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/m ^{ 3 } )
ρ _{ i } = inside chimney density of air (kg/m ^{ 3 } )
a _{ g } = acceleration of gravity (9.81 m/s ^{ 2 } )
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/m ^{ 3 } .
The draft in the chimney can be calculated with (3) as
dp _{ ch } = (20 m) (1.292 kg/m ^{ 3 } ) ( 1  (273 K) / (473 K) ) (9.81 m/s ^{ 2 } )
<
= 107 Pa
Chimney Draft Calculator
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)
where
A = crosssectional area of chimney (m ^{ 2 } )
Q = volume of flue gases at chimney temperature (m ^{ 3 } /s)
v = velocity (m/s)
In general 1 kW boiler heat requires 1100 mm ^{ 2 } of chimney area.
Chimney Vent Capacity Chart  Btu/h and kW
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