Many yrs ago when I first had hi-efficiency installed, I had the same problem a couple of times. I'd also wake up to a cold house with the scavenger draft fan only running. Apparently this problem is more common than one thinks?

My solution for MY furnace (I don't have a CCA therefore I make NO recommendations) was to cut my intake pvc in half, only a few feet from the furnace. I then took one of those rubber sleeves that they use to join pvc together & I drilled a 1" hole in the side of it. I slipped the sleeve over the pipes & made sure pvc pipes stayed spread far enough apart to allow the hole to breathe. Then I tightened the hose clamps. Now, when/if the intake freezes on the outside of the house, it will draw air from within the basement. However, if it's not frozen over, there seems to be less resistance to the outside air source & I feel very little air entering the hole via a venturi effect.

That was MY solution & I do NOT recommend anyone else modifying their furnace without proper supervision from a qualified installer.

The photo below is my unauthorized modification. You can see the hole in the slipjoint. The additional slipjoint to the right does nothing now. It has NO hole in it. I slipped it over the pvc when I had them split just in case my modification did not work. I wanted to be able to quickly join the pvc again & go to plan B. That was probably 15 yrs ago. There's been no further freeze problems since the mod.

Jay, you might see the same idea where I got it from. The crankcase breather tube on most recip airplane engines has a "whistle tube" in it to prevent total line freeze over when flying in sub-zero temps.