Buyer's Guide

Chimney Principles

fireplace & chimney specialists | hyper fires

The terms flue and chimney are often used interchangeably.

The flue is the working part of the chimney, conveying the products 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 the appliance.

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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 route of chimney - the straighter and more vertical the chimney the better
  • The quality of insulation along the length of the chimney.

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:

  • 1
    Single skin liners for use with gas appliances. Single skin liners must never be used with wood or multi-fuel applications.
  • 2
    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 multi-fuel appliances
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Chimney Position

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.

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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 & 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. Positive pressure is created from the side which the wind blows from, once it hits your houses roof. Normally smoke will be emitted when the wind blows from the side where the chimney is situated, if the chimney is not high enough. This problem is worse when the angle of the roof exceeds 30 degrees, this creates a higher pressure situation and requires the flue to be taller. This may vary from house to house depending on the surroundings and close by obstructions which the wind may blow over or around.


Fuel Burning

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.

Regular Inspection

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 be dangerous.