the instruction in the mr heater "instructions" say that for horiz venting only 3" single wall galv 26 gau is required. the optional accesory kit comes with 4" 26 gauge and also 4"*36" bvent...first why are they trying to sell you bigger pipe then required, 2ndly why the bvent if its not required??? i'm thinking the bigger pipe would be a little cooler then the smaller but is it really an issue? any disadvantages to the bigger size? i'm guessing draft is not an issue as its fan forced vent?