Determining Chimney Heights


Clean Air Acts


The Clean Air Acts of 1956 and 1968 were introduced to control air pollution in England, Wales and Scotland and cover all combustion plant (except motor vehicles) in effect virtually all boilers, air heaters, driers and general processes using fuel must comply with the Clean Air Acts.


Anyone considering new or modified plant should ensure that it will meet the relevant requirements of the Acts.

In Northern Ireland chimney height control is covered by Section 10 of The Clean Air Act (Northern Ireland) 1964.


Sulphur Dioxide (So2) Levels


The Clean Air Acts 1965 & 1968 give procedure for calculating chimney heights based on ground level SO2.


The method is based on research by Sutton:


          P max            =     153.7  x  ( CZ / CY )  x    ( S / m  . H )



P max        =          maximum additional SO2 permissible (p.p.m.)

S               =          weight of sulphur emitted (kg/h)

m               =          wind speed (m/s)

CZ/CY     =          ratio of horizontal to vertical diffusion coefficients of  SO2 in the air.

H              =          effective chimney height (m).


Chimney Heights


The ‘Memorandum on Chimney Heights published by HMSO provides a simple guide to chimney heights for most plant.

This takes into account the maximum rate of emission of sulphur dioxide, the height of adjacent buildings and the general background pollution of the district. 

Where the sulphur content of a fuel is negligible, the emissions of other pollutants in the waste gases may be significant.


A local council (borough or urban) will not approve the height of a chimney unless satisfied that it will prevent the smoke, grit, dust, gases or fumes emitted from the chimney from becoming prejudicial to health or a nuisance. 

In built up areas this can simply mean that the outlet of the chimney should be at least 1 metre above the roof (ridge) of the building or roofs of adjacent buildings.


For small to medium sized oil-fired plant consult other methods of chimney height sizing.

Also the ‘Memorandum on Chimney Heights method is not used for no / low  sulphur fuels such as natural gas.


Large Plant


The method of calculating chimney heights in the ‘Memorandum on Chimney Heights is based on:


1.       The maximum rate of emission of SO2 calculated from the sulphur content of the fuel and the maximum rate at which the fuel will be burnt.


2.       A waste gas efflux velocity at full load of not less than 6m/s for small installations with forced draught fans,  7.5m/s with induced draught fans to a maximum of 15 m/s for installations rated at 128 MW or equivalent.


3.       The character of the surrounding area, e.g. rural, residential or industrial.


4.       The height of the adjacent buildings.


Small / Medium Plant


All chimneys produce a suction at their base due to the difference in density of the hot flue gases rising in the chimney.


Suction Effect          =    Weight of a column of air at ambient temperature   -    weight of column of hot flue gases.




Dp    =       H  x   g    (  r air    -    r  flue gas )



Dp          =          suction pressure or draught (N/m2) obtained from boiler catalogue.

H            =          height of column (m) or chimney height

g             =          acceleration due to gravity (9.81 m/s2)

r             =          density (kg/m2)


Chimney or Flue height can be determined from the following;


H   =   Dp    /  [  g    ( r air    -    r  flue gas ) ]


Example 1


Calculation the minimum flue height for an oil fired boiler and determine a suitable termination height for the flue given the following information;


Building height at eaves level = 6 metres.

Height of roof ridge = 8 metres.

Height of highest openable window = 3.2 metres.

Outside air density = 1.2 kg/m3.

Flue gas density = 1.0 kg/m3.

Minimum flue gas suction pressure at boiler = 6 Pa (N/m2).

Pressure loss in straight flue and flue fittings  =  4 Pa.




Flue Gas Suction Pressure required  =   minimum boiler suction pressure + Flue pressure losses.

Flue gas Pressure required at boiler   =  6   +   4     =    10 Pa


Minimum flue height:


H   =    Dp   /  [  g    ( r air    -    r  flue gas ) ]


H   =  10   /  [  9.81   (  1.2 – 1.0) ]


H   =  10   /   1.962          =       5.097 metres.


A suitable termination height would be 6 metres since this is; more than the calculated minimum height, above openable windows, lower than the ridge height and at eaves level.


Note - See Building Regulations (Combustion Appliances and Fuel Storage) for details of flue terminal positions.



Plume Rise


Plume rise is the height above a chimney outlet of the flue gases, as shown below.



















Plume rise provides an advantage in chimney systems in that the flue gas moves higher as it exits thus reducing the possibility of polluting the area beneath.


Plume rise depends on:


1.      Wind speed – higher wind speeds reduces plume rise and can in extreme cases lead to downdraught.

2.      Flue gas velocity – plume rise increases with increasing efflux velocity.

      Typical values: 6 m/s natural draught

      7.5 to 10.0 m/s forced & induced draught plant.

3.   Temperature – the higher the flue gas discharge temperature, the more the buoyancy effect and the higher the plume rise.

4.   Volume of flue gas – the greater the volumetric flow rate, the greater the plume rise. This is due to air infiltration becoming slower which reduces the rate of cooling of the flue gas.