In your location, It seems to me that louver vents on the wall would be better. As snow builds up on the roof, the whirly birds will hold snow and could possibly add even more problems. The wall louvers would be protected from the chimney, the weather and from ice.
90% of the time, the best way is the hardest way lol
[URL="http://www.houserepairtalk.com/announcement.php?f=39&a=6"][size=3]The Ten Commandments of House Repair Talk[/size][/URL]
[URL="http://home.bellsouth.net/p/s/community.dll?ep=16&groupid=289528&ck="]Square Eye's home page[/URL]