Jadnashua
Retired Defense Industry Engineer xxx
Pressurizing the makeup air would likely mess up the air/fuel mixture, so I wouldn't do it. Now, if that path was too restrictive, then you'd need to ensure it was big enough. A 6" duct probably doesn't meet the manufacturer's requirements for free air intake. Often, they want one square in per K-BTU - check to be sure. For a typical gas dryer, it would use about 45K-BTU, and need 45 sqin of inlet. An 8" duct without too many elbows would just barely qualify...a 6" is too small, especially if it is coaxial, and part of that volume is overtaken by the exhaust. Code-wise, I think you might have problems with what you've done...performance-wise as well.