Twin wall flue is also known as DW (double wall) or HT (High Temperature) flue.
Twin wall flue pipe cannot be cut to length by you to make it fit. So one needs to plan carefully when installing a twin wall flue chimney.
Twin wall flue pipe is a “clip-together” chimney system that can be used to safely take the combustion gases from a wood burning stove and to the atmosphere.
Twin wall flue is a metal tube (flue) insulated with approx. an inch of insulation (two layers of stainless steel with insulation in between). The insulation is necessary to keep the gases hot. Were you to use non-insulated flue pipe for the whole of your chimney then the gases would likely cool too much and start to slow rather than rise – this causing the smoke underneath to “dam” forcing the smoke below to exit out of the vents in your stove instead of the top of your chimney. Cooling gases also form condensation which forms on the inner walls of the flue and runs down into the stove (mixed with soot it is a disgusting brown liquid).
With regard to diameter, twin wall with an inner diameter of 5″ has an outer diameter of 7″ whilst twin wall with an inner diameter of 6″ has an outer diameter of 8″.
You will most likely be fitting flue that has an inner diameter of 5″ or 6″ but check what diameter flue your stove requires. You can fit 5″ if the stove is DEFRA approved and the manufacturer does not state 6″ as a minimum (see articles on DEFRA stoves). The narrower diameter of 5″ can look less obtrusive if taking it all the way down to the stove and can prove desirable if combustible materials ensure the route is difficult (e.g. if the gaps between joists are too close for a 6″ flue to pass through with the required safety air gap then 5″ may be the answer).
Twin wall flue is generally available in silver (reflective chrome-like) and black (satin/matt finish). The black is a little more expensive.
Quality twin wall flue is expensive at £75+ a metre (but cheap when compared to building a new chimney).
Twin wall flue can travel internally within a building or externally on an outside wall.
You must notify Building Control prior to installing any chimney (unless you are approved to self certify, as are Hetas engineers).
Note: steel vitreous stove pipe (non-insulated) MUST NOT be used to go through a wall, except directly into a chimney. If you need to exit the room, whether out through a wall or through a ceiling then you must use twin wall flue pipe and then stay with twin wall flue pipe to the cowl.
Twin wall flue pipe should always be kept a minimum of Xcm from any combustible materials. “X” is usually 5 or 6cm but can be more (check with supplier). Dinak Twin Wall Flue, for example, should be kept 6cm from combustible materials.
Twin wall flue pipe is not difficult to fit (it all clips together); you just have to know your Building Regulations.
A chimney cannot have more than four bends and no bend can be more than 45 degrees (except if you use a 90 degree “T” off the rear of the stove). If you are are going out through a wall then you will be using two 45 degree bends straight away and have two left if you need them. Use two more to clear the eaves and that’s your maximum! Note that coming off the back of the stove with a T is classed as, for Building Regulation purposes, equivalent to two bends even though it is actually one 90 degree bend.
With the exception of the T off the back of a stove the maximum angle of bend is 45 degrees. Bends of 15 and 30 degrees are also available.
DON’T GET CAUGHT OUT BY: ceilings are almost always classed as combustible and any steel vitreous pipe must be kept well away (425mm or 17″ covers all situations likely to be encountered e.g. with 5″ and 6″ diameter steel vitreous pipe).
The top of the chimney should be a minimum 4.5m above the top of the stove (so “recommend” Building regs – but some stove manufacturers suggest 4m above so check with your Building Control Officer). A chimney should also pass the 2.3m rule or/and be 60cm higher than the ridge of your building. Read “Regulations For the Chimney“.
In the UK flues are always connected with the male pointing down (is the opposite in Europe). This is so that condensation/thin tar etc. running down the tubes cannot escape.
To ensure you are installing correctly take your cowl and look at it: obviously the joint points down and goes into the piece below it – every joint will work the same. You cannot fit a cowl upside down so the jointing process becomes apparent when you examine this particular item.
Note: other examples of installations are detailed here
Let’s look at some pictures:
My initial plan with this article was to treat the actual stove fit completely separately, in the stove fit section of this site, and just detail a twin wall chimney install here. But you really have to have the stove-fit in mind at all times when “building the chimney” and it is difficult to separate the two processes.
