You may want to consider using a "concentric vent" that handles both combustion air and exhaust through the same 4" opening in the outside wall. I just installed a 95% Lennox using this vent and it has performed well during the coldest period this winter ( lows to -20F ). The vent is Lennox part #71M80 and is called a 1.5" concentric vent kit. I am familiar with ice buildup on the standard 2" exhaust vents in a cold climate, I used several Lennox Pulse in Fargo, ND in the 1980's but can't say they ever iced up to the point of shutting down the furnace.
The concentric vent ties in to both the 2" combustion air line and the 2" exhaust right before it goes out of the wall through a 4' diameter hole. The exhaust goes out through the middle of this concentric vent and once outside is reduced down to 1 1/2", at a length of 8". The fresh air intake is right near the siding and is the outer portion of this vent, the exhaust keeps this whole unit warm. One other important point is the whole venting system is sloped back to the furnace so no condensation drips from the exhaust vent outside.
