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How to Install Flexible Flue Liner, costs and questions answered

Note: I've researched, talked to HETAS fitters and had word from the BFCMA (British Flue and Chimney Manufacturer's Association) regarding what problems might be caused by not sealing the top of the liner to the plate. No one is able to identify a problem with safety of the installation I've covered here. There are however other potential issues, detailed below, that lead me to the conclusion using the Pot Hanger Cowl method is the best method to employ.

As with all my articles, I present the facts as I see them and leave you to make up your own mind, do further research or get a professional in. The video has received many comments on You Tube if you would like to add to the debate please leave a comment on the BLOG. The more input we get, the closer we can get to a confident consensus of opinion.


Flue liner top plate and clamp

Installing a flexible flue liner is often the most economic option to upgrade an existing chimney flue so a multi fuel stove or wood burning stove can be fitted.

There is much conflicting advice surrounding flue liners and the best method of installation. This page shows the method used for our stove that has been heavily used with no problems for the last 4 years.  The video and pictures have been taken as we used the same method to install two liners at our Whitby cottage renovation. Knowing what I know now I would do it very slightly differently - all is explained.

Before I go in to details of How to install a Flexible flue liner (inc. Video), the notes below might help you decide on how to tackle the job and the flue parts, cost involved.

The LEGAL bit...

Installing flues and stoves comes under building regulations, this is to ensure that installations are to the relevant standards and are "safe". Without the necessary approval you may be at risk of prosecution for non-compliance. (It would be interesting to know about what this procedure entails, comments on the blog are most welcome). It's also a certificate that any future house buyer will want to see.

You Local Authority will advise and give you the cost (£180 - £300 ?)to get the approval directly from them. Alternatively HETAS trained and authorised installers can self certify and inform the Local Authority of their own installations. There is current no market completion to HEATAS...

This leaves the DIY er in a dilemma when the budget is tight. It's a rock and a hard place problem. You might be able to find able to find a reasonable and friendly HETAS fitter that is prepared to let you be "labour" on the job. Do respect that they have paid a small fortune for the training, registration, insurance and they need to make a living but it might save you a shilling or two.

If anyone has the information about the total number of meters of flexible flue pipe sold and the number of registered installations let us know! It'll be interesting to see now many installation are likely to be un-certified.

OPTIONS For Flue Liners

What is a Flue Liner? and Does a Wood Burner need a flue liner?

What is a Flue Liner? A flue liner is a smooth bore duct that flue gasses from combustion travel up. They can be made from anything that will resist the heat and corrosive chemicals in the smoke. Ceramic and concrete liners are available but for most retrofit jobs the Stainless Steel Flexible Flue Liner is the most practical and economic solution.

Is a Flue Liner necessary? Do you need a flue liner? Well no. if the only consideration is to provide a path for the gasses to exit above your roof. However for a modern wood burning stove as opposed to an open fire, the flue performs an important additional function. Hot gasses rise up a smooth flexible flue much faster than gasses that cool down in a rough old chimney stack. The rising smoke is what provides a "draw" for the stove. Modern wood burners need a good draw to work effectively and efficiently and in the extremes, safely. So yes a flue liner is a good idea even though it might not be required by building regulations.

To insulate or not to insulate a flue liner? 

This is where there seems to most conflicting advice. It is true that that flue gasses can and will condense on the inside of a cold flue liner.  This can be a problem if tarry deposits build up as this will increase the risk of a flue fire. The acidic condensate can also eat in to the flue liner causing premature failure.  These arguments are used to sell insulation sleeves or backfilling material even when it might not be required. 

I take a less dramatic view of the potential problem, supported by my own experience.  Insulation of the flue liner is not essential if you use your stove properly and burn only well-seasoned timber, unless there is something peculiar to your chimney that would cause excessive cooling of the liner (e.g. It's against an exposed external wall, lots of bends, very long.) So for this job I'm not using any flue liner insulation.

What Size Flue Liner Do I Need for a Woodburner? 

Simple answer 6” 150mm minimum diameter if you are burning wood. The regulations. do allow 125mm diameter flue to be used on Defra Exempt stoves if the flue outlet from the stove is 125mm.  Many stoves have 125mm diameter outlets but these should be stepped up to 150mm at the join from the flue pipe to the liner. The stove manufacturer will give a maximum flue diameter or larger minimum if required.

Flue liner close up for solid fuel or wood / log burning stoveWhat Flue Liner Grades?

Stainless steel Twin Wall flexible flue liners are available made from different grades of material. 316 grade is usually guaranteed for 10 years and 904 grade guaranteed for 20 years.  There are also liners with the outer skin in 316 and the inner 904.  Although the 904 is almost twice the price of 316, I take the view that it’s a job I only want to do once so I use 904 grade. Prices for Flexible liners varies from supplier to supplier. The liners for the cottage (2013) cost £30 per meter from an on-line retailer.


Which Method to fit a Flexible Flue Liner?

