They seem very fussy about acceptable terminations on vents and inlets. Trane requires a vent coming straight out of the wall to be 12" min and 14" max. What would the harm be in running the vent out 30"? All I can think of is condensation, but they let you angle it down, so any condensation would run out.

I am sure there is a really good reason, but its not clear to me what that might be.