Terracotta flue blocks.

[Robbo]

Hello to you all - I am installing a 14kw woodburner into the first floor living room of our barn conversion - there is no existing fireplace hence I am building an internal flue using double skinned terracotta blocks, these are rated at 1000 degrees celsius max. The blocks are supported on a metal frame cemented into the stone wall at about 1.5m above the hearth and rising 4m up out of the roof.

I have two burning questions -

1) Do I still need to line the terracotta blocks with a single skin stainless steel system or are the blocks suitable for woodburners in their present form.

2) With what cement material should I backfill the flue against the stone wall (about 1 - 2 inches gap). I was going to use a basic 1 - 5 cement sand mortar but have read there are specialist compounds designed for the job.

Got windy about this job hence any advice would be gratefully appreciated - Thanks - Bruce

[Théière]

I believe the only flue pipe that can be used for wood burners is twin wall and the cavity between your terracotta should also be filled with an insulation like vermiculite. Single skin should only be used for gas appliances.

As the mortar is outside the flue a general mix should be ok but may still crack with the heat if not warmed up slowly. Refractory cement is often used for BBq's where the heat is direct.

The best people to ask is Poujoulat

[Robbo]

Thanks Theiere,

You are perfectly correct as a I have discovered - apparently the twin walled terracotta blocks are equivalent to the stainless steel piping in function - but at a fraction of the cost - bit more work but heh, what else have I got to do!!

I will source the vermiculite and use that. Thanks again for your time and advice.

Bruce

[Araucaria]

Our builder and our heating engineer said that the twin walled terra-cotta blocks cannot be used directly as a flue: they must be lined, though I gather a single-skinned flexible stainless steel flue is OK as a lining inside these.

I don't see how you'll fill the small cavities in the blocks with vermiculite: at each of the mortared joints you'll have a potential blockage, though perhaps if you fill each block before mortaring the next one up you might succeed.

Sorry to post a contrary view, but this quite definitely what we've been told by the local professionals. And what we have had to do, too.

Last thing: the blocks are marked with a small arrow showing which way is up. Our not very competent builders put all ours in upside down, but the heating chap said it didn't matter much.

[Théière]

No you don't have to fill the small cavities, only the large one outside the S.S. liner and inside the main cavity of the terracotta blocks.

As I said, I am not sure of the latest reg on twin walled/single walled so would check with Poujoulet. I know wood burners must use twin walled under the HETAS regs but friends recently had a HETAS installer fit single wall but tested the chimney first. [8-)]

[pachapapa]

In Gourgé the boisseau  are installed to conform to NF DTU 24-1 and 24-2.

http://boutique.cstb.fr/dyn/cstb/upload/fichiers/ext_cheminee_2.pdf

[pachapapa]

€ 18,01 HT could be a pedagogic investment.

Guide Pratique " Construction d'une cheminée "
Atres, appareils à foyer ouvert et inserts. Conduits maçonnés et métalliques Édition 2007
Format papier	124,37 FF HT	18,96 Euros HT
Offre spéciale jusqu'au 31/10/2010	118,14 FF HT	18,01 Euros HT

[Robbo]

Thank you for all your replies, I have taken the plunge and startyed the chimney. Not as hard as I thought.