That was my problem, Just relining the existing chimeny, if it was feasible, since it has a jog in the middle, would have cost at least $500 or more. I could have saved $200 and not gotten the circulation, and could have gone with one size smaller for $100 less.
Simple payback over a standard tank is about 5-7 years after the utility rebate. Service life is 20 years for the tankless, HE warranty is 15 years. But I think we waste quite a bit of water waitign for it to get warm, and now we can actually wash our hands with warm water all the time.
Most problems come from improper gas perssure or gas line sizes, inproper venting, or improper water piping. Many users try to still use 120 or 125F water temps and then mix at hte faucets so their flow rate is low. The whole point is to set it lower and temepra it as little as possible. You also have ot have descent water quality and do regular PM's. But even a tank type should be flushed once a year. The overall function is no more complicted than a newer HE furnace and those are known to run reliably for years. The first generation Naviens had some issues, but thsi model has been out for 2 years and has a pretty large market share now.
As many bad reports as I have I also hear how much people love them... as above.
Just met with the plumber at lunch. Looks like I can save seom installation costs and mount it at an existing abandoned cold water drop, that right next to where the gas service comes in and under a hopper window that already has an abandoned clothes dryer outlet, so I can use the smalelr 2" PVC and it will be <5 in length.
Again, with my install I get instant hot water, endless hot water for 2 showers + 1 faucet simulateously at even 40F incomming water temps and there's no cold water "sandwhich". We're also repalcing a few sections of older galvanized piping while I'm at it.
I'm also going to keep the old tank unit as an emergency, short term back-up. Just drain it and shut off the gas. I'll cap the chimney, but leave a vent over that section.
I'll report bakc how start-up and installation goes.
Previous home - 1968 single story Ranch/Colonial, 1200sqft - 11 windows
Current home - 1925 2 story Beaux Arts Neo-classical overlooking the Mississippi River, 3200sqft - 48 Windows