as just an amateur, some thoughts come to mind. check your operating specs and see if the unit is designed to shut down in that cold of climate. Also, if the exhaust of the heater contains steam condensate, it might cause abnormal build up in the intake. Are there any installation instructions that might give you a clue as to a proper/recommended way to install the vents? Sometimes the installation pictures make it obvious. Just some thoughts. Maybe find another plumber to come and look at it for a second opinion. Take a pic of it and see if the manufacturer agrees with the install.