The HVAC Company Fabricated the stack on this fan to protect the swamper and grease that was blowing all over the roof. This place is in a high wind area. that’s all.
Gotcha. The first time I looked at your post I didn't see any pictures. We have a customer that has a similar "extension" on thier upblast fan. I think that the only thing that it accomplishes is to....wait I don't think it actually does anything.
I guess I still don't get it. To me it appears to be a typical aft-shoot which is in place to keep proper distance from roof lines, etc. We see them all the time. A real pain.