The most common methods of fitting a twin-wall flue system involve using vitreous steel pipe for the first section “off the stove” with a change to twin wall just before exiting the room (via the ceiling or wall). It is not the only way though – one can take the twin wall all the way to the stove if one wishes (twin wall all the way to the stove can look a little “fat” depending on stove size, this due to the extra diameter).
I used to take this as meaning that the bottom length of vitreous pipe should be adjustable and always used to make my vertical steel-vitreous an adjustable section. The problem with this is that you then get a join in the main vitreous that customers do not like to see. But having an adjustable section is great – the stove can be added/removed easily at will (keeps Building Regs man sweet and helps when you realise that you forgot to add the cosmetic rosette during installation). If you do not add the adjustable section and just keep stacking each item on top of another then it is usually not possible to remove the stove at all because the first section of pipe drops an inch into the stove collar and each pipe above then drops an inch or so into the one below. When you attempt to lift the bottom section of vitreous up out of the stove collar (to remove the stove) you realise that nothing moves a single mm.
BUT… speaking to others in the industry everybody seems to just ignore this rule about “stove and chimney dismantling”. Am I the only one in the world bothering? One fitter said to me “if the legs can come off the stove then one can remove a stove by unbolting the legs and dropping the stove so that gets around it”. Another fitter says “just cut through the vitreous and remove the stove and the chimney has not exactly been dismantled has it” (re-fitting would then require an adjustable). Another just said… well something rather rude about people who make the rules so we won’t go there.
So I leave it up to you to decide and maybe I am overthinking this one.
Update: The last two twin wall jobs I have fitted I have not used an adjustable vitreous and I do not believe that one needs to do so.
It is good practice, if possible, that the first length of pipe exiting the stove does not have any bends for the first 600mm. Theoretically this assists flue draft (Building Regs “recommend” this and an inspector may insist on it). This does mean that our hole in our wall is quite high but luckily it usually looks good at this height (in art there is something called the rule of thirds which suggests that the flue should exit a third from the floor or one third from the ceiling). If you do come off of the back of the stove then less of your pipe is seen in the room. I prefer coming off the top as the draft is somewhat better but both can work fine (off the back can mean the stove is more difficult to light without getting a little smoke in the face during lighting).
To create our hole we first have to work out where the hole needs to be. Back to the “off the top” method the best approach is to assemble your stove on its hearth with a length of 500mm pipe on top and a 45 degree elbow on top of that (the two combined provide your 600mm height). From this mockup we can project the route pf the 45 degree flue and mark on the wall where the hole will be.
Drill a pilot hole first using a very long masonry bit (I use a 15mm bit about 3 ft. long attached to an SDS drill). To ensure you are at 45 degrees use a longish spirit level with the 45 degree facility (ideally a friend holds the spirit level close to your drill bit). Once you have your hole you can just follow it with an SDS breaker with hammer chisel.
A: is a length of vitreous flue pipe 500mm. B is twin wall (adjustable in this pic but I don’t recommend adjustable as it’s not nice to work with and the join spoils the look). C is a vitreous elbow with soot door. D is a “twin wall to vitreous adaptor”. E is a rosette that is glues to the wall.
The stove must NEVER bear the weight of the chimney (weight will be taken outside with a bracket and maybe also in the hole with mortar). Steel vitreous pipe is secured together using small self tappers to ensure a firm pipe (happiness is a firm pipe…) and you can lightly spray any unsightly screw heads with black stove paint (mask off pipe first if pipe has a “vinyl sheen” as stove spray is matt black or you can quickly spray all the pipe matt black as does a stovefitter mate of mine).
In the majority of installs the section through the wall is a fixed 0.5m length. I find this fixed length is ideal for walls 250-330mm thick.
Before you slide your expensive bit of twin wall flue through your wall you may want to insert a metal wall sleeve within the cavity. This protects the property from the twin wall flue but also prevents jagged brickwork scratching the flue pipe. Any gaps around the sleeve can be filled with mortar. Note that most fitters do not use a sleeve but just push the pipe through and fill the gap with a mortar/vermiculite mix (just stating fact). I do not always mortar the gap around the pipe at the end of the job but fill the gap with Rockwool insulation (available from many merchants but it MUST be Rockwool brand as other brands are not fireproof). I hear that most stovefitters just slap mortar all around the pipe at the end of the job (some add Vermiculite beads to lighten the mix).
