft
P
Barometric pressure, psia
Ta
Ambient, °R
Tg
Flue gas, °R

Flue area

Area in² = flue gas CFM / 900 x 144

Worked example

With the default values below, Chimney Sizing Calculator returns Theoretical draft of 0.1167. Change any input above and every figure here is recalculated in your browser.

Inputs used

Chimney heightminimum 1 ft
20 ft
Flue gas temperatureminimum 100 °F
430 °F
Ambient temperatureminimum -40 °F
70 °F
Appliance inputminimum 1 BTU/h
150,000 BTU/h

Results

Theoretical draft
0.1167 in.wg
Flue gas volume
47.8 ft³/min
Flue area required
7.64 in²
Equivalent round flue
3.12 in

Step by step

  1. 1Values used
  2. 2Theoretical draft
  3. 3Flue area
  4. 4Theoretical draft= 0.1167 in.wg
  5. 5Flue gas volume= 47.8 ft³/min
  6. 6Flue area required= 7.64 in²
  7. 7Equivalent round flue= 3.12 in

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Venting must be sized to the appliance manufacturer's tables and the applicable venting code, which account for connector rise, offsets and combined venting.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Venting must be sized to the appliance manufacturer's tables and the applicable venting code, which account for connector rise, offsets and combined venting.

Frequently Asked Questions

How is the Chimney Sizing figure worked out?

D = 0.52 x H x P x (1/Ta - 1/Tg). Natural draft comes from the density difference between hot flue gas and outside air: 0.52 x height x pressure x (1/Tambient - 1/Tflue) with absolute temperatures.

Why does a cold chimney back-draft?

Draft depends on the flue being hotter than outside air. Until the masonry warms up the density difference is small, so a cold external chimney may spill products at start-up.

What values do I need to enter?

This calculator takes 4 inputs: Chimney height, Flue gas temperature, Ambient temperature, Appliance input. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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Chimney Sizing Calculator – Calculate Theoretical draft