I'm looking at the installation manual of a Rheem condensing tank water heater, and I see a contradiction. The intake and vent are 2", but they show a 2" to 3" reducer within a few inches of the unit, and they say to use only a 3" concentric. But in a table they say less than 30 feet can use 2" piping. Can anyone tell me why they would have 2" intake and outlet on the heater and then go up to 3" pipe and concentric? Is there an advantage? Can I keep everything 2" concentric and all? Thanks for any input, but if you're going to tell me to use a tankless, please don't respond..