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Green Building Bible, Fourth Edition
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    • CommentAuthorDabbler
    • CommentTimeNov 26th 2017
     
    I am just beginning the technical design stage of a retrofit and am looking at providing an air supply for the existing wood stove.
    I've noticed a couple of firms supplying chimney systems that draw the air for the stove down the chimney alongside the exahust flue (eg POUJOULAT efficience). These are all rigid systems for new build. Has anyone seen a flexible flue system for retrofit?
    I'd imagine them having a specialist terminal, then dropping a second smaller pipe for the air supply down alongside the flue liner.
    There are probably very good reasons why they don't, the care needed to make certain the flue is large enough, the care needed to avoid kinks when fitting etc. But it would be a lot simpler than running a air pipe through the sub floor to the outside.
    • CommentAuthorgyrogear
    • CommentTimeNov 26th 2017
     
    Posted By: DabblerHas anyone seen a flexible flue system for retrofit?


    FWIW, I have a flexible French retrofit flue, it is twin-wall stainless steel, smooth on the inside and corrugated on the outside, inside diameter 153 mm. It is called "polylisse" and costs around 28 euros per linear meter.
    Other diameters available.

    (The important thing with a retrofit, is to ensure that the existing flue is not just CLEAN, but totally free of coke, as high temps in a retrofit flue can cause offgassing, with potentially fatal consequences...).
    =====
    (for the moment) I get my outside air via a wall penetration in the basement, then an (open) hearth penetration through the lounge floor.

    With your proposal, I'd be worried about dragging combustion gases back down the air feed, with dubious consequences on stove performance...

    gg
    • CommentAuthorowlman
    • CommentTimeNov 26th 2017
     
    I once fitted a "hole in the wall/picture frame surround", type gas fire for a client. Quite a big machine and very heavy with remote controls. I had to construct a fire resistant chamber in the false stud wall to accommodate it, I digress; however, the balanced flue was just as you describe, with one twinwall flexible pipe inside a larger one. I can't remember what the terminal was like but it all worked fine, I think the exhaust flue was the inner one. In the case of a woodstove I guess it would have to be to facilitate sweeping, and with many wood stoves requiring 5" or 6" flues that would be a start point, and work up. Then deciding if the area remaining of your outer was enough to provide sufficient combustion air.

    The bottom, stove end finishing detail may be fun and games though. :bigsmile::wink:
    • CommentAuthorgravelld
    • CommentTimeNov 27th 2017
     
    I don't understand how this type of system would really differ for a retrofit compared to a new build? i.e. what makes it unsuitable for a retrofit?

    It's all about the design of the dwelling surely.
  1.  
    I read a translated Finnish study on using external supply air for stoves. The efficiency of the stove dropped by 15% when O Degree external air was used, the conclusion was that there was no advantage to using external air.
    • CommentAuthorDabbler
    • CommentTimeNov 27th 2017
     
    Many thanks for looking at this for me.
    It's definately having proper fittings top and bottom that are the key to avoid the mixing of gases. So unless it's commercially available with the proper testing this idea is a dead duck.
    On the Scandinavian front I've also heard they can take a different view, but haven't steeled myself to do the reading.
    • CommentAuthorowlman
    • CommentTimeNov 28th 2017
     
    Posted By: Viking HouseI read a translated Finnish study on using external supply air for stoves. The efficiency of the stove dropped by 15% when O Degree external air was used, the conclusion was that there was no advantage to using external air.


    I guess that would be a plus for the balanced "flue in flue" approach, as the incoming air would be pre-warmed.
    • CommentAuthorgyrogear
    • CommentTimeNov 28th 2017
     
    Posted By: owlmanthe incoming air would be pre-warmed.


    therefore less dense, therefore less oxygen (one supposes...)

    http://wiki.gekgasifier.com/w/page/6123822/Stoichiometric%20Combustion%20Ratios

    gg
    •  
      CommentAuthordjh
    • CommentTimeNov 28th 2017
     
    Posted By: gyrogeartherefore less dense, therefore less oxygen (one supposes...)

    Yes, normally one would expect combustion to go better with colder, denser air so the Finnish report is unusual. One would have to look at the exact circumstances to understand why.
    • CommentAuthorgravelld
    • CommentTimeNov 28th 2017
     
    Posted By: Viking House[...] the conclusion was that there was no advantage to using external air.
    (assuming you mean a passive air brick or vent)... while the stove is running? Pretty clear advantage when it isn't running (which is going to be 99% of the time in a low energy house) imo.
  2.  
    Posted By: gravelld
    Posted By: Viking House[...] the conclusion was that there was no advantage to using external air.
    (assuming you mean a passive air brick or vent)... while the stove is running? Pretty clear advantage when it isn't running (which is going to be 99% of the time in a low energy house) imo.

    Yes, I'd agree. Having changed the passive seal on the outside of the chimney, it seems that there is a constant warm air draft through the chimney itself when the stove is not in use.
    •  
      CommentAuthordjh
    • CommentTimeNov 28th 2017
     
    Posted By: ComeOnPilgrimYes, I'd agree. Having changed the passive seal on the outside of the chimney, it seems that there is a constant warm air draft through the chimney itself when the stove is not in use.

    That's why chimney balloons were invented!
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