Twin Wall Flue Systems

So you have no existing chimney stack and you like the look of a free standing stove, perhaps one of the more contemporary designs. What do you need to do for a chimney and flue?

One option is to build a new traditional chimney stack and line it but the costs are high. The quicker and less expensive solution is to install a twin wall stainless steel flue system. These systems are insulated to maintain flue temperature and also to reduce the heat on the external surfaces. The systems are relatively quick to install though the materials are expensive.

Twin Wall Flue Systems

Twin wall flue off top of Contura stove.

External installation

Typically, this will service a wood burning stove in the corner of a room in a new extension. The flue can rise off the top of the appliance or the rear and then bend through the outside wall before returning to the vertical and rising up to the roof and further. Depending on the roof structure the flue pipe can bend round any overhang or pass through it.  Alternatively, if the overhang is not too prominent, the flue can simply be fixed on extended wall brackets and pass by.

Where the flue pipe penetrates the wall a stainless steel wall sleeve is used and once installed this is sealed to the flue pipe and set with in the wall. If sweeping of the flue is not possible through the stove and there is no access in the stove flue pipe, sweep access can be included in the component on the outside wall. This keeps the soot from the vertical section of flue pipe outside the building but the section of flue from the stove to the vertical section will still have to accessed from inside. In our opinion it always best if the whole height of the flue can be swept from the inside of the stove ensuring that there can be no section of flue pipe left unattended to.

At the top of the flue a terminal is fitted to protect the flue from bird nesting and rain. There is also the option of an anti splash / anti down draught terminal that might need to be considered.

Internal installation

Twin Wall Flue Systems Diagram
This requires greater thought and planning than an external installation because the there are additional regulations that take into account the potential risk of fire. All twin wall flue systems are rated to indicate the distance they can be installed from combustible materials. Generally this is 50mm but can be 60mm or greater. This may require the trimming of joists and floor boards to ensure that this safety requirement is met. Should the flue travel into a first floor room the section that passes through the void between ceiling and floor above will need to be ventilated and a suitable firestop / support/ ventilation component used. This ensures that the required clearances are achieved and also that there is no potential for heat to build up in the void.

Regulations,(again!) insist that the twin wall flue projects a minimum of 450mm below the ceiling. The transition from single skin flue pipe to twin wall flue can be aesthetically jarring and if the chosen stove suits it is worth considering bringing the twin wall down to the top of the stove and avoiding any single skin flue pipe.

It is essential that the flue pipe cannot come into contact with anything that has the potential to catch fire and often this is neglected in a loft which so often starts as an empty space but ends up cluttered. To reduce the risk of something coming into contact with the flue pipe a flue enclosure can be installed to ensure that this cannot happen by some casual action. Alternatively the flue pipe can be boxed in using traditional materials.

With an internal installation the flue pipe may exit through the roof tiles and this area needs to be made weatherproof. At Billing Chimneys we have a specialist roofer and lead worker on the team and a purpose-made lead slate will be made to mimic the pitch of the roof, to sit within the existing tiles and to rise up as a tube to fit inside the lip of the storm collar. This will prevent any rain water arriving in the interior of the building. The disturbed area of roof is made good.

With both types of installation it is important that the flue is supported throughout its height. With the first example the flue pipe is supported largely on the outside wall with a wall support and this is strong enough for several metres of flue pipe. At higher levels the flue pipe is steadied with wall bands. With an internal installation there are a number of supports available but in most cases the flue pipe is supported with the ceiling supports mentioned and also a roof support which adjusts to the pitch of the roof.

Twin wall flue systems in thatched properties.

Our preferred method of lining in thatched properties is to use the Schiedel Isokern pumice product which is well insulated and robust. In thatched properties insulation is the key to a safe flue so that thin materials like flexible steel and resin based linings are completely unsuitable. However, the current regulations demand that there is a 200mm distance from the inside of the flue to the thatch. That is, unless the flue lining has a different designation that allows a closer proximity to combustibles. With twin wall flue requiring just 50 or 60mm clearance from the outside wall of the flue pipe and with the insulation thickness generally of 25mm, on occasions using twin wall flue pipe where the flue passes through the thatch can allow the regulations to be met.

In time, all steel products will rot, though, depending on use and maintenance, this can be many years. When it comes to the time to replace this flue there will always be disruption to the fabric of the building. On occasions, where the chimney stack was also showing signs of dilapidation we have been able, on rebuilding the chimney, to increase its dimensions sufficiently to meet the requirements and allow the pumice product to be installed.

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