Which is the better chimney liner – 316 or 904?

Which is the better chimney liner – 316 or 904?

There is nothing in Building Regulations Document J (or its NI or ROI equivalents) which stipulates that you should use a particular grade of stainless steel flexible liner for solid fuel stove applications. Further, Hetas recently stated "Hetas do not perceive the benefits of the 904 grade of liner to be greater than those of 316; so long as installation, maintenance and use of the liner / appliance is within guidance supplied by the manufacturer / user instructions, both are accepted as suitable".

Are there any particular reasons that I should choose one grade over the other?

Stainless steel is used for chimney liners because of its high chromium content which provides greater resistance to corrosion from acidic condensation caused by incomplete combustion from either slumber burning, burning wet wood, burning a mixture of wood and coal or burning coal with a high sulphur content (eg bituminous house coal aks smoky coal). However, prolonged exposure to condensates could still eventually make a stainless steel liner rust.

The two stainless steel grades used in flexible chimney liners, to give them their correct classification, are 316L and 904L. The L is important as 316 and 316L for example, have different chemical compositions and physical characteristics making them suitable for different types of applications. The same is also true for 316L and 904L where the proportion of iron, chromium, nickel, molybdenum (a white metal chemical element highly resistant to corrosion), copper and other ingredients varies between the two to make 904L slightly more corrosion resistant to sulphuric acid than 316L. The difference between the two has absolutely nothing to do with how thick the stainless steel is. Flexible liner skins are all of approximately the same thickness, somewhere between 0.10mm and 0.12mm thick for each skin.

The slightly superior corrosion resistance of 904L grade flexible liners has led to certain assumptions within the stove industry – that because a 904L liner will take a little longer to corrode in the same conditions as a 316L liner it is therefore much better, consequently worth more and also worthy of carrying a longer guarantee. But consider this: since 316L and 904L are both highly resistant to condensate corrosion and since destructive acidic condensates can generally be eliminated by avoiding incomplete combustion and burning the wrong fuels, then why choose the more expensive 904L grade liner? If you don't operate your stove correctly then you're eventually going to damage your liner, whether it's 316L or 904L.  

One further observation: all domestic factory made twin wall insulated stainless steel chimney systems use 316L for the inner lining (and 304 for the outer) and to the best of our knowledge none uses 904L on the inside, the part which is in contact with the flue gases and condensates, and would be most likely to corrode.

Flexible liners are not seen within the industry as a permanent solution either, so in our opinion, you should take the various warranty periods offered by manufacturers and their onerous terms and conditions with a large pinch of salt. Ask yourself, is the manufacturer likely to be in business in 30 years? Will you actually be living in that house for the next 20 or 30 years? In addition, if you upgrade the appliance at some point in the future then an installer will most likely need to install a new liner, irrespective of its condition, to ensure that the correct lining is used and, more importantly, to enable them to certify the installation.

To sum up...

If you insist on slumber burning for long periods, burning wet wood, burning poorer quality coals (none of which is a good idea) or operating a boiler stove where the flue gas temperatures are cooler and condensation is more likely, then you may feel there is some justification for specifying a 904L grade chimney liner. Other than that, a good quality well made branded 316L liner conforming to BS EN1856-2, when properly cared for and regularly swept, will suffice for most domestic stove installations.