The vent line on a condensing boiler needs to slope back to the boiler so the condensate can be collected and drained away. It depends on how high that is where it needs to exit the house. You also want it to be above the minimum from the ground and within the limits of the manufacturer's recommended spacing from windows, doors, etc. So, the actual height it exits will depend on both the height of the installation and the distance from the wall so you maintain at least the minimum slope. It's not a real big deal to use a coring drill to make a hole later, if you prefer. It can go through the rim joist I think as well.