[Myrddin]

My heater is in the basement, dead center. The vents go up through the basement, 1st floor, 2nd floor, then to the roof. Much cutting to make new room for larger diameter pipes all the way up. I didn't figure the extra cost would be amortized over the life of the heater. It would have been exceptionally cheap to do the same upgrade to my old house in Mira Mesa. I replaced that one as well. The "extra" work that time was getting the building permit, adding the earthquake bracing and having the inspector come out. No big deal. I've replaced 5 or 6 heaters since I was a teen at home.

[steve86]

I seem to recall that building code lets you use smaller diameter vent piping if “powered” venting is used — but then, powered venting may be required anyway if a multistory vent is used. Have never dealt with it.

The space that became available after replacing my old tank with the tankless heater got used real quick for a solar power backup system for the house (which I installed). Battery banks outside, inverter inside. Around 800 amp hours.