# Chimney Sizing

## Calculate chimney draft and required chimney area

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:

dpch = h (ρo - ρi) ag         (1)

where

dpch = pressure draft in the chimney (Pa)

h = height of chimney (m)

ρo = ambient outside density of air (kg/m3)

ρi = chimney inside density of air (kg/m3)

ag = 9.81 - acceleration of gravity (m/s2)

(1) can alternatively be expressed as:

dpch = 0.0465 dt h         (2)

where

dt = temperature difference between inside and outside air (K, oC)

### 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 - Boiler house topics - fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency

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