This is where the subsequent detailed research comes in. Hopefully at some point flue suppliers will consider and maybe adopt this advice. At the moment their information is sketchy at best and possibly misleading in some cases. This comparison may help you draw your own conclusions.Illustration two methods to fit flue liner

Method A to fit a flue liner

Down the pot. The simplest way to fit a liner is by using a pot hanger cowl. The liner is dropped down (or pulled up) the chimney pot and suspended from the top of the pot. 

Pot Hanger PROs

It's relatively quick and easy.

It'll be relatively easy to pull out and replace at the end of its life

The flue liner goes right to top of the pot

The top of the pot and inside of the chimney stack is fairly well protected from water ingress

Pot Hanger CONs

The Liner will need to be longer

Loose fill insulation takes longer to feed in

Pot Hanger Cowls cost more

The liner has to be mobile (move up and down a bit) to fit or remove the cowl. The bottom adaptor will have to under the register plate if loose fill insulation is used

The top of the liner will be cooled by the cowl acting as a heat sink.

Method B to fit a flue liner.

With the pot off.  If the chimney pot  has to be taken off for repairs to the chimney top or the flaunching (mortar around the pot) needs replacing or the chimney has been blanked off, this method is possible.  The flue can be pulled up from inside or dropped down from the top. The liner is secured with a blanking plate (or top plate) and clamp under the pot. You might see this also done with the clamp set in to the brickwork below the plate, more about that later.

Plate and Clamp PROs

Parts cost is marginally lower

The larger opening before the top plate is fitted can make installation and insulating easier.

Wider range of cowls available.

The very top of the flue will remain warmer

Plate and Clamp Cons

The pot will have to be removed when the liner needs replacing.

The top of the liner should be flaunched with acid resistant mortar so water getting past the cowl is directed down the inside of the liner.

Effective, long lasting, water tight flaunching will be difficult due to different rate of thermal expansion etc.


new illustrations to go here

What other bits do I need to fit a flexible flue liner?

Components to fit a flexible flue linerIt fit the flexible liner using Method A all you need is the a pot hanger cowl. Method B requires the Blanking top plate and clamp. Top inserts are also available that are intended to stop the clamp from crushing the liner (more about that later).  A nose cone can also help smooth the process of inserting the liner.

You will also need the parts for the bottom end so you might want to order a register plate and adapter/ connecter to suit the stove flue pipe at the same time.  I’ll be covering the stove installation in a further article.

A pot hanger cowl should cost around £60, the alternative plate and clamp will be around £20 plus around £25 for a rain cowl and bird guard.  A register plate and adapter will add £50 ish to the order.

How long should the Flexible Flu Liner Be?

You aren’t supposed to have any joints in the liner so it’s important to make sure you get the measurement right.

This is how I do it.
Determine the height of the register plate. This would normally be no more than a couple of inches above the fireplace opening. Then measure up to ceiling height and add on the depth of the floor above. Add the height of the next room(s) above including any other floor depths and the final ceiling depth to the loft space. In the loft add the height of the chimney stack to the underside of the roof.  From outside count the number bricks from the roof to the pot (take account for flashing that will cover some bricks, then count out and measure the same number of courses of similar brickwork within easy reach.  Add a little extra for any bends in the flue, 6” 150mm for a sideways bend of 2’ 600mm should be plenty.

75mm 3” will be needed for method B to fit the clamp

For method A you will have to add the height of the pot.  This can be very deceptive to gauge from ground level but comparing it to the brickwork of the stack should get you close.
Flue Liner is sold by the meter so you will have to over order and cut it back when fitted. You could always ask if they can sell to the length required. In this case make sure you add a bit on for any uncertainties.

How Much Will the Flue Liner Installation Cost?

As a rough guide the Component cost would be £310 for a 7m flue using method B. Method A would be an additional £40 ish.  Add or subtract £30 for every meter difference. 

So that would be the DIY price but what if you don’t fancy working at height and would rather someone else did the high level work.  A simple method A installation can be done from ladders and roof crawlers so a jobbing builder may only charge around £100.

Method B is a different job altogether and will be dependent on what access to chimney is like and if the builder is prepared to work without scaffolding. I would think £200 – £300 would be closer to the mark with maybe an additional £350 - £400 for scaffolding.  If this is this case then it’s worth seeing what other jobs might need doing whilst he’s up there to make it more economical. 

If you don’t fancy doing the bottom end (inside work) yourself then the cost will increase dependant on what state the opening and chimney stack is in. For a simple job another £300 ish should finish of the job and paying for building regulations approval, if you are not using a register HEATAS installer will bring the total to around £1000 plus whatever the stove costs.

For our Cottage renovation we are installing two liners. The Chimney stack was in poor condition, requiring a partial rebuild so the pots where off and we had scaffolding up as an extra to what we needed for the re-roofing work.  The builders, Trevor and Terry agreed to let me film as they put the liners in for me.

NEXT: Full explanation and details of flue liner installation here

Any Questions or Comments? please leave them over on the blog

For more about Log Burning Stoves "Living with a wood burner, are they worth it?" please start here

Various other information and details about the Whitby Cottage renovation here