Soot is a sign of a rich (oxygen deficient) flame. Do you know the flame color of the old WH? Flame should be blue with little or no yellow. Yellow is a sign of unburned carbon with associated soot production likely.
A cubic foot of natural gas needs just over 10 cf of air to get a complete burn. 40,000 BTU/hour requires a little over 400 cf of air/hour through the combustion chamber plus air drawn in at the hood.