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. Furan-Flex liners are made of a polymer type material which has a 25 year life time guarantee but being expensive is 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 looks & 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 just 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 try 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.

The German Connection

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.

The award-winning German chimney component manufacturer Schiedel 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 ??

Condensation and other issues that show up in cold weather.

At this time of year when the weather gets cold enough to start affecting your fire or stove you may be surprised or even shocked when things don’t seem to work the way they normally do.

Appliances have Running Temperatures

Stoves have a running temperature and take time to reach this point , you may find they won’t start up so easily and/or are Smokey for the first 10-15 minutes. This is due to the cold air dropping down the flue but once up to running temp the hot air rising displaces the cold air as it pushes its way up and out the chimney. You may need to try heating the metal baffle plate with a blow torch before attempting to light your fire or if it is not metal , you could try the Scandinavian method of building your fire upside down i.e. paper on top> kindling>wood/coal.

Tar or Black Liquid

Today a customer called worried about black liquid tar coming down into their stove. You too may have found a sticky , black liquid in your wood-burning stove and you might have spotted it running down the inside walls of your appliance or settled somewhere in the firebox. The consistency reminds you of tar because that is exactly what it is.

If you have seen this black liquid, it indicates that you are not operating your stove correctly and in particular , burning unsuitable (damp) fuel.

Usually tar is created as a by-product of burning logs on a wood burner when the moisture level of the wood is too high. Since energy is being wasted evaporating water, the stove doesn’t get up to high enough temperature, which leads to cooler gases going up your chimney. Since they are cooler, they are prone to condense when they touch the cold metal of the flue liner.

The condensed gases will either solidify on the inside or the flue or start to drip back down into the stove in the form of the sticky liquid. Whenever the tar does get the chance to solidify it will turn into creosote. A build-up of highly flammable creosote in your stove system significantly increases the risk of chimney fires. This is especially true if you burn soft wood such as pine , the resin being the main culprit.

How to stop the black liquid appearing in your wood burner

There are a few ways to stop the black liquid appearing in your wood-burning stove. The first is to ensure that the only logs you burn are ones that have been correctly seasoned and have low moisture content (<15-20%), this is easily achieved by using a moisture meter.

Ensuring there is a strong draw up your chimney, operating your stove at full capacity and burning it with the air intake fully open for 10-15 minutes once or twice a day will blast off the vapor and burn any sediment that settles on the flue. You can also try using some table salt on the burning wood to try to help absorb any water that is created. Finally , not slumbering your stove over night and also ensuring your room is well ventilated (especially if you keep damp logs in the room too) will minimise the chance of flue gases condensing in your chimney.

It is also important to get your chimney swept regularly as this will remove any residual creosote that has built up in your chimney and prevent it seeping back down your chimney and into the stove. Regular sweeping will also protect your liner from corrosion and eventual destruction.

Cool Fuel Rules

There are many fuel types available to the solid fuel user: coal, peat, wood logs, compressed wood products, smokeless coals. With so much negative information in the press these days … which is the right fuel for you to burn?

Firstly , you must look at your appliance and follow the manufacturer guidelines as burning the wrong fuel type can be dangerous and at the very least invalidate your warranty. Good sweeps are generally a hive of knowledge.

If you care about the environment, then you would want to focus more on renewable fuels such as wood logs.

Selecting and purchasing wood logs

Ash and beech are some of the best woods logs to burn however most logs burn perfectly well (see our burning wood post) if they are dry enough and have been adequately seasoned.

Wood should be purchased from a reputable supplier who should have to comply to regulations and have a vested interest in planting more trees to replace what has been felled.

Wood logs must have less than 20% water content when they are burned. This can ascertained several ways, (see our post on moisture meters) but the easiest and probably most reliable at a price is to purchase logs from woodsure who will have done all checks and tested the moisture content in random batches. You will also be able to complain if the wood you purchased is too high in water content.

If you choose to purchase unseasoned logs then you must season them (typically for 18 months or more) and after seasoning you must test the moisture content with an electrical conductivity moisture tester prior to using them. (again for more on this see our post on moisture meters) Users burning wet wood are one of the biggest problems and challenges our industry faces but it is one of the easiest fixes too.

Why mixing fuels is a problem

Wood and coal burn quite differently. Wood burns through a process of gasification where the volatile oils within become heated, turn to a gas, mix with oxygen above the log and then eventually combust as a mixture of gases.

