Mon, 17 Aug 2026 03:18:16 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301500#Comment_301500 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301500#Comment_301500 Wed, 10 May 2023 18:46:45 +0100 blubb
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!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301502#Comment_301502 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301502#Comment_301502 Wed, 10 May 2023 20:25:53 +0100 djh
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301503#Comment_301503 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301503#Comment_301503 Wed, 10 May 2023 20:45:22 +0100 blubb 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 :)]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301504#Comment_301504 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301504#Comment_301504 Wed, 10 May 2023 21:19:18 +0100 djh 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?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301505#Comment_301505 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301505#Comment_301505 Wed, 10 May 2023 21:24:26 +0100 Peter_in_Hungary Edit to say cross posted with djh]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301512#Comment_301512 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301512#Comment_301512 Thu, 11 May 2023 10:31:03 +0100 blubb 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!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301531#Comment_301531 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17833&Focus=301531#Comment_301531 Fri, 12 May 2023 08:54:55 +0100 GreenPaddy
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.]]>