Install a wood burner and save £££'s
Twin wall flue is also known as DW (double wall) or HT (High Temperature) flue.
Twin wall 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” and come 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.
With regard to diameter, 5″ internal twin wall flue is 7″ external and 6″ internal twin wall flue is 8″ external. You will most likely be fitting 6″ internal but check what diameter flue your stove requires. You can fit 5″ internal 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 combustibles 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).
Twin wall flue is generally available in silver (reflective chrome like) and black (satin/matt finish). The black is a little more expensive. Dinak also offer a colour-matching service.
Good 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: standard 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 have to 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.
For more information on chimney regulations please first read Regulations For the Chimney. For now a few key points to bear in mind when planning the route of your chimney:
You are allowed no more than four bends in the chimney and there must be a sweep-access point before your third consecutive bend (so you cannot do “bend,bend, bend bend” or “bend, bend, bend, door, bend” but you can do bend, bend, DOOR, bend, bend).
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 higher than the top of the stove (so “recommend” Building regs – but some stove manufacturers say 4m above stove so check with your Building Control Officer) and should 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. 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 item.
Let’s look at some pictures (note that many pictures can be clicked on for a larger version):
My initial plan was to treat the actual stove fit completely separately, in the stove fit section of this site, and just build the twin wall chimney. 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 (as in the picture above). It is not the only way though – one can take the twin wall all the way to the stove of one wishes.
Now let’s get something out of the way early on: Building Regulations state that you must be able to remove the stove without dismantling the chimney.
I used to take this as meaning that the bottom length of vitreous pipe should be adjustable and have always made 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 always 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 you forgot to add the cosmetic rosette). If you do not add the adjustable section and just keep stacking each item on top of another then it is often not possible to remove the stove at all because each pipe drops an inch or so into the other and you have to lift the bottom one up out of the collar but nothing above it 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 the stoves by unbolting the legs and dropping the stove so that gets around it”. Another fitter says “just saw through the vitreous and remove the stove and the chimney has not exactly been dismantled has it”. 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.
Note: The last two twin wall jobs I have fitted I have not used an adjustable vitreous.
It is good practice, if possible, that the first length of pipe from the stove not have any bends in for the first 600mm as this assists flue draft (best practice but not a Building Regulation but an inspector may insist on it). This does mean that our hole in our wall is quite high. If you do come off of the back of the stove then it can be a little lower (I prefer coming off the top as the draft is a little better but both can work fine as long as you have access to the rear of the stove after the install is completed for sweeping purposes).
To create our hole we first have to work out where the hole needs to be. The best method is to assemble your stove on its hearth with a length of 500mm pipe on top and a 45 degree ellbow on top of that. From this we can project the route 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 spirit level with this 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 adjustable vitreous flue pipe (or a solid piece see article in orange above). The top part, the male, is connected to the bottom of the elbow above it and the bottom part, the female, can slide up and down around the male so it lifts up and hence the stove can be removed. Without this function you may not be able to remove the stove without dismantling the chimney or sawing the vitreous.
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). The male adjustable vitreous length is secured to the female with one or two self tapping metal-to-metal screws. I connect all steel vitreous pipe using these self tappers to ensure a firm pipe (happiness is a firm pipe…) and you can spray any unsightly screw heads with black stove paint.
C: Is a 45 degree elbow with a soot door. If the stove is one designed to be swept through the stove door then this elbow-plus-door is not necessary but it does give the sweep another option. The soot door could also be in the 500mm vertical length.
B: In the picture i have used an adjustable by a company called Midtherm (customer supplied materials). Usually I use a single length of twin wall flue pipe passing through the wall (not an adjustable). One can use an adjustable section but the join on some adjustable sections can be downright ugly so a set length is always better (Dinak adjustable is not designed to be on show near the stove).
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. I normally only need an adjustable if the wall is a bit thin e.g. 200-250mm.
D: Is an adaptor, vitreous steel to twin wall.
E: Is an oval wall plate, stuck to the wall using a very thin layer of Silicon adhesive (can be sanded and painted to match the wall).
Before you slide your adjustable twin wall flue through your wall you may want to insert a 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/Mica mix (just stating fact). I often fill the gap with Rockwool insulation (available from many merchants but it MUST be Rockwool brand as other brands are not fireproof).
Now we move outside. Let’s start with a diagram:
This is the industry standard method of doing things. I use Thunderbolts (I can supply) to secure the wall support and other brackets. The offset to clear the eaves may or may not be required.
With Dinak twin wall flue one does not require the Base Wall Support as the wall brackets are very strong and supportive. 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).
But back to the job we started earlier:
The two elbows are 45 degrees and have a short length (250mm) twin wall pipe between them. The exterior rosette is made from 12mm Hardiebacker concrete board. I used the interior rosette to draw the oval for the hole (with a little extra work with the grinder as well). I suggest maybe using a second rosette outside – to make your Hardiebacker oval look better (it is not easy to do a neat elipse in Hardiebacker). 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 in the air. I spoke to the flue manufacturers and they informed me that the flue could be unsupported by up to 1.5m only.
Note: Dinak inform me that their flue can be unsupported up to 2.5 metres.
So my colleague and I fashioned an adjustable 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.
Adjustable wall brackets sold by twin wall flue manufacturers provide strength and useful adjustment (e.g. adjust to allow pipe to be 7-12cm away from wall).
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!
Here are a few more pictures from a later job (using Dinak twin wall flue):
Sunday 13 October 2013: This article is still not quite complete as within a few days I will be popping back to finish off (I am awaiting internal and external rosettes to finish off).
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. Dinak do not have a support plate but rather a “slab support”, a bar that clamps around the pipe and then is screwed to the joists (nice and easy).
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. Remember that you are only allowed four bends in total. 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. 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.
The ceiling above your stove is probably the easiest place to start: 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. 15, 30 and 45 degree offsets provide differing offset amounts and you can also add sections between the elbows if requred.
IMPORTANT NOTE: Although your twin wall pipe can be as close as 5cm to combustibles (Dinak 6cm) your steel vitreous pipe, coming off of the stove, has to be a long way away (3x its own diameter so 15″ for a 5″ pipe and 18″ for a 6″ pipe). Plasterboard walls are classed by Hetas as combustible but I have never checked what Building Control think (check with your Building Control Officer). 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.
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.
Let’s look at a picture sequence:
How to secure the flue inside the building at the rafters? 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: Dinak do 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 shortly.
The plate 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).
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. You are not allowed to use the stove to support the chimney according to building regs. Here’s a picture:
You can click on the picture to see a larger version. 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. Another method (rather than the great triangular contraption above) is to use two standard wall brackets one above the other (with at least 6″ between the two to provide lateral stability). But you can only do this if your flue is close to the wall.
Whilst we have this picture in front of us: 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). In the picture we also have a plasterboard wall (classed as combustible according to Hetas) and you can see, if you look closely, that the flue is protected (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 wooden frame with chicken wire.
Here’s a picture of a flue boxed in (ensuring a minimum 5cm clearance from the pipe).
Google “twin wall brackets” and “twin wall rafter brackets” for some ideas.
By Julian Patrick