If you do not use a metal wall sleeve make sure you protect your pipe by putting strong cardboard or something similar in the hole whilst “playing around” with your pipe. If you do not do this your pipe, especially if black, will be scratched and your partner will say you are a muppet.
Now we move outside. Let’s start with a diagram:
This is the industry standard method of doing things. I use Thunderbolts to secure the Base Wall Support and other brackets. The offset to clear the eaves may or may not be required depending on the severity of your eaves.
I would suggest that one does not really require the Base Wall Support as the wall brackets are very strong and supportive (to save cost I would add an extra flue pipe bracket and lose the Base Wall Support). Base Wall Support is however full “belt and braces” if cost is not a concern. Check with your twin wall supplier as to whether this support is required (many fitters do not fit one suggesting the standard brackets are enough). Let the bracket take the strain.
But back to the job we started earlier:
The two elbows at eaves height are 45 degrees (15 degree and 30 degree elbows are also available) and we have a short length (250mm) of twin wall pipe between them due to the depth of these eaves. Note that I could also have used a 500mm or even a 1000mm between the two elbows. One can then just twist the bottom elbow to bring the upper vertical section back to close to the eaves (looking at the house end on you would then see a dog leg to the right (or left if you twist the other way). In effect you have a dog leg to clear the eaves AND a dog leg left or right. If you understand the hell what I’m burbling on about you will see that it is hard to go wrong when striving to clear the eaves as “a little bit of a twist” will sort it out.
The exterior rosette is made by myself (easy job if you have a pattern for the elipse) from 12mm Hardiebacker concrete board. I used my interior rosette (purchased from the twin wall flue company) to draw the oval for the hole (using the grinder to make the hole). I also made it in two halves.
Use Decorator’s Caulking to fill in gaps in joins or countersunk screw heads – and everything looks better when it is painted. The Hardiebacker board is connected to the wall using Thunderbolts.
This job was not quite textbook due to the flue height of 4.5m ensuring that there was going to be a lot of unsupported flue waving around, unsupported, in the air. I spoke to the flue manufacturers for this flue brand (not the brand I now supply as this can go 2.5 metres) and they informed me that the flue could be unsupported by up to 1.5m only.
Note: Some twin wall flue can be unsupported up to 2.5 metres. I have installations with 2.5m of freestanding flue but I would not wish to go to more than this.
So back to our job… because of the unsupported flue my colleague and I fashioned an adjustable support bracket in his workshop, making sure it was also strong enough to take the weight of the chimney above it and provide lateral stability. We painted this bracket with a primer and silver Hammerite.
What’s the 45 degree T with cap for? This is where the sweep inserts his brushes to sweep the bulk of the chimney. He will also do some sweeping inside but it is good to get the majority done outside the building. Mucky pups some of these sweeps.
Here are a few more pictures from a later job:
Note: other examples of installations are detailed here
Once again a picture is worth a thousand words:
Let’s take a look at where we go through any ceiling (usually downstairs ceiling and attic ceiling):
I believe that the graphics above explain everything very well.
The twin wall flue can be black or silver. Note that there is a Building Regulation recommendation that says, for a stove outside of a fireplace, that the first length of pipe from the stove must not have any bends in for the first 600mm whether the pipe exits the stove from the rear or top.
Your first job will be to plan your route. This is very important to get right. You are only allowed four bends in total (a ninety degree “T” off the rear of the stove is classed as two bends so if you use this method you only have two left and note that an offset uses two bends). Let’s take a look at one of my scribblings:
Above are just a few examples of the routes you might take. In your mind at all times will be the Building Regulations (especially “no more than four bends”and your termination point and the 2.3m rule: Regulations For the Chimney). Another key issue is the gaps between your joists and the gaps between your roof rafters (twin wall flue requires a safety air gap – usually 5 or 6 cm depending on brand). If these are close together consider a stove that is allowed to have a 5″ twin wall flue (external 7″ due to insulation). See Defra-approved stoves articles as only DEFRA-approved stoves can use 5″ flues.
The ceiling above your stove is probably the easiest place to start when designing your chimney: you can position your stove directly beneath the gap between two joists if this works for you – or you can use steel vitreous elbows to offset your stove (the problem here is that you will use two of your four bends ration).
