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Installing a twin wall flue chimney - a stove installer details all

Stovefitter's Manual> Installing a twin wall chimney

This page starts with an introduction to twin wall chimneys followed by general advice that may be useful for any install. We then look at two of the most common installs: an external and an internal example and show variants of these designs.

Feel free to let me know what info is missing from this page that you feel would be useful via info@stovefitterswarehouse.co.uk (Julian)

My favourite flue brand: There is quite a lot of cheap twin wall on the market and I have seen sagging brackets, discolouration and rust. You get what you pay for. Duraflue Twin Wall was launched to the market during 2014 and, due to its high quality and innovative design, is already a favourite amongst professional stove fitters. Brackets are strong whereas other brands may be spindly and quality is premium. Do it once - do it good.

Shed, shack, boat, van?

After reading this article you might wish to order one of our 3-metre chimney kits suitable for sheds, shacks, vans, caravans, home offices etc.

 

Conservatory or extension?

After reading this article you might wish to read more about chimneys for conservatories and extensions.

Introduction to twin wall flue

So, no chimney in the room where you want your stove? Why not create one? It's easy - clip-together parts from top to bottom. Take advantage of a FREE design service and telephone assistance OR order from our extensive parts menu.

Feel free to email your design sketch to us for our initial thoughts: info@stovefitterswarehouse.co.uk

Twin wall flue is also known as DW (double wall) or HT (High Temperature) flue and is a clip-together chimney system enabling a wood burning stove to be added almost anywhere in a property. No adhesives or sealing potions at joints - slide one part into another and add a locking band.

Twin wall flue pipe cannot be cut to length by you to make it fit (although there are adjustable lengths) and one therefore 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 outside to the atmosphere.

Industry standard diagrams for most situations can be found here(opens new window).

Some excellent documents by the British Chimney Manufacturer’s Association on this page (really worth a look as easy to read with lots of diagrams) here.

Want a fast and excellent method for building a false chimney breast? Use Skamol: https://www.opiesuk.co.uk/skamo-enclosure.

Twin wall flue pipe is a metal tube (flue) insulated with approx. an inch of insulation (two layers of stainless steel with insulation 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 with this causing the smoke underneath to “dam”. This might cause 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).

So keeping gases hot is good.

Note the difference between a chimney and a flue: if the chimney is a polo mint then the flue is the hole (I really enjoy that definition).

With regard to diameter, twin wall with an inner diameter of 5″ has an outer diameter of approx. 7″ whilst twin wall with an inner diameter of 6″ has an outer diameter of approx. 8″. Our twin wall flue has an exact outer diameter of 191mm (5" internal) and 217mm (6" internal).

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 only fit 5″ internal diameter if the stove is DEFRA approved and the manufacturer does not state 6″ as a minimum (see articles on DEFRA stoves). Otherwise choose 6" internal. 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 (for example, if the gaps between joists are close)

Twin wall chimneys can be surprisingly close to combustible materials (wood usually). Duratwin (the brand we sell) can be as close as 5cm (flue open to the air) or 7cm (boxed in flue) but different manufacturers' specs may vary.

Twin wall is generally available in silver (reflective chrome-like) and black (powder-coated matt finish). The black is a little more expensive.

Quality twin wall flue is about £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.

Twin wall flue pipe is not difficult to fit (it all clips together using locking bands); you just have to know your Building Regulations (or let us do the design for you).

The wider diameter pipe exiting the room in the next image is black twin wall flue. Underneath this is a metre of steel “vitreous” stove pipe. Both types of pipe are 5″ (inner diameter) but the black twin wall flue is 7″ external diameter as it is insulated.

You can run twin wall all the way to the stove if you wish (can look “fat” on small stoves). If you are not bracketing to a wall, for example if the the room is open-vaulted, then "twin wall all the way to the stove" is stronger and superior. Why? Because the joint between single skin steel pipe and twin wall pipe is just "located" and has no strength. One can fix this by adding self tappers to the joint but the tapper heads will be seen. In our image (with the Boxer dog on the rug) the flue is well bracketed so single skin on the lower section is fine.

Note that there is no perfect design for any given situation. You have an available list of parts and as long as you don't break any rules all is fine.

