The terms flue and chimney are often used interchangeably. The flue is the working part of the chimney,
conveying the pr
oducts of combustion safely to the atmosphere. The chimney includes the shaft within
which the flue is housed. A flue works under negative pressure drawing the product of combustion from
A successful “chimney draw” is dependent on the following principles:
- The difference in air pressure between the appliance and the top of the chimney. (created by the height
of the chimney)
- The difference in temperature between the appliance’s exhaust gas and the outside temperature.
- The quality of insulation along the length of the chimney.
- The route of chimney. (The straighter and more vertical the chimney the better).
It is essential to choose a flue diameter that matches the output from the appliance. An adequate air supply
is also required for the appliance to operate safely and efficiently
Stainless Steel Flexible Liners
Flexible liners are used to reline an existing chimney.
Care must be taken when selecting a flexible liner.
There are two types of flexible liners:
- Single skin liners for use with gas appliances and
- Twin skin liners, manufactured from overlapping strips
of high grade stainless steel to give a smooth sealed
flue-way. These liners should be used with wood and
Single skin liners must never be used with wood or
The ideal location for a chimney is on the inside of the building.
That way it can benefit from being kept warm. Chimneys situated
outside the building can be affected by cold weather causing
poor up draught and condensation, particularly if they are
un-insulated. It is therefore important that a cavity wall is
continued around a lined masonry chimney or a factory made
insulated chimney system is used for external applications.
Connection to the Chimney
For open fires a suitable throated front lintel and gather must be installed above
the fire opening, so that the front,
back and sides slope up smoothly into the
flue opening in the chimney at an angle no greater than 45° from the vertical.
Flat surfaces or shelves must be avoided as these can cause turbulence and
smoky fires. Most flue and chimney manufacturers provide standard gather
and fireplace components. Precast fire chambers or Firechests are also available
for standard and larger fire openings.
Bends in the Chimney
Both the Regulations and the Standards recommend that bends in the
chimney be avoided, as a straight vertical chimney performs better. If bends
are necessary there must be no more than four in the
length of the chimney.
The angle of the bend should be no greater than 45° from the vertical, with
the exception that 90° factory made
bends or tees may be treated as being
equal to two 45° bends.
Where System Chimneys are used, always use
the standard offset components which are available from the chimney
manufacturer. For stainless steel chimneys the distance between bends must be no greater than 20% of the total
chimney length. It is recommended that a vertical rise of 600mm should
be allowed immediately above the appliance before any change of
direction. An inspection hatch is required between each offset.
Chimney Heights and Termination
The minimum chimney height recommended for minimum performance of wood burning and multi fuel
appliances is 4.5 m from the top of the appliance to the top of the chimney. It is best to position the chimney,
so that it goes straight up as near to the roof ridge as possible.
A chimney operates on the principle of having a natural up draught. One factor in creating the up draught is
maintaining a warm flue gas temperature, of between 150°C and 450°C. Burning wood or multi fuel slowly with
insufficient air supply, particularly on stoves or closed appliances must be avoided. Low flue gas temperatures
will cause condensation and greatly increases the
risk of producing excessive tar and corrosive soot deposits.
This is a common problem, particularly
when burning wet wood or coal and should be avoided. If soot and
condensate deposits are allowed to accumulate in a flue, the deposits can ignite causing a chimney fire. These
deposits can also be very corrosive and if they are not regularly removed can cause corrosion of the metal
parts of both the chimney and the appliance.When burning wood it is important to ensure that it is dry and well-seasoned.
The chimney should be swept regularly to remove soot and tar. At the very least the chimney should be swept
at the start of the heating season. It is not recommended that the appliance is over fired, (allowed to burn fiercely
and out of control), or chimney fires be started in an attempt to clean the chimney. Deposits of soot and tar will
be greatly increased if unseasoned wood is burnt. Should a chimney fire occur, the chimney and appliance should
be checked for damage before using them again.
It is also good practise to check at least every year or two the exposed parts of a chimney, flashings and terminals for
signs of damage. Just like the outside of a house chimneys can suffer from the wear and tear of extreme weathering.
If at any time smoke or fumes are apparent or suspected from the appliance, chimney or flue, seek advice
immediately from the installer or chimney expert in case there is a blockage or failure. Do not use the appliance
or chimney until they have been thoroughly checked for safety and soundness. The escape of fumes can