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# Chimney Sizing

## Calculate chimney draft and required chimney area - online calculator.

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.

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:

dp ch = h (ρ o - ρ i ) a g (1)

where

dp ch = pressure draft in the chimney (Pa)

h = height of chimney (m)

ρ o = ambient outside density of air (kg/m3 )

ρ i = inside chimney density of air (kg/m3 )

a g = acceleration of gravity (9.81 m/s2)

Since the Ideal Gas Law states that

ρ o T o = ρ i T i

or

ρ i = ρ o T o / T i (2)

where

T = t + 273 = absolute temperature (K)

where

t = temperature (°C)

- (1) can alternatively be expressed as:

dp ch = h ( ρ o < - ρ o T o / T i ) a g

or

dp ch = h ρ o ( 1 - T o / T i ) a g (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

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

= 107 Pa

### 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)

where

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.

## Related Topics

• ### Combustion

Combustion processes and their efficiency. Boiler house and chimney topics. Properties of fuels like oil, gas, coal and wood and more. Safety valves and tanks.

## Related Documents

• ### Chimney's Explosion Doors

Recommended size of explosion doors or draught stabilizers in oil fired installations.
• ### Chimney's Size vs. Boiler Power

The size of chimney vs. boiler's heat load.
• ### Chimneys - Roof Termination

Chimneys and single wall vents roof termination.
• ### Chimneys and Fireplace Sizing

Chimneys and fireplaces for stoves burning wood or coal as fuel.
• ### Natural Draft - Air Flow Volume and Velocity

Air flow - volume and velocity - due to stack or flue effect caused by indoor hot and outdoor cold temperature difference.

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## Citation

This page can be cited as

• The Engineering ToolBox (2003). Chimney Sizing. [online] Available at: https://www.engineeringtoolbox.com/chimney-sizing-d_175.html [Accessed Day Month Year].

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6.3.17

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