It is common practice these days to line your chimney with a stainless steel liner (Flexi-Flue). There are various reasons for doing this but the majority are used in conjunction with stoves. Some stove manufacturers recommend a flexi liner, some will only issue extended warranty if one is used!
There are two types or "grades" of liner commonly available, 316 and 904.
316 grade is only recommended if you want to burn wood only.
904 grade is a bit more corrosion resistant and can be used on multifuel stoves but is more expensive.
We recommend you only go for 904 grade otherwise you may find it's a false economy.
Here's the problem:
We have come across many flexi liners that have corroded in a very short time from new (2 to 3 years!!)
The flue gases and soot from smokeless coal is VERY CORROSIVE. It attacks your stove, your liner and any other metal it comes into contact with.
Your stove manufacturer may indicate that smokeless fuel can be burnt on their stoves.
The flexi liner manufacturer may indicate that smokeless fuel can be used with their liners.
Your coal supplier may tell you that it's fine to burn smokeless fuel on a multi-fuel stove.
We can tell you that if you burn smokeless fuel with a stainless steel liner you WILL be replacing it far sooner rather than later!
Our best advise : DO NOT BURN SMOKLESS FUEL WITH A STEEL LINER !!
It's usual to have a 10 or 20 year warranty on a stainless steel flexi liner. However, the manufacturer is only liable to "replace the part", ie the liner, they almost certainly won't be liable for the cost of removing your corroded one and fitting the new one!
We've even seen stainless steel twin walled flues corroded by smokeless fuels.
Regular sweeping helps as it removes built up acidic soot from the flue but it cannot prevent corrosion ocurring in the end.
Also we should point out that if you have a liner coated in creosote/tar from burning wood it is unlikely that the manufacturer will replace it under warranty, they will state that clearly you haven't maintained it with regular sweeping!
This photo shows what happens to a flexi liner when it corrodes. Smokeless fuel was used (as per the installers advise) and 2 years after installation this is the result. The chimney had been swept twice a year. The liner was replaced free but the customer had to pay for the old one to be removed and the new one installed!!