It's become obvious from reading here at Hearth.com that a 4" vent will aid in creating a draft and a good burn when electric power is unavailable. However, i have a tight install to make therefore 3" pipe would work better for locating the stove. So....are there any benefits when power is unavailable if i use 3" pvp for the first leg and then convert to 4" about 8' up, and run the last 15' with 4"? Thanks for any feedback.