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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 to rust from the outside or 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: choose 904 grade chimney flue liner if you want belt and braces quality or you are going to burn smokeless fuel or coal. Choose 316 grade chimney flue liner if you are on a budget and you only intend to burn seasoned (dry) wood.
Some liners are a mix of 904 and 316 (904 on the inside and 316 on the outside). This is quite acceptable as the inner protects from corrosives whilst the outer protects from rust.
Personally, for the small additional cost I would choose 904 grade liner. I do appreciate however that finances can dictate decisions and 316 grade liner is not necessarily a poor choice.
Liner does not last for ever. If you burn wood with paint on then the corrosives from the paint will shorten the life of the liner quite dramatically.
DO choose 904 grade chimney flue liner if you are installing a boiler stove: a boiler stove 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). Stoves approved by Defra to burn wood in a smoke control area do not generally slumber quite so easily (slumbering is bad for the environment).
Different manufacturers offer different guarantees on liner, with 904 getting a longer guarantee than 316 (25 or 30 years as opposed to 15 years as a rule).
I would not get too excited about the guarantees: if your liner does get damaged in under that time then you will have done something wrong anyway and manufacturer’s can, if they wish, prove this with tests. Even if a manufacturer does replace your liner he is not going to pay for any labour or scaffolding costs which is often the greater expense in these matters.
What is not in doubt is that 904 flue liner will last considerably longer than 316.
If you are self installing and it would not be a big chore to replace a liner then you might consider 316 grade. For example if you live in a bungalow then replacing a liner in the future may not seem such a big issue. But if your property is multiple storeys and scaffolding or a cherry picker is required then, for me, it is 904 all the way.
Still not sure whether to go for 316 or 904? Go for 904 – it should last 30 years plus as long as you burn the correct fuel (smokeless fuel or dry wood with no paint surfaces) and the price difference is not that huge.
Oh… and choose a good brand of liner. Cheapest Internet liner is not worth it.
As a professional stove fitter I would choose Duraflue Silvacore for my own home. Click the image below to find it in the Stove Fitter’s Warehouse (free £100-£300 liner with 99%+ of stoves).