Person who thinks they're saving the world from greenhouse gases here.
I got a Navien 180S without recirc a year ago, after a lot of research, and it's been great. Wish I'd had something like this long ago. It's just -- bam, hot water, no problem. I don't live in a mansion, and I don't have four people trying to take showers at once, so the 180 works fine here, and I don't mind waiting the five to 15 seconds it takes for the hot water to get to the faucet. The limescale problem is solved by doing maintenance, but it helps to soften the water before you put it through the heat exchanger -- I have a Stiebel-Eltron TAