Quote Originally Posted by bigsby
OK. You have convinced me and confirmed what I already suspected. 2" isn't going to work it. I've come up with a different solution that isn't as good due to physical constraints but I believe I can make it work. First, please note that I will use a fully enclosed hood with a 4" exhaust so no reduction from 6" to 4" and so no back pressure there.

Consider this: I could enlarge my external exit to accommodate a 4" exhaust and then Y join my 2" vacuum PVC exhaust into the 4" duct. It shouldn't impact my vacuum. So that would give me a 4" exhaust all the way through the system. Will that work for the setup that I'm considering? 3'6" x 3'6" x 7' with 400w HPS.
Good to see that you have the option of going with the 4" duct. As long as your incoming air is a few degrees below your target temp, you should be fine. The 2" ducting and hardware is for specialized applications, you ain't gonna find it at the local hardware store. Besides being kinda pricey, a single 2" line is really on the ragged edge as far as venting a 400 HPS. But, you've already figured that out.

For sheer stealth, albeit at a significantly larger initial investment, LED has some undeniable advantages. You still need to vent, but you can get away with using fans that are significantly quieter than a 4" inline.