I am getting a new wood stove and want to install the outside air kit. There is very little information out there regarding this install. It appears since I will not be right against an outside wall that I need 4" pipe instead of 3" due to the length and a couple of elbows. Am I okay if the first 3 feet are steel thru the wall and then plastic flexible from there? Any thoughts? I am getting a Harman TL300 and Harman doesn't actually seem to have a kit as an option. Replacing Englander wood stove.