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