Coal on the other hand is primarily a carbon, to burn efficiently oxygen must enter and mix from below the fuel. Combustion takes place inside and not above the coal itself.

There are also distinct differences between wood and coal burning types of appliances, such as a grate below or just flat metal base.

Some issues associated with the incorrect use of fuels

  1. Wood on top of coal

If you try to burn wood on a bed of coal you would need to open the top air intake, thus drawing air above the coal and not through it from below. Therefore, the coal would burn poorly and produce high levels of carbon monoxide and carbon dioxide, which acts as a fire suppressant/extinguisher and the effect is compounded by the inadequate heat from the coal.
Wood burns by first emitting volatile hydrocarbons which then become gaseous.
Normally these hot gasses mix with oxygen and burn with the yellow flame we associate with wood. However, in the reduced oxygen environment above a coal bed the volatile gas emitted by the wood log will instead travel into the chimney where it then deposits as a flammable creosote.
Another issue is when combustion temperatures and oxygen levels are too low, then the later hotter stages of gaseous combustion will not effectively take place. During these later phases, carbon soot particles should be burned.
If these particles are not burned they will then exit the chimney as fine dust pollution harming our air quality with all the associated risks.

2. Coal on top of wood

So we understand that coal requires oxygen from below to burn effectively, think bellows in a foundry. If we burn wood below a coal bed we will have a situation where our coal is bathed in carbon dioxide leaving an incomplete burn. The second issue here is that wood produces lots of ash which in turn blocks the ash grate, further lowering the amount of available oxygen to the coal.
If the coal used is smokeless, such as anthracite then high levels of sulphurous compounds will also be emitted. These compounds are highly corrosive and will eat through stainless steel liners and most pot coverings in next to no time. The one exception to the wood underneath coal scenario, is when kindling a fire with wood. Here it is very likely that the air intakes or even the door will be open in order to provide enough oxygen for both fuels for a limited time.


The term slumbering describes the process of the user loading the already burning appliance and then closing down the air supply. This is in an attempt to make the fire burn for long periods, typically overnight.

The big problem here is the fact that fuel requires oxygen for combustion. When oxygen is limited in a combustion reaction, there are several results that occur as are shown below.

  • Much of the volatile content within the fuel is emitted due to the reduced heat but it does not combust. It rises up the flue coating the chimney with volatile fuel, greatly increasing the risk of chimney fire and the requirement for sweeping.
  • Carbon requires lots of oxygen and heat for the particles to combust. When the oxygen supply is limited, dangerous sized carbon particles are released. Many of these particles make it to the outside air as fine dust pollutants , polluting the environment and causing health problems such as respiratory and lung diseases.
  • Heat or the lack of. In order for a fuel to release its stored heat, it must react with oxygen, it being the catalyst for a combustion reaction.
  • Cost. Slumbering will shorten the life of stainless steel liners, increase the need for servicing, sweeping and maintenance. It will reduce the heat value that you should get from the fuel, making it less cost effective too.

In conclusion

Ideally you would only burn dry wood logs in an environment that has adequate air/O2 for moderate flaming combustion to take place. This is achieved by adjusting the air supply so that flames are not sucking up the chimney but not limited too much. Smoke should not be visible in the firebox, only in the fire.
Also by adding only as much fuel as is needed to fill the fire box with flames, because burning too much or too little fuel at a given time is another mistake to be avoided.

Your chimney sweep should give tips on fuels and how to achieve adequate flaming combustion, fuel storage, types, sweeping frequencies and many other important factors surrounding good use practices.

Multi Fuel or Wood Burning stove.. What’s the difference?

Traditionally, stoves were intended to burn wood and only wood, then multi-fuel stoves were created to give you the option of heating your home with either coal or wood.      So , ..    Multi Fuel or Wood Burning stove ?
First off, you should know that it is NOT advisable to burn both coal and wood in your stove at the same time as this can and will damage your flue lining and stove itself. 

Why is it not wise to mix fuels 

Why can’t you mix fuels ? because corrosive materials such as halogens and sulphur found in coal will combine with the moisture in wood creating nasty, Sulphurous acid & other solutions that stick to and then erodes your whole stove system.
Whilst multi-fuel stoves bring with them the versatility of being able to choose which fuel you’d rather use at a particular time , their one limitation is the use of  ONLY One fuel at a time! Mixing fuels is not cool ! 

