# 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:

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}) aρ_{o}T_{o }/T_{i}_{g}

or

dp_{ch}=(3)h(ρ_{o}1 - T_{o }/T_{i}) a_{g}

#### 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 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.