I have some venting questions about certain appliances. A furnace can be a natural draft vent, which is B-vent that is a Category I. I was told Category II is not used anymore. Category III is stainless steel venting under positive pressure and can be terminated horizontally and vertically. I think this is for hot water heaters and fireplaces. Category IV is direct venting where you can use PVC with a furnace. My confusion is if there is a Category IV with like a hot water heater or furnace or is it simple just a Category III? Also, because of the low gas temperatures in direct venting can you use an existing chimney or run it through the roof?