I want to install my Harman P68A in the lower level of my house which is below grade. In order to get the vent outside and above grade I will need to use 3 elbows (one will be the cleanout T) Should I upgrade to 4" or is 3" OK. BTW, the total run of pipe will be approx 9 feet, 10 tops. The P68 manual says to keep elbows to a minimum but offers no further info. Thanx