COSI is run by Eur.Ing. Graham Thornhill C.Eng.


Since 1966 all chimneys built had to be lined when they were constructed. There are many different methods and materials and all have their uses, advantages and disadvantages. Chimney lining can be roughly divided into three categories; metallic, ceramic, & refractory concrete.


This is the commonest method for newly built houses. Terracotta low fired square and round liners with socket ends are used. These should be installed (as all liners should) with the male downwards, and back filled with an insulating lightweight concrete. The disadvantage of these is that they have a very low insulation value and so tar up easily. A chimney fire can cause them to crack and shatter, the only solution is to remove them and reline, quite a major job and one that COSI is having to do more and more of. Top quality ceramic liners are available for the relining market that can withstand thermal shock and high temperatures but, of course, these cost considerably more. In short the terracotta lining is the bargain basement of the chimney linings and is only suitable for low cost new build.


This can be divided into three sections;-

The cast-in-situ method is a method of relining or lining an unlined chimney. It gives a high insulation value helping to reduce tar and condensation formation and can withstand some thermal shock of a chimney fire should the worse happen. With this system the chimney is cleaned and opened up every 6 feet and at all bends if the operator is following the Nation Association of Chimney Engineers code of practice. A pneumatic former (giant sausage) is lowered down the chimney and inflated. Spacers of pre-cast material are used to hold the former in the centre of the chimney. A very fluid refractory concrete is then pumped around the former, & the holes sealed and plaster made good. 12hours later the former can be removed. The problem with this system is that some companies don’t cut enough or intact any holes. One national company has even been heard saying that tube floats to the middle of the mix!.

Main advantages

Refractory concrete is a material that should withstand minor chimney fires, high insulation factor should help reduce tar build up. ‘Soft’ mix should help reduce noise.


Can considerably reduce the cross sectional area of the chimney. Especially with a 225mm X 335mm (9″ X 14″) chimney which can only end up with a 7.25″ diameter flue too small for most open fires. Can only be used (without extensive opening of the chimney breast) if the mid-feather bricks are in ‘reasonable’ condition. The mixture is very wet, it has to be to pumped, and to flow 30 feet down the side of the former without sticking and forming voids. Approximately 35 litres of water to every 75 litres of mix, in other words about 125 gallons or 1/2 ton per chimney. The hydrostatic pressure at the bottom of the chimney can blow the wall out on a very large opening (yes some people in the past have tried to fill up inglenooks) and mid-feather brick can catastrophically fail just as you pump the last few litres in. This system is very operator dependant and is a bit like the ‘little girl with the curl in the middle of her forehead…….’

For the above reasons COSI have made it a policy to only cast 150mm (6″) liners for wood stoves & solid fuel room heaters. Where a solid fuel room heater is installed in a standard chimney (9″ X 9″) chimney this method would probably give the longest life if done properly compared to 904 grade stainless flex liners. It is essential that the company is NACE registered for this type of lining. Short cuts cannot be taken with this method.

Pre-cast; Lining sections can be purchased for new build that have the same properties and cast-in-situ and form better chimney linings especially for wood burning. They are more expensive and will add about £200.00 to the cost of a chimney in comparison to terracotta. A price well worth paying if it is your own house. These liners are available from COSI, please ask for further details.

Cold Set Ceramic Refractory Resin;

Some new materials have been developed in Sweden that will form a thin coating over most surfaces. With this system the brick, concrete or cracked clay liners are cleaned with wire brushes. A cavlar cloth backed with shaped foam is placed at the base of the chimney and the premixed resin liquid poured down the chimney. The sponge and cloth are then winched up as more resin is poured on top. This process is then repeated as required. It sounds crude and a bit hit and miss, but camera surveys always reveal good results.

Main advantages;- Does not alter the cross-sectional area of the chimney. It is therefore ideal for open fires, especially the larger ones. Does not need Building Regs. as it is so thin, but is guaranteed for 10 years.

Main disadvantages;- As with the Cast-in-Situ it requires a fairly sound chimney to start with. We always camera survey first and any holes have to be repaired by cutting into the chimney first and bricking up. Scaffolding is always required as you have to stand and winch.


This can be divided into four sections;- single skin flexible, twin wall / smooth lined flexible, solid single skin and solid twin wall.

Single skin flexible.

Used for gas or oil installations. Made of grade 316 stainless steel these have a good life expectancy and are only usually replaced when the appliance is changed. For gas the void around the liner must be sealed top and bottom and a suitable terminal fitted. For oil the void should be filled with a dry insulating material, (vermiculite, perlite or leca) unless it is too big, when insulation (rockwool or fibreglass) can be wrapped round the liner as it is lowered.

Twin wall flexible

A twin wall smooth bore multi-fuel flexible liner is available for relining. Care must be taken when sweeping this type of liner and only brushes with a rounded top should be used.  It has a very limited insulation value, and if not back filled with a dry insulation, (vermiculite, perlite or leca), can tar up reducing its useful life. COSI install many flexible liners each year but we always insist on back filling/insulating to ensure a long life.

Main advantages

Cost, often no holes need to be opened in the chimney breast, manufacturers ten year reducing guarantee. Can sometimes withstand a chimney fire, easily & quickly removed and replaced after a chimney fire. Ideal & most commonly used for wood and multi fuel stoves.

When using with boiler stoves, the higher, more expensive 904 grade stainless should be specified.

Main disadvantages

Fairly delicate until inside chimney, on-site care and careful handling is required by installers..

Solid single skin

Flue pipe section in 1mm thick grade 316 stainless steel make an incredibly strong and long lasting liner that can withstand minor chimney fires. More expensive and more difficult to install than the flexible.

Main advantages;- Easily available, very strong, should withstand chimney fire, last few sections only need to be replaced in the event of a fire. Very smooth so easily cleaned.

Main disadvantages;- Fiddly to fit if a lot of bends, fairly large hole needs to be cut in a 9 X 9 chimney to get length down.

Solid twin wall

If you have no chimney a solid insulated twin wall stainless flue can be erected inside or outside your building and can be used for any appliance or fuel. With an inch of mineral insulation, tar will not be a problem if dry wood is burned.

Which method should be used?

In conclusion there is no perfect method, and certainly no one method stands out above the rest. Probably the best is the solid stainless, the strongest, the most likely to withstand fires. The cast-in-situ refractory concrete method when done well is excellent, far superior to the pre-cast sections, but they are also the two most difficult to replace in the event of a fault, failure or fire. The flexible is the easiest to replace but care has to be taken with it, it is the least disruptive, dry, and 95% of all liners done by COSI are flexible grade 316 Stainless steel.

In other words the operator/contractor is more important than the method used. They should be NACE registered as a “competent” installer. All liners and stove installations require building regs. approval. You must inform your Council and register the work yourself (approx. £200.00) or you use a “competent chimney installer” who will register the work for you.

Each chimney should be looked at in its own right and the method chosen to line should be reasoned out. Reliable contractors must be used who have full knowledge of all systems and not biased towards one method alone.

The architect should apply basic quality control, and the work of new contractors checked carefully to ensure they are competent.

All chimneys must be thoroughly swept first, power cleaned if tarred up. Pumped liners must be spaced, pre-cast liners jointed properly and cement washed to reduce porosity. Steel and all pre-formed liners must be back filled and put in the right way up. The method of starting and termination must be thought out.

We at COSI mostly fit flexible single wall stainless steel liners for oil or gas appliances, and flexible twin wall stainless steel liners for wood and solid fuel appliances.