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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.

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    • CommentAuthorblubb
    • CommentTimeMay 10th 2023
    Hi all,

    I am retrofitting an MVHR system (1927 two-storey property with Zehnder Q350 and Lindab ducts).

    I was thinking of using the disused chimney flue (18 x 18 cm) from a former gas boiler as the return duct to bring the exhaust air from the ground floor to the MVHR system in the loft. Are there strong arguments speaking against this solution? Has anyone tried doing this?

    I'd appreciate hearing additional thoughts about this!

    Many thanks!
    • CommentTimeMay 10th 2023
    Using the route of the disused chimney sounds like a sensible idea. But if you're talking about using the actual chimney flue then the obvious question is the condition of the flue. Is it clean? Free from smells etc?

    Note that you need easy access to the MVHR unit every few months to clean or change the filters, so think carefully about where you plan to place it and access to it. It'll also need careful mounting to avoid transmitting noise into bedrooms etc.
    • CommentAuthorblubb
    • CommentTimeMay 10th 2023
    Thanks @djh!
    Yes, I am talking about the actual chimney flue.
    - On the positive side the flue appears airtight, it "only" will need custom-made adaptor plates for the 160mm ducts on both ends.
    - On the negative side, the flue is not clean from the inside and still smells a bit of old soot. This is why I was thinking of using it as the exhaust (return) duct -- the smell should not transfer to the fresh air part of the system, right? Did anybody try this?

    Thanks for the warning about the MVHR positioning, both boxes you mentioned are ticked :)
    • CommentTimeMay 10th 2023
    Posted By: blubbOn the negative side, the flue is not clean from the inside and still smells a bit of old soot. This is why I was thinking of using it as the exhaust (return) duct -- the smell should not transfer to the fresh air part of the system, right?
    No the smell won't transfer but deliberately polluting your MVHR's heat exchanger is probably not best practice. Maybe remove the flue and insert a new duct instead.

    Is the chimney within the heated envelope, by the way, or is it on an outside wall?
    blubb - if you have access to the flue it might be worth considering a liner in the flue to act as the duct to avoid any nasties from the flue getting to the heat exchanger of the mvhr.
    Edit to say cross posted with djh
    • CommentAuthorblubb
    • CommentTimeMay 11th 2023
    Many thanks djh and Peter,
    flue liner is indeed a very good thought. Two related questions:
    - do you think grade 316 steel is good enough, or should I go with grade 904?
    - any advice how to join the 6" flue liner to 160mm Lindab spiral duct?
    Many thanks!
    • CommentAuthorGreenPaddy
    • CommentTimeMay 12th 2023
    Can't see any need for st st at all, other than that's maybe what's avail as a large diam flexy duct, to follow the contours of the existing flue.

    For me the much more important point was raised by DJH above. Is the existing flue inside the thermal envelope, or very well insulated? Heat loss over the large surface area of the duct, will mean a much reduced heat transfer to incoming air. This is not just a numerical heat loss issue, but could lead to cool/cold air dropping into the supply air rooms from the outlets - very uncomfortable. The reverse is also true, if the flue could get heated say by summer sun.

    MVHR in a loft (that's outside the thermal envelope) does have issues around the loft temperatures (freezing to roasting), all of which can impact the air temp into your rooms. I would be very wary about installing in a loft unless the MVHR and associated ducts are insulated to effectively bring them inside the thermal envelope of the house. That's not so easy, whilst still giving maintenance access to the MVHR unit. Don't underestimate the importance of the summer by-pass feature, for bringing cool summer evening/night air, which could be impeded if a roasting loft adds to the ducted air temps.
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