Water being sucked into cold air return from humidifier pad
There should be a damper on the 6" pipe from the hot air plenum to the humidifier, closing it part way will help but the most probable cause is not enough return air openings in your house. If you have wall returns, take the grills off and make sure the air is going through and nobody forgot to cut the openings in the bottom of the wall or in the ductwork itself. Also get a copy of the heating design submitted in the building department to make sure it matches what's in your house, including the furnace size. It wouldn't be the first time a furnace was put in that was all they had in stock instead of buying the correct size. Mine was replace with one at double the btu output and I had to modify the orifices to half the output to get proper heating. High efficiency furnaces with too high of an output only wastes fuel since the design only brings them up to the proper efficiency after running for 20 minutes so you don't want it with a much bigger input than called for in the heat loss design. If everything is it's supposed to be and you closed the bypass damper and still having issues, you will need to add more returns in your house, even if it's a floor return grille in your return duct, or if the Return duct running down the sie of your furnce is under 10 to 12" on the narrow side and under 23 or 24" on the width, it should be changed to something around 10 x23" to 12 x 23", if it's still bad then you are really starving for Return airs!