I am considering having a Harman P68 installed. My installer wants to include two 45 degree elbows in the venting to avoid the ice dam on the roof. In reading about Equivalent Vertical Lengths, the configuration that the installer is recommending will have an EVL of 17 feet. I've read that anything over 15 feet would require switching to 4 inch piping. How strict is this rule of thumb? Am I OK with 3 inch piping or should I insist he install 4 inch piping? Any advice will be greatly appreciated. Thanks.