Every time I flush the toilet on the 2nd floor, there is a sewer smell that comes through the heating vents. What could be causing this promblem? How can I correct it?

Check the actual main vent line and see if its not cracked from ice freezing in it and the sewer gas is pushed out by the heat vents.
Of course the heat vent itself must not be to tight either to allow sewer gas to get in but its not impossible.

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