I plan to purchase a tankless water heater to replace a failing water tank. The current vent is a 6" diameter vent pipe. I need a 7.4 GPM unit (4 bathroom house with 3 showers) and considering a RheemGT199PVN. I noticed some tankless heaters have two separate vent pipes (one intake and the other exhaust). Some have one vent pipe with one pipe inside the other. If I choose the later I will need an additional hole in the side of the house (no chimney). What are the pros and cons of two separate pipes than just one pipe with an interior pipe? Are two separate pipes functionally better than one pipe inside the other? I'm a bit concerned when I need to replace the new heater (hopefully in 10+ years) that I may have to deal with patching or resizing holes. I've seen so many different kinds and configurations in water heaters and vents. Unemployed right now, I don't have $1600 to spend on installation, I got the time to do it myself. Any tips on installation are appreciated.