I haven't done one yet and I have always been worried about snow blocking a boiler vent through the roof, but I have never heard about the problem. has anyone experienced a problem? I live in northern Massachusetts.
Snow can block the intake so mount it up as high as it can go. Also I extend the center pipe up another 12 to 18 inches to stop reversion on very cold days.
On very cold days the exhaust gasses are not hot enough to rise and can fall down and into the intake.
Code requires a minimum of 12" above roof measured from bottom vent cap at air intake.
You can extend, do not use couplings, only full length pipe of the same diameter. Maximum that can be extended is 60" total. Dont forget to add new length to equivalent totals for exhaust run from boiler to determine if you are under 100' total length.
This is from cocentric exhaust manual that comes with boiler.