Only coal as fuel? ..Simple use Smokeless

You should always check your owner’s manual for the final word from the manufacturer on what fuels are recommended for use. Although most multi-fuel stoves are equipped to burn normal house coal, (aka bituminous coal) often most stove manufacturers will advise against this because of the high amount of soot in house coal.
**High soot content results in your stove system becoming rapidly clogged up**.
Using smokeless coal avoids this problem by agglomerating particles and reducing the amount of smoke and soot going up your flue , is also better for the environment and thus suitable for use in smoke control areas.

Knowing which stove is which , just by what you can see 

Multi Fuel or Wood Burning stove , the most obvious examples you can see immediately by eye are the:

1. External primary PRC & SRC secondary air flow regulator controls and 
2. Internal raised grate of multi fuel burners or the flat bottom non grate of wood burners.

Coal burns best on a ‘raised grate’ since it needs an air supply from below. Therefore , the primary airflow intake is below and its manually adjustable Primary Regulator Control outside (usually, somewhere near the bottom of the stove door & is normally either a sliding slat or turning knob/disc) is used to burn it effectively.

multi fuel burner showing SRC & PRC annotated in green   Multi Fuel or Wood Burning stove

1. The PRC [below discs]  &  SRC [above slats] Regulatory Controls of a multi-burner

Wood doesn’t need this additional air supply below therefore , wood burners come with only a ‘flat bottom. This limits the air supply to the fuel, resulting in a slower burn and you will see they ONLY have a top secondary air flow intake with manually adjustable Secondary Regulator Control. 

****        The adjustable multifuel grate in the picture below shows it can be both raised (top) allowing upwards air flow or it can be flattened (bottom) stopping upwards air flow from below.    ****

multi fuel adjustable grate Multi Fuel or Wood Burning stove

2. Multifuel adjustable Stove Grate

You will find that wood burns faster on a multifuel stove than on a normal wood-burning stove because of the extra oxygen and air flow from below. 

side view of multifuel stove showing air intakes Multi Fuel or Wood Burning stove

3. Primary, Secondary & Tertiary air intakes .. stove door to Left of pic

So what do I want to buy then?

In short , what to buy depends on what you are going to burn? If you’re thinking of using wood as your main fuel then it’s best to buy a wood-burning stove. By buying a multi-fuel stove, you are already almost committed to coal being your main fuel (but keeping the option open to burn wood if need be, albeit at a lower efficiency) and wood as an option.

Therefore , for the reasons already mentioned above , smokeless coal is your prime candidate fuel for a multi fuel stove. So you will be considering Anthracite, as well as a host of brand name alternatives such as Taybrite and Phurnacite. 
However , as you will see in our article on stainless steel liners there are some drawbacks to using these smokeless fuels. 

 Things to keep in mind::

1. Normal house coal is cheaper but too dirty for stoves 
2. Smokeless is expensive and more corrosive but burns clean enough & is better to use in stoves than bituminous
3. Unless you have a free source of weathered hard woods then having a multifuel means using smokeless coal IS the option. 
4. Don’t ever use wood and coal together at same time, water in wood dissolves sulphur in coal creating sulphurous acid !! ALL coal has sulphur , some worse than others.
5. Best to have a china hat/cowl to deflect water ingress. All lum/flue dampness causes sulphurous acid corrosion, esp with smokeless fuel
6. Smaller (within stove capacity) hotter hardwood fires are cleaner and don’t ruin baffle-plates or warp stoves
7. Read about liners on this site, 316 SS for wood and 906 for Anthracites NB/ sulphur ~ halogen corrosives
8. In olden days damper plates were used to control burning[often slumbering Tutt Tutt!!] by choking the fumes exhaust rather than the air intake itself reducing combustion. This is dangerous due to the accumulation of toxins rather than reducing their creation in combustion control itself. 

Using a wood [you see SRC only ! Flat Grate] only stove:-
You would be controlling the flames ONLY by the Secondary air flow intake [see picture 1 above] by manually adjusting the Secondary Regulator Control. Correct settings would be seen as curling yellow flames emanating from the mixture of gases combusting above the fuel on the grate.

Using a multi [you see PRC & SRC Raised Grate] fuel stove:-
You would control the main flames by using the Primary Regulator Control [see picture 1 above] then adjust further also with more tighter control to satisfactory conditions by the SRC [see picture 1 above]. Visually blue/orange flames would be seen below and yellow ones above the fuel.