This is why my friend the furnace installer has started making coaxial vents.  He uses ... I think 3" PVC, maybe 4" a combustion air source, then cuts a hole in a floor drain to hold the smaller exhaust tube in the middle.  I forget how he riggs a Y fitting inside, but it pulls the cold air through the outer pipe and vents the exhaust through the inner. His theory is if the intake and exhaust are going outside in the same preseure zone the furnace will work better. Seems to make his customers happy.