[Monarch Butterfly]

If you have a high efficiency furnace or boiler, the intake vents probably were installed to be 30 centimetres (12 inches) above the ground or snow. It's the "snow" part you should be concerned about. With all the snow we have been getting, I think there is little or no clearance between the snow and your furnace vents.

Please clear the snow away from your furnace vents.

 

[Jarrek]

Good tip.

However, because most vents exhaust heat anyways, most of the time the problem never occurs because the snow around the vent melts.

It becomes a bigger problem when snow is pushed against the vent by a plow, snow shovel, etc ...

 

[Monarch Butterfly]

The exhaust must not be sucked back into the intake vent. That is why there is an additional space between the two.
In my case, the snow on Saturday was so much that the intake was covered with the white stuff. The exhaust vent was open, but not the intake, which was closer to the ground (snow). What can't go in, can't go out.