Know your water heater, know your venting options.
Growing up in the plumbing profession, venting a gas-fired water heater was always an easy process: You looked at the top of the water heater, determined the size of the outlet, connected that size vent connector (smoke pipe), and tied it to the chimney or vent. Ah, the good old days - they don't exist anymore for venting a water heater.
In the early 1990s, the gas industry decided to redefine venting requirements of gas-fired appliances. One of the first steps was to develop new categories for classifying the venting of gas-fired appliances. The categories were simply Category I, II, III and IV.