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Chimney Sizing

Calculate chimney draft and required chimney area - online calculator.

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The capacity of a chimney depends on the potential chimney draft - the pressure difference created by the inside and outside air temperature difference and chimney height - and the chimney area.

Chimney Draft

The pressure difference in the chimney pulls air and fuel gas from the boiler or fireplace through the chimney and out of the building.

Chimney Draft

The pressure difference or chimney draft can be expressed as:

dpch = h (ρo - ρi) ag         (1)


dpch = pressure draft in the chimney (Pa)

h = height of chimney (m)

ρo = ambient outside density of air (kg/m3)

ρi = inside chimney density of air (kg/m3)

ag = acceleration of gravity (9.81 m/s2)

Since the Ideal Gas Law states that  

ρo To = ρi Ti              


ρi = ρo To / Ti                  (2)


T = t + 273 = absolute temperature (K)


t = temperature (°C)

- (1) can alternatively be expressed as:

dpchh (ρo - ρo To / Ti) ag            


dpchh ρo ( 1 - To / Ti) ag               (3)

Example - Chimney Draft

A 20 m high chimney operates with flue gas temperature 200 °C (473 K). The outside temperature is 0 °C (273 K). The density of the outside air is 1.292 kg/m3.

The draft in the chimney can be calculated with (3) as

dpch = (20 m) (1.292 kg/m3) ( 1 - (273 K)(473 K)) (9.81 m/s2)

       = 107 Pa

Chimney Draft Calculator

h - height of chimney (m)

ρodensity outside air (kg/m3)

ti - chimney inside temperature (oC)

to - outside air temperature (oC)

Chimney Draft Chart

Chimney Draft Chart

Chimney Area

The velocity of air and flue gas in a smaller furnace should not exceed 2 m/s. Design velocities of larger furnaces should not exceed 10 m/s.

Required chimney area can be calculated as

A = Q / v         (3)


A = cross-sectional area of chimney (m2)

Q = volume of flue gases at chimney temperature (m3/s)

v = velocity (m/s)

In general 1 kW boiler heat requires 1100 mm2 of chimney area.

Chimney Vent Capacity Chart - Btu/h and kW

Chimney Venet Capacity Chart

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