How do I get an exterior vent to stop frosting over in a cold Canadian winter?
I have experienced the same problem. My intake frosts over and the furnace shuts down with a differential pressure fault. The cause is; when the intake air temperature falls below the dew point the relative humidity of the intake air becomes greater than 100% which means the moisture in the air will condense or precipitate out of the air onto anything that is colder than the dew point. When the furnace fires it draws a huge amount of air in through the cold air intake port. If the air temperature is less than the dew point, the excess moisture in the air immediately condenses on the cold intake screen (if you have one) and the inside of the intake pipe. As long as you draw this moisture saturated air across the cold surface of the intake port and the subsequent frost layer the moisture will continue precipitating out and make the layer(s) of frost thicker and thicker until the pipe is completely closed off or the screen is completely plugged.
The fix; First item of business was to remove the bird/bug screen. This eliminated about 80% of my issues. But every once in a while when it would get really cold (-20 to -50F) and the dew point was above 0, the intake pipe would completely fill with frost. So, I reconfigured my intake pipe to be as close to the building as possible and constructed a foam insulated hat that covered the top, sides and outboard surfaces of the intake, leaving the space between the building and the intake uninsulated. This allowed the intake to be "heated" by the escaping heat from the side of the building. I was only looking to get the intake pipe temperature above the dew point and I believe this has worked.