General advice for installing a twin wall flue

You must notify Building Control (local council) prior to installing any chimney (unless you are approved to self certify, as are Hetas engineers). In Scotland this may not be necessary. If the building is not attached to a main dwelling (e.g. shed, shack, home office, boat, garage etc. then notification usually not required).
More info on self-installing and Building Control.

The video is about six minutes long and really worth a viewing (narrated by myself, Julian Patrick)

Steel or Twin Wall?

Steel vitreous stove pipe (non-insulated) is often used for the first section of pipe on top of the stove. It MUST NOT be used to go through a wall or ceiling (except directly into a brick/stone chimney). To exit the room, whether out through a wall or through a ceiling, you must use twin wall flue pipe and then stay with twin wall flue pipe to the cowl.

Lengths of twin wall are 250mm, 500mm, 1000mm (same for steel vitreous pipe).

Elbows are 15, 30 or 45 degrees (same for steel vitreous pipe).

Adjustable lengths are:

380-585mm
590-1005mm

Combustible surfaces & boxing

Steel vitreous pipe has to be at least 3x its own diameter away from combustible materials. So a 5" pipe must be 15" away from a ceiling.

Our twin wall flue can be as close as 5cm to a combustible material (or 7cm if "boxed in").

If boxing in a flue system joints must be accessible for inspection. A hatch, minimum 30cm x 30cm needs to be in place, within 1.5m of any joint.

Twin wall flue does not have to be boxed in or have guarding unless it is in a storage area e.g. cupboard or loft storage area. In lofts a wood frame and chicken wire is often used.

Bends

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.

Bends of 15 and 30 degrees are also available.

Watch the video :-)

Chimney height

The top of the chimney should be a recommended minimum 4.5m above the top of the stove according to Building regs – but as long as the chimney works properly there is no legal minimum. With modern twin wall flue being so well insulated a 3.5m is generally accepted as being tall enough.

A chimney should also pass the 2.3m rule or be 60cm higher than the ridge of your building.

Watch the video :-)

Male on top

Flue pipes, whether steel vitreous or twin wall, are always "male on top", as in the top pipe slides into the lower pipe. This is to ensure any condensation runs down the system eventually evaporating or entering into the stove.

If fitted the other way around this condensation can escape at joints (and it is usually brown and horrible and would burn and sizzle making an awful stink).

Pictured elbow is a steel uninsulated vitreous pipe.

Unsure with twin wall which end is which? Grab the cowl - it only has a "bottom".

Strength

Each joint in our twin wall system is fixed with a narrow locking band.

To add extra strength to a joint we can also choose to add a wide locking band on top of the narrow locking band. Most likely place for this treatment is the first joint above the highest bracket.

Up to 1.5m of unsupported twin wall is fine. Up to two-metres if wide locking bands are used at the necessary weak point. Above this you need telescopic leg supports (guy wire bracket plus telescopic legs).

Base support brackets are used when the weight of a number of lengths of twin wall might cause standard brackets to sag under the weight.

Custom bracket

Back in the days before long brackets existed we made our own using box-section steel and a welder.

Unsupported

Our twin wall can go as high as 2 metres above the highest bracket (this job shows more but the householder wanted to risk it and it is still standing today. Extra locking band security will be required.

Thunderbolts for exterior

Perfect for brick, stone or block. Just drill a hole and let these little monsters wind themselves in.

Brick or stone: if the Thunderbolt shank is 5mm then drill a 5mm hole.

Block: if the Thunderbolt shank is 5mm then drill a 4mm hole as block is a little more "crumbly".

Do a test or two first.

Easier with scaffolding

Twin wall flue is quite heavy. Scaffolding makes life safer and easier. Remember to offset the scaffolding away from the wall so as not to interfere with the flue.

Distance to combustibles

It is important to ensure your stove and flue are a safe distance away from combustible materials and it is therefore very important to understand this subject. Rules are different for stove, vitreous single-skin pipe and twin wall pipe. It can be possible to shield combustible materials.

Removable stove?

Now let’s get something out of the way early on: it is mentioned in Doc J of building regulations that you be able to remove the stove without dismantling the chimney. Why? Well you might have a warranty issue with the stove for example and need to swap it over. Or you might choose to buy a new stove.

But then again hardly anybody worries about this and stoves are generally installed “for good”.

