I have a Buderus. WHen the installers put in the air pipes (pvc), they didn't follow the manufacturer's diagrams and had the hot and cold reversed to the vent termination fixture. Maybe counterintuitively, on the vent hood I have, the hot goes out the lower pipe and the cold intake comes in the upper. One might think that the hot would pollute the intake air. ANyway, they had it backwards, and the condensation from the exhaust, instead of falling harmlessly down and not blocking the intake, when the intake was below, got all iced over and the thing stopped working on a very cold day! After a call the manufactuer to verify, I had them back out to rearrange the pipes.
If you don't have the proper slope to the lines, or have the arrangement setup properly, ice clogs could be an issue. Most of these have a condensate drain, but there's still hot moist air that hasn't release all of its moisture going out the pipe. SOme of it will condense, and could freeze. Enough of it, and you have problems.
