Avon Forest Chimneys Ltd. Unit 3,Hurn Bridge Business Park,
    Hurn, Christchurch, Dorset BH23 6AD T:01202 064639

Twin Wall Stainless Steel Rigid Flue Liner - Dispatched Same Day

We have over 10 years direct experience of supplying & installing chimney liner products to our UK customers.

If you need technical help, please call us on 01202 064639.

Free delivery on orders over 100 (Nett). Same Day Dispatch if Order Placed by 2pm.

We stock the industry leading Dinak twin wall, insulated, stainless steel modular chimney systems, which are fully Hetas approved.

Double wall stainless steel with 30mm rockwool insulation
Suitable for use with Gas, Diesel, Wood, Biofuel and Pellets
Designed for condensing (with joint) and dry (without joint) applications
10 year product guarantee
Available in internal diameters of 130mm (5 inch) & 150mm (6 inch) External diameters 190mm & 210mm respectively.


What is Twin Wall Flue?

Twin wall insulated flue is a chimney system, designed for use with wood burning fires, multi fuel stoves, and high temperature oil systems. Constructed from two skins of stainless steel, with a layer of rockwool fire resistant insulation in between. Generally this layer is 25 -30mm thick, depending on the manufacturer (e.g Selkirk is 25mm, Dinak is 30mm). The insulation has two roles ; to keep the outside cool and therefore avoid any potentially high heat sparking combustion with the building, and keep the inside warm to allow efficient evacuation of exhaust gases.

Where is it used?

In domestic situations, twin wall flue is used in situations where no usable chimney exists, or if it is too short. In some situation, chimneys may have been truncated, either for aesthetic reasons or due to poor condition, and central heating installed instead. Subsequent decisions to install wood burning or multi fuel stoves, means that there is insufficient chimney height, so double skin flue needs to be used.

Other more modern properties may not have a chimney at all, therefore twin wall flue is required to construct a new, free standing chimney.

There are many other commercial and industrial situations where double skin flue is used, including heating systems, biomass boilers and hot air generators.

Chimney System Design

A double wall chimney should be installed as straight as possible, to allow proper ventilation of exhaust smoke. Bends should only be used where absolutely necessary. Building regulations currently advocate a minimum height of 4.5m.

The following are key twin wall flue design & installation points (please refer to stove manufacturers instructions and Document J of building regulations)

  • A vertical section of 600mm minimum should be installed above the appliance before any change of direction.
  • A maximum of two offsets in any one chimney
  • No bends of angle greater than 45° from the vertical
  • When passing through a floor or ceiling, an air gap clearance must be maintained (as defined by manufacturer)
  • No flue joints can be made within the thickness of floor or ceiling .
  • An adjustable length should be used immediately above the stove or appliance to facilitate easy removal of the appliance

Flue Size

The diameter of twin wall chimney flue used is dictated by the stove manufacturer, and should never be less than the outlet diameter.

Where open fireplaces are used, openings larger than 500mm x 550mm require a flue size with a cross section area equal to 15% of the total face area of the opening. Appliances burning smokeless fuel require a minimum 125mm diameter flue. Multi-fuel and wood burning stoves of up to 30kw rated output generally require a minimum flue size of 150mm internal diameter.

Chimney Flue Regulations

Twin wall flue chimney installations are governed by Document J of the Building Regulations in England and Wales and in Scotland by part F of the Building Regulations.

Sweeping, Cleaning & Maintenance Access

The design of the system should allow for access to clean and inspect the chimney, especially on solid fuel installations. Chimney flues should be swept at least once a year, more if heavily used.

double skinstainless steel tee section      twin wall flue elbow section