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!

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 :)

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

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.