Chimney Sizing

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

h - height of chimney (m)

ρ _o - density outside air (kg/m ³ )

t _i - chimney inside temperature ( ^o C)

t _o - outside air temperature ( ^o C )

Chimney Draft Chart

• Chimney Draft Chart (pdf)

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 (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

• Chimney Vent Capacity Chart - Btu/h and kW (pdf)

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm