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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorwookey
    • CommentTimeJun 13th 2019 edited
    My boiler flue needs to get about 160mm longer when adding EWI. The obvious thing to do is buy an extension piece, but whether that goes on the inside or the outside you will end up with a join inside the wall structure. Seems to me there there is likely to be a rule against this (non-inspectability, leak-risk inside structure) but I haven't actually found one yet. A continuous pipe is obviously better. Anyone have chapter and verse?

    (On the other hand, there is a telescopic flue kit, so there is not an absolute prohibition: https://viessmanndirect.co.uk/Catalogue/Flues/Flue-File/Flue-60100mm-Telescopic-Flue-Kit-7411961 )

    The Viessman technical flue guide seems to be silent on this matter: https://viessmanndirect.co.uk/files//4086178a-0fae-403b-8a53-a8eb0114ba8a/1805%20Vitodens%20Flue%20Technical%20Guide.PDF

    On mine (viessman) it's easy to buy extension pieces, but hard to buy a new 'balanced flue external wall connection' (i.e just the final piece through the wall that the cowl fits into0mmm (which I probably cut down by about 160mm when it was originally fitted...). That only seems to be available as a whole 'horizontal flue kit' so you end up with a set of pointless spare bits (elbow, boiler connection). So just adding an extension is cheaper and easier with less wastage.

    Plenty of people must have done this/had it done. How was it done on your place?
    Sorry, Wookey, I cannot answer your question, but will 'listen' attentively for answers, since a client will have a similar issue with a Worcester-Bosch boiler. I had provisionally arranged a gas-fitter to do the extension, and knew jolly well in the back of my mind that I would not be connecting wastes within the thickness of the EWI, but had not made the connection (sorry) in order to raise the question.
    • CommentAuthorbhommels
    • CommentTimeJun 13th 2019
    Hi, I have had a Viessmann installed last November. I have no EWI at the moment on the wall where the flue sits, but will install 200mm later this year. The installer was going to leave the flue pipe 210mm longer, but accidentally did what he always does and cut it down to fit the existing wall perfectly :-(
    He left the extension bit with me and did not see a problem with fitting it in afterwards. We'll see though.
    From taking a peek at the installation process it looks like the extension pipe can be used to replace the "middle bit" that sits between the elbow on top of the boiler and the flue cap at the end of the assembly - or am I wrong here?
    • CommentAuthorwookey
    • CommentTimeJun 14th 2019 edited
    From taking a peek at the installation process it looks like the extension pipe can be used to replace the "middle bit" that sits between the elbow on top of the boiler and the flue cap at the end of the assembly - or am I wrong here

    The end piece 'external wall connection' has holes at the end to hold the end-cap piece on (it clips in). Extension pieces have one plain end, but so far as I know they don't have the holes. Now you could probably drill them and make the end-piece fit but the design is that extension pieces go _between_ the elbow and the external wall connection which always has to be the last piece. Which is kind of annoying because a 1m extension piece (plenty long enough) is £30 whilst a flue kit (ext wall connection + elbow + terminal + hole-covers + boiler connection) is £78.

    I phoned the supplier and they confirmed that the final piece in the chain needs to be the external wall connection, which you can't buy on its own. Grrr.

    I'm still tempted to try doing it with just an extension piece to avoid wastage, but it may not work. The spec says that the pipe+terminal is 1120mm but it also says that the max wall thickness is <475mm. This makes no sense to me as it's just a pipe, and you need 140mm to connect to the elbow internally (for a boiler bolted directly to the wall), and the terminal is 155mm, so why can't the wall thickness be up to 825mm (1120-155-140)?
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