I used to take “removable” as meaning that the bottom length of flue should be adjustable and always used to make my lowest length an adjustable section. The problem with this with steel vitreous pipe is that you then get a join in the main vitreous that some customers do not like to see. Our twin wall range has a removable section available for purchase so all is well (see image).

But having an adjustable or removable section is great – the stove can be added/removed easily at will (also 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.

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 ” (re-fitting would then require an adjustable). Another fitter told me to get a life :-0

I reckon "cutting the vitreous" is so easy that this will suffice. Twin wall I tend to add a removable section unless the chimney is a short one (almost as easy to dismantle a 3m chimney as any other method).At the end of the day ask whoever is signing it off.

So I leave it up to you to decide and maybe I am overthinking this one.

Base support

Base support unit (adjustable) for the foot of multiple lengths of flue on an outside wall. Can be fitted upside down quite happily. Two sizes available" short and long

Brackets

Adjustable brackets: 90-135mm, 130-210mm, 225-385mm, 370-590mm, 550-770mm

Roof/rafter support

Roof/rafter support for securing chimney to a flat surface or to rafters. Angle is adjustable.

Ventilated firestop

Ventilated firestop support unit for passing through a floor from one room to another.

Lowest twin wall part

Single to twin wall adaptor: is always the lowest twin wall part. Can sit directly onto the stove (inserts into collar) or slides inside single skin vitreous pipe. Use fire cement around snout or fireproof webbing (webbing best!)

On top of the stump

Chimney chopped off in loft? Sit one of these atop your stack and continue on in twin wall. Hang a liner off the bottom, attached by a Jubilee clip.

Typical Install: twin wall flue outside the property, off top of stove

Go vertical for 600mm

It is good practice (but not legally required), if possible, that the first length of pipe exiting the stove does not have any bends for the first 600mm (500mm pipe then a 45 degree elbow is fine). Theoretically this assists flue draft. This does mean that any hole in a wall is high, but luckily it usually looks good at this height (in art and design 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).

More than 600mm is fine but no more than 2000mm of steel vitreous pipe should be used to prevent the gases cooling (insulated twin wall pipe keeps those gases hot).

If your flue is off of the rear of the stove (on a 45 degree pipe to outside) 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 draw is less and the stove is more difficult to light without getting a little smoke in the face during lighting).

Variants (chimney outside property)

You can see these in more detail on this page

The most common method of fitting a twin wall system involves 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 but it does make for a very strong chimney and advisable if limited bracketing points available (e.g. vaulted ceiling).

 

Hole through the wall

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 minimum 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 of the 45 degree flue and mark on the wall where the centre of 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 – Silverline, at the time of writing, do a set of three super-long long masonry-bits for under £25). 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 whilst you are drilling).

Or do what one of our customers (Nick G) did - created a "jig" (see image).

Once you have your hole you can just follow it with an SDS breaker with hammer chisel function making the hole gradually larger until you are happy.

In the majority of installs the section of twin wall through the wall will be adjustable. My favourite is the 135 Degree T With Long Adjustable Snout as it has the smoothest transition between the two parts. You can also choose to purchase a Short 135 degree T and add a fixed or adjustable length to it. If a joint ends up within the wall, as it invariably will, don't worry about it.

Before you slide your expensive bit of twin wall flue through your wall you may temporarily want to insert heavy-duty cardboard or old carpet inside the cavity or find another way to protect your twin wall pipe. If you do not do this your pipe, especially if black, will be scratched and your partner will say you are a muppet.

You might decide to use a metal sleeve through the wall. 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 even though Building regulations state that there should be a sleeve "if the pipe is likely, through heat expansion, to damage the building or the pipe".

I have used a sleeve on some jobs and not on other jobs. If not using a shield I fill the gap between the pipe and the building with Rockwool insulation (available from many merchants but it MUST be Rockwool brand as other brands are not fireproof). My thoughts here are that, as there is no expansion issue, no sleeve is required.

Note that adjustable parts are provided with extra insulation for inside the twin wall as when you extend a part you create a cavity within the part.

Through and out

The offset to clear the eaves may or may not be required depending on the severity of your eaves. 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, as long as your offset is longer rather than shorter, it is hard to go wrong when striving to clear the eaves as “a little bit of a twist” will sort it out.

Note that the highest bracket should be above the offset.

