Twin wall flue systems are used when you don’t have a chimney. The flue is made up of components that fit together. Just like Lego – only more expensive, and slightly more complicated to fit!
What is a twin wall flue?
Twin wall flue is exactly as its name implies. It’s twin-walled. There are two walls or casings: an inside layer and an outside layer. Between the layers, it is insulated – typically with Rockwool. Both layers are made from stainless steel. Because of the insulation, it is less hot to touch, although it can still get hot in places. Typically the higher up the flue, the less heat there is.
When do you need a twin wall flue?
When you need to create a chimney where one doesn’t already exist! Because it isn’t as hot as other solid fuel flues – such as vitreous or flexible flue liners – the twin wall flue is used when passing between floors, roofs, and walls. Typically it can pass as close as 50mm/2″ to combustible material such as plastic, timber, roof felt, and plasterboard. Therefore in new builds that are timber-framed internally and finished with plasterboard, twin-wall flues will be used unless a heat shield is used.
The flue can be designed to run either through the inside or on the outside of your property – aka twin-wall internal or external flue systems.
What does a twin wall flue look like?
It’s quite a substantial industrial-looking piece of kit. It is cylindrical, rigid, and large insomuch that both your hands can’t fit around it – It is between 200 and 220mm in diameter which is about 8″.
There are many components required to make up an installation that will include adapters, flue lengths & bends, locking bands, supports, finishing plates, cowls and flashings. And so depending on where the stove will be sited and what run the flue will take, the amount and type of twin wall used will alter accordingly to each specific job. Needless to say, we will use the correct parts in order to connect all the components and form a new flue that is safe, sealed, and well supported.
Chrome is the standard finish/colour as it is stainless steel. However, it can also be powder-coated (typically black) to change its appearance.
Chrome or black – what colour would you recommend?
Keeping the flue chrome is good when light is limited in a room or if you like a chrome finish. If the flue will not be visible such as in a loft space or boxed in a bedroom, the chrome should be used since it is cheaper. We always recommend black flues externally since they weather better in our salty Cornish air – no matter the quality of the stainless steel, our Cornish air will always cause rust spots to appear, and so for a small amount extra, we think it is always worth upgrading to black externally.
Where do you get your flues from?
There are many manufacturers for this type of flue. Some are quite cheap and not practical to use and there are several brands that are well made. We’ve tried many types over the years we have been operating and have stuck with the brand we currently use for the last 4 years as we simply think it is the best; it looks fantastic, has excellent performance statistics, and is very practical to use on installations. It is DTW – or Dura Twin Wall supplied by Flue and Ducting Ltd.
What are the types of twin wall flue installation?
Twin Wall Internal – The flue runs inside your property and out of your roof;
Twin Wall External – The flue passes through the external wall and fixes to the wall outside your property
Flexi to twin wall – if you have an existing chimney, but it stops in the loft, we will combine a flexible flue liner and twin wall internal installation
Twin wall internal
This flue runs up through your house and exits through your roof.
The twin wall flue must pass through the ceilings and roof. If the flue passes through bedrooms it should be “boxed in” with an inspection hatch situated in a specific place. In the loft, it will likely need some type of mesh protection. Specialised fire-stop plates, finishing plates and supports will also be used along the way. As the flue penetrates the roof, a flashing kit will be used to ensure it is watertight.
The twin wall flue has a layer of rockwool in between the two layers of flue and outer casing. This makes it about 210mm in diameter. The twin wall can be powder coated black for extra cost. We recommend this externally as it weathers better in our salty Cornish air.
Passing through the ceiling
As with all twin wall flue systems, the terminal point of the flue will be dictated by building regulations, your roof and any obstacles close to your property.
We will take measurements so that we can make a hole in the ceiling where the twin wall flue will pass through. We will start tentatively investigating as we go along to ensure there are no nasty surprises awaiting us – such as electrical wires and wooden joists. If we encounter any obstacles we will inform you immediately so that a plan can be made and implemented. For example if the flue has to use this path and electrical wires are obstructing it and cannot be moved, we will call an electrician to rectify the situation. This will be charged to the customer, as this is an unforeseen circumstance. This situation rarely happens.
Eventually a hole in the ceiling will be made that is wide enough to allow the flue to pass through with a 50mm air gap surrounding it.
