Hi guys. I'm doing an addition above my garage and I've decided to run a 4" supply duct through a return air space to avoid a nasty bulkhead that would effect the garage door movement. I understand that's it's not conventional but there's a lots of room in the area that going through that wouldn't effect flow. The inspector noted that I can't have a supply duct within a return space. I'm a tinner and I ran a shop for over 10 years so I know smacna and ashrae code well and couldn't find anything with in those codes for this issue and I've been over the OBC (Ontario building code) and can't find anything there either. Is anyone aware of code (possibly fire code?) that supports the inspectors note? I should also add that I responded immediately to the inspector asking if I might be able to insulate the 4" duct and sheath the pipe with a 6" duct to avoid any issues with heat loss/gain. I have not received any response and that was two days ago..
I think it would come down to the design and if the supply vent interferes with the size of the return air. You could just use some thermopan and box around the supply duct, so it's no longer technically inside the return air. If you have an HVAC designer sign off I'm sure there's nothing the inspector can do about it.