Flue liner for larger chimney, necessary or not?

[tomfc]

Hello, more questions from me! This time it's about getting a multifuel stove for our newbuild. We suggested and pushed for putting in a smaller chimney flue, but our builder was indignant and said he always put in 9in flues... so we now have a 9in (clay?) flue. The builder told us this would work with whatever size of stove we ended up wanting, and wouldn't need a liner. Having spoken to a stove fitter he said that this flue is too large and we'd have to get a 5-6in liner fitted (ie extra cost).

 

I am at the point of giving up taking advice from 'professionals' given that at pretty much every turn so far we have been given literally contradictory advice! But I suppose this is the curse of self-building!

 

It seems the internet too has conflicting advice, from what I've read so far. Part J of building regs seems pretty complicated too. So I come here to ask if anyone can offer any guidance. Do we need a liner? Or if it's not a simple yes/no, then what are the pros and cons? And also do we need a HETAS certificate for the stove and fitting (the fitter mentioned this in passing)?

[PeterW]

Stove fitter is wrong. You need a 5-6” flue coming off the stove itself but that can increase to 9” with no issues in concrete or clay liners. Depending on how the chimney and stove are laid out, you either use a register plate which is a big horizontal plate of steel with a hole though it, or a clay adapter which looks like a stainless cone with two ribbed straight sections that match the stove outlet size and your chin bet size. 
 

Technically you can fit your own but it needs signing off by building control. You must fit a stove plate near the stove with the correct details about the flue, but they are not difficult to do if you take your time. You may need to ensure it’s on you Building Regs application too so it gets signed off with the build. 
 

 

[Crofter]

I would always follow the stove manufacturer's instructions over those of a builder or stove fitter.

I've seen an installation fail where a 6" flue was led in to a 9" clay lined chimney. Stoves can have quite cool flue gasses compared to open fires (which is why they're so much more efficient) so you can get condensation in the large 9" clay chimney flue, because it cools down on its way up. Problem was apparent when brown stains started appearing through the brickwork on the outside of the chimney. Problem solved by fitting 6" flexi steel flue liner and insulating between that and the clay.

 

That was for a house where the stove was used daily for six months of the year. If it's only being used occasionally, you'd probably be fine going straight in to clay, at least for several years.

[tomfc]

Thanks guys, slightly worrying that the larger chimney flue might cause condensation, as we'll probably use the stove fairly often in winter at least.

[PeterW]

6 minutes ago, tomfc said:

Thanks guys, slightly worrying that the larger chimney flue might cause condensation, as we'll probably use the stove fairly often in winter at least.

If it’s new and has been correctly insulated with vermiculite then the clay liner will be fine. What normally happens when they cool down is due to poor install of liners - they should be lipped correctly and sealed - and they aren’t insulated. Used to occur often with brickwork chimneys.