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.
The pressure difference or chimney draft can be expressed as:
dpch = h (ρo - ρi) g (1)
dpch = pressure draft in the chimney (Pa)
h = height of chimney (m)
ρo = density of outside air (kg/m3)
ρi = density of inside air (kg/m3)
g = 9.81 - acceleration of gravity (m/s2)
(1) can alternatively be expressed as:
dpch = 0.0465 dt h (2)
dt = temperature difference between inside and outside air (K, oC)
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.