Stainless Steel Flexible Liners & Smokeless fuels: What you need to know.

Occasionally a chimney flue will be in such a bad condition that a liner will be required. This might be due in part to the age of the building but is definitely more to do with the user (or abuser as the case may be) of the flue: as in what they burn, how often and how they burn it.

Prior to the 1950’s , the mortar between the bricks generally consists of old sand and lime material mixes. This tends to disintegrate and dissolve away when the sulphur in the soot mixes with water to create Sulphurous acid (H2S03) which is [NB/only one oxygen away from Sulphuric acid (H2SO4) ] pretty acidic stuff. It is also responsible for the acid rain effects environmentalists have been banging on about for a few decades now.

All coal contains sulphur in it to a greater or lesser extent , with the higher (smokeless) grades having much more & often halogens too unfortunately. I personally , have seen a china hat I fitted to a flue that uses only smokeless fuel last just 18 months (these often last more than 10 years in other conditions) before it fell to pieces.
As a sweep , for me personally it doesn’t take very long to realise I am sweeping a smokeless burning appliance. After the stuff gets on my face , I can literally count to 30 before my skin feels like it is melting off , the soot is indeed horrible stuff to a chimney sweeps skin complexion , never mind his health if inhaled.

However , on the plus side smokeless fuel does create a lot less soot & other pollutions overall.
It can be useful when used occasionally on very dirty open fires. Fires that burn lots of cheap house (bituminous) coal and/or a lot of wood too can benefit by using an odd bag here & there (such as one bag per season) as it reduces the soot volume by aggregating it together.

Chimney Liners

Chimney liners come in many forms , for example in the USA most chimneys are ceramic block types that are rectangular and slot together. Here in the UK some are concrete tubes that have been poured and allowed to set. Some can be made of a polymer type material called Furan-Flex which has a 25 year life time guarantee but this is expensive and not very common , I have never come across one yet. You may often see twin wall (shiny steel or matt black) rigid pipe sticking out of an external wall or through a roof. All liners have their own properties and as such , pros and cons or strengths and weaknesses.

For the most part here in the UK though , flexible Copex style stainless steel liners which come in 2 grades are used. The 316 which is suggested for use in wood burning only stoves and the 904 which is suggested for multifuel stoves. If you don’t know what the difference between wood burning and multifuel types of stoves are , you can read about them here in our previous article from 2015. Although these stainless steel liners are suggested for use in stoves , they are occasionally found in normal open fires too. In this case they are often connected to a hood up inside the fireplace which acts as a fume collector not unlike the hoods you often see above kitchen cookers.

These types of stainless steel liners are composed of two layers an INSIDE and OUTSIDE layer so not all are made the same (read on to schiedel information below).
Some liners are 904 with an external layer of 316 on the outside. The inner 904 protects from corrosives (found in smoke and gases) whilst the outer 316 is a solid extra barrier to rust from a damp chimney. I would always choose a liner with a 904 internal core but I appreciate that finances can dictate decisions and a 316 grade internal core is not necessarily a poor choice if you intend to only burn dry hard wood.

Stainless Steel Grades

The difference is purely one of quality. The 904 grade is a higher grade of stainless steel than the 316 grade and is less likely over time to corrode from the inside. The terms 904 and 316 are terms used within the stainless steel industry and are not peculiar to the wood-burning stove market. If you do not want to read any further: just choose the 904 grade high quality flue liner especially if you are a “heavy user” and DEFINITELY IF you are going to burn smokeless fuel. Choose the 316 grade flue liner if you are on a budget and you intend to burn ONLY seasoned (dry) wood as a light to medium user.

As I said above in the opening paragraph , the state of your flue is more to do with the user/abuser as the case may be , liners do not last for ever. Even if just burning wood which is damp or with paint on the surface , the dampness & corrosives from the paint will shorten the life of the liner quite dramatically.

Definitely choose 904 grade chimney flue liner if you are installing a boiler stove because it is more likely to be slumbered overnight and when slumbering , less of the nasties in the gases are burned away. Any stove that is likely to be slumbered will benefit from a 904 grade liner (slumbering is when you turn the air supply right down until the fire is almost going out but not quite and is NOT recommended nor is shutting off exhaust vents by use of a damper plate in the flue pipe, for many reasons but mainly because it is blocking the monoxide escape path!).
Slumbering is also bad for the environment , puts you at risk from monoxide and tars up your flue , so your chimney sweep won’t be too happy either.

Different manufacturers offer different guarantees on liners. Those with a 904 core get a longer guarantee than a liner with a 316 core , generally 25 or 30 years as opposed to 15 years as a rule.

However , don’t get too excited about the guarantees: if your liner does get damaged in less that time then it is probably likely YOU have done something wrong and manufacturer’s can , if they wish prove this with tests. So this kind of situation can quickly descend into a blame game.

You should get your liner swept regularly (your chimney sweep can help by producing their records to show this if required) and take all possible precautions (burn a correct fuel , in the correct way , have a china hat fitted to deflect rain and use ALL other measures to avoid dampness in your flue) to ensure your liner is protected. Even if a manufacturer does replace your liner , they are not going to pay for any labour or scaffolding costs which is often the greater expense in these matters.

Not all liners are made the same

Liners with a 904 core should last considerably longer than liners with a 316 core , although some sweeps would definitely disagree with that statement and are of the opinion there is not much difference between the two grades of steel , especially once corrosion has set in. NB// Bear in mind not all liners are made the same so it always pays to buy a better quality brand than a cheapo version!! Also , DIYers do NOT be tempted to save money by using a Gas fire liner on a solid fuel appliance , as they are single cored and are in no way rated for solid fuel use.

In the chimney sweeping world you would be hard pressed to find any country more obsessed with high standards than Germany. The Schornsteinfeger is quite an important person and indeed has a lot of power as you can see here in Wikipedia.


Schiedel is an award-winning German chimney component manufacturer and its premium TecnoFlex liner is widely used by HETAS and Oftec installers all over the UK. Formed in a completely different way than most other liners , TecnoFlex has an unbreakable lock between layers which prevents it from pulling apart and keeps the inside super smooth for easy down drainage of condensates. This super smooth layer also helps prevent soot and tar from building up on the inside of the liner potentially preventing blockages. Schiedel liner material is imported from Germany and finished off here in the UK by skilled technicians.

They make and sell the highest grade liner sold in the UK – 904/904 grade liner – NOT 904/316 like many other manufacturers – which is perfect for wood and smokeless coal burning but NOT at the same time !!!

Why?? because the moisture in the wood will mix with the sulphur in the coal soot creating sulphurous acid…. remember ??