Base support brackets are important

Some fitters do not fit Base Support Brackets (to save money). I used to be the same. Then I ran into problems with one job where I used “long adjustable brackets” as the flue needed to be “away from the wall” by quite a lot. As I progressed skywards the brackets started to sag. Lesson learned. Do it once, do it good.

Typical Install: twin wall flue inside the property (off top of stove)

Key areas

Running a twin wall chimney up through the property and out of the roof is standard practice and quite straightforward. Please watch the video earlier in this article as it includes important information on bends and sweep points etc.

There are three key areas to look at:

- passing though a floor from room to room
- passing through from a room to a loft
- passing through a roof to outside

Of course, the first two points may not be relevant as some properties will have vaulted ceilings or be single storey with no upper floor or loft space.

Do you need brackets besides that shown on my drawing? I say not but you can add more if you wish (not often possible if the flue is not near anything to bracket to of course). Why do I suggest this? Because in open-vaulted properties where a stove is in the middle of the room there are often no brackets between the stove and the ceiling. Stovefitter's office is an old Chapel and the only two brackets are in the ceiling void some 5 metres above the stove. The chimney is strong.

Variants (chimney inside property)

You can see these in more detail on this page

Passing through a floor to another room

If passing through a floor from room to room you will chose a Ventilated Support Unit (pictured). This allows ventilation into the floor void and comprises three parts:

- Top plate: sits on top of rafters or floor surface. Tabs show how close combustible material (usually wood) can be to flue pipe and act as a "stop" point.
- Ceiling plate: screws up to ceiling.
- Clamp ring: clamps around flue pipe and prevents pipe being able to drop downwards even if everything below it were removed.

Obviously your joists might not be conveniently spaced. In such a case you will add one or more “noggins” (extra pieces of wood) where required.

Note that when looking up at the ceiling of the lower room you will be able to see through the vents into the void. It is possible to hide this using a Masking Plate which just clips on (few people bother).

Passing from a room to a loft

If you are passing through a ceiling into a loft with no floor surface (exposed beams) then you do not need to ventilate. In this case you can add a bracket inside the loft (usually a roof/rafter support). On the ceiling you would choose a Finishing plate.

Passing through the roof

If you are passing through a weather surface (roof tiles, felt, plastic etc.) you will need a "flashing".

Simple guide:

  Tiled surface with flat tiles: Aluminium Seldeck (cheap) or Nulead (best)

  Tiled surface with concrete "corrugated" tiles: Nulead

  Non-tiled surface flat or "corrugated": EDPM

Nulead moulds to uneven surfaces. Aluminium Seldeck does not.

EDPM moulds to surfaces surprisingly well. Can be bonded using silicon adhesive. For additional security add roofing screws or stainless steel bolts. Many of our customers have just used silicon adhesive.

Going out through the roof is often more straightforward than it sounds and the only difficult bit is fitting the roof flashing on a tiled roof. 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.

Planning your route

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 still 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.

Note that if you have four bends in your system you MUST have a sweep point on one of the bends or on a straight between the bends. Twin wall flue is not available with soot doors on bends.

Please watch the video from earlier in this article.

Vitreous single skin?

If you look at the image of our office (it is not this tidy - this was moments before we moved into it) you can see two chimneys. The chimney at the rear is single-skin Vitreous pipe for the first metre and a half, changing to twin wall just before half height. The chimney in the foreground is "twin wall all the way to the stove".

Some people prefer the narrow single-skin vitreous pipe.

But only use Vitreous pipe in your system if the twin wall is bracketed securely (at least two brackets more than a metre apart). Why? Vitreous pipe has no locking mechanism - one part slides into another about 2cm depth. No jointing compound, no clamp. That chimney at the rear in our office was "wobbly" because the twin wall is only located once at roof height. Since this picture was taken an extra bracket has been fitted and all is well.

The stove in the foreground could not have been fitted using Vitreous pipe for the lower section because there was nowhere to bracket the twin wall to and make it strong. In one continuous length, because each joint has a locking band, it is good and sturdy.

Article by Julian Patrick, author of The Stovefitter's Manual. Wood burning stove installer (including solid fuel heating systems). Laid down tools in 2013 to write The Stove Fitter's Manual and open a small shop in North Wales (the Wood Stove Hut). Launched Stovefitter's Warehouse soon after due to fast growth of sales. Own stove is a DG Ivar 5.