# Chimney Sizing

## Calculating chimney draft and chimney area

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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}) g(1)

where

dp_{ch}= pressure draft in the chimney (Pa)

h= height of chimney (m)

ρ_{o}= density of outside air (kg/m^{3})

ρ_{i}= density of inside air (kg/m^{3})

g = 9.81 - acceleration of gravity (m/s^{2})

(1) can alternatively be expressed as:

dp_{ch}=0.0465 dt h(2)

where

dt= temperature difference between inside and outside air (K,^{o}C)

#### 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= 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

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