Chimney Sizing

Calculate chimney draft and required chimney area based on height, temperature difference, air density, and flue gas volume.

Chimney Sizing 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.

Chimney Draft Formula

dp_ch = h × ρo × (1 - To / Ti) × ag

Where:

  • dp_ch = pressure draft in the chimney (Pa)
  • h = height of chimney (m)
  • ρo = ambient outside density of air (kg/m³)
  • Ti = absolute inside chimney temperature (K)
  • To = absolute outside air temperature (K)
  • ag = acceleration of gravity (9.81 m/s²)

Chimney Area Formula

A = Q / v

Where:

  • A = cross-sectional area of chimney (m²)
  • Q = volume of flue gases at chimney temperature (m³/s)
  • v = velocity (m/s)
Calculator

Inputs

Results

Inside Temperature (Absolute)-- K
Outside Temperature (Absolute)-- K
Chimney Draft Pressure-- Pa
Required Chimney Area-- m²
Required Chimney Area-- mm²
ParameterValueUnitDescription
Chimney Height20mHeight of chimney structure
Outside Air Density1.292kg/m³Ambient outside air density
Inside Temperature200°CFlue gas temperature inside chimney
Outside Temperature0°CAmbient outside air temperature
Gravity Acceleration9.81m/s²Standard acceleration due to gravity
Design Velocity (min)2m/sMinimum velocity for smaller furnaces
Design Velocity (max)10m/sMaximum velocity for larger furnaces
Area per kW1100mm²/kWStandard chimney area requirement per kW of boiler heat