If you have two ceilings to pass through then you will likely want to position your stove directly underneath the gap between the lower ceiling joists. This means that you have four bends in the bank: two bends for aligning to get through the second ceiling, and two more bends available to you should you need an offset in the attic to align the route through the gap in the roof rafters (the black line in our graphic above: flue routes).
When creating offsets using elbows you can vary the offset with 15, 30 and 45 degree offsets provide differing offset amounts. You can also add sections between the elbows if required (25/50/100cm lengths available).
IMPORTANT NOTE: Although your twin wall pipe can be as close as 5cm to combustibles (some brands 6cm) your steel vitreous pipe, coming off of the stove, has to be a long way away from combustibles (3x its own diameter – so 15″ for a 5″ pipe and 18″ for a 6″ pipe). Plasterboard walls are classed as combustible in Building Regulations. See distance to combustibles. You may wish to take your twin wall pipe all the way to the stove if you are having problems in this area (twin wall can look very big and fat on smaller stoves though – not such a problem if 5″ internal is used as outer diameter is 7″). Or you can replace your plasterboard with 12mm Hardiebacker board (pink fire-resistant plasterboard does not pass regs).
For travelling through floors you will find that your chosen brand of twin wall flue will have proprietory and certified parts e.g. “firestop” to seal pipe against ceiling and some kind of support to connect the pipe to the floor (or joists). Don’t fret about this as these items are usually very easy to fit.
Going out through the roof is often more straightforward than it sounds and the only difficult bit is fitting the roof flashing (the item the flue passes through). This is where you might choose to employ a roofer (I have used a roofer in the past and paid £120 for him to fit the roof flashing that is required, not including the cost of the flashing unit). If you are happy with a bit of roofing then you might wish to do it yourself.
Various roof flashings are available. Some leave an air gap around the flue pipe and rely on the storm collar to prevent water ingress. Others have a high temperature rubber seal.
Let’s look at a picture sequence of a flue going through a vaulted ceiling:
How to aesthetically seal the flue inside the building at the rafters on a vaulted ceiling if the brand used does not offer a suitable rosette? There are special brackets for this that work very well in the loft but are unsightly with a vaulted ceiling as they intrude into the room and require boxing in. Here’s what I do if everything is on show:
The material is 12mm Hardiebacker concrete board cut and shaped with an angle grinder. It is difficult to get a perfect oval. Update: I can now supply a wonderful sealing plate for this that is in two parts that slide together to surround the twin wall pipe and is a doddle to fit… pictures and article shortly…
The plate pictured is created in two halves. Because of the thickness of the material (12mm) you will need to grind both ends of the oval (on different sides of the board). Update: 6mm Hardiebacker is easier to work with
If you are not passing through a ceiling before exiting the building (and therefore not using a support plate), but are going straight out through a vaulted ceiling then you have to ensure that your chimney is supported somehow at the rafters. You are not allowed to use the stove to support the chimney according to building regs. Here’s a picture of a real belt and braces way of doing this:
The chimney is supported by a wall support bracket and it is this taking the weight. I know it’s rather utilitarian! Leave it on show or box it in. Boxing in actually looks fine but needs a bit of joinery and plasterboarding.
Now that Wall Support Bracket is rather utilitarian if unboxed. Another method and the more common is to add a rafter support at rafter level. A rafter support is hidden above the plasterboard. It does mean that you only have one point where the chimney is attached (unless you include the stove) so best for shorter chimneys but it is neater because your bracket is hidden.
Please ensure you understand the Building Regulations with regards to distance to combustibles. The ceiling of a property is invariably classed as combustible. That’s why the twin wall flue pipe in our picture protrudes so far into the room from above – to keep the steel vitreous pipe away from the ceiling (42.5cm is enough protruding to keep 5″ and 6″ diameter vitreous far enough away from the ceiling for Building Regs). If you have a plasterboard wall (classed as combustible according to Hetas) you will also need to ensure your vitreous pipe is far enough from that wall (this protection documented in distance to combustibles).
You only have to box in your twin wall flue if it is passing through a storage area or a loft. You can use stud wall (wood frame and plasterboard) as long as your minimum Xcm clearance is adhered to (usually 5cm or 6cm).
In lofts you can use a cheap and cheerful wooden frame with chicken wire.
Here’s a picture of a flue boxed in (ensuring a minimum 5cm clearance from the pipe).
By Julian Patrick