Passing through floors and lofts
The flue will connect to the top of the stove and run vertically. As it passes through the ceiling, the internal twin wall flue system needs to fit between the joists. We will not cut through joists as it is not within the scope of our practice (a joiner should be employed separately for any structural trimming purpose). However, any combustible material such as non-structural timber, or plasterboard will be cut away from the flue allowing for a gap of 50mm surrounding it. A ceiling plate will be fitted over the hole we have created. This plate will vary depending on the ceiling and the location. In 2-storey houses, the first plate/ceiling will be ventilated and also act as a support for the flue. Ceilings bridging the loft tend to be solid/non-ventilated plates since most lofts are ventilated (or cold roofs). Any single storey building tends to have a solid cover plate on the ceiling.
Any flue passing through a first floor must be boxed-in (i.e. the flue surrounded by a box such as timber or plasterboard with an access hatch), and flue passing through an accessible/usable loft space must have mesh protection surrounding it.
We are happy to leave this to our customers to fulfil and can offer guidance, or a price for fitting if preferred.
Depending on what ceiling and what type of property you have, a plate will be used encircling the flue and covering up the hole we have created. This plate will either be a ventilated fire-stop or a finishing plate. There are plates available for every roof pitch.
If the flue passes through a first floor, it must be boxed in. You could choose to do this if you wanted under our guidance. We typically do this with plaster-board and then skim it to make good. An inspection hatch will also be fitted that will be about midway on the boxing on one side.
Supporting the flue
Where possible, the flue will be supported above every ceiling and also at rafter height. This is above the standard expected but it adds stability to the system and helps to prevent any issues with water ingress and vibrational noise.
Passing through the roof
As the flue passes through the roof, a flashing kit will be used which will include a suitable flashing (typically lead-type based with EPDM) and a storm collar. This is the best way we have found to reduce water ingress from any weak points.
If extra supports are required at roof level, these will be described on your estimate.
A standard storm cowl will be fitted to allow waste products to be easily removed, but to prevent excess water ingress (n.b. as standard, water will pass from the top of the flue and throughout the internal flue into your stove)
As the flue goes through the roof, a flashing kit will be installed around the flue and this will be made watertight depending on your roof structure. If you have tiles or slate, these will be tiled around it. We tend to use a lead flashing coupled with a storm collar in this instance as it looks and performs well. If you have a flat roof, then the flashing and products used to waterproof this will vary accordingly.
Your flue system will be designed following building regulations.
On rare occasions, we may have to return to modify the flue which will be chargeable.
Twin wall external
This flue passes through the external wall and then runs up the side of your house.
The angle of the twin wall flue as it passes through the external wall will be 45 degrees. There will be a wall sleeve encompassing the flue as it passes through the wall cavity. A cover plate will be used internally to ensure a neat finish.
Specialist supports and wall brackets will be used at intervals ensuring the flue is well supported as it passes up the side of your house. The height of the flue above the gutter/roof will be determined by building regulations.
Making a hole in the wall
Measurements will be made to ensure the hole we create is accurate to the nearest centimetre. The distance of the stove against the wall or its heat shield (required if the stove is located against a “combustible” wall such as plasterboard – including pink board) is taken into account.
The hole through the wall will be carefully drilled out creating an opening that is oval shaped and that will allow the twin wall flue to pass through on a 45° angle. Minimising the effects of dust-fall will be taken in consideration and implemented.
The hearth will be fixed and the stove will be situated on it directly in the middle (unless otherwise stated).
Vitreous enamel flue pipe or twin wall flue pipe will start from the top of the stove (this creates a better draw), and then a 45-degree bend will be used to direct the flue through the wall. A wall sleeve, insulation and various methods and materials will be used to create a watertight seal that does not leak into your house. Internally a finishing plate (typically oval shaped) will be attached to the wall surrounding the flue to complete the internal aspect of the installation.
Outside
Externally a pair of triangular arms will support a base plate that will take the weight of the entire flue system. Fixings to the support arms are normally achieved with thunderbolts.
The flue will be perfectly level and at the right distance from the wall to bypass any combustible objects (such as gutters) without having to use any unnecessary bends and thus compromise the draw. The simple rule being that the taller and straighter the flue, the better the draw. Why include expensive bends that compromise the draw if they are not required?
Ample support will be given to the flue along its journey upwards. Let’s not forget that we live by the coast and are regularly subjected to Mother Nature and her windy ways! We like to lock that flue in place and ensure it stays locked down.
Flexi to twin wall
If you have a chimney that stops in the loft, we will use a flexible flue liner combined with a twin wall flue to install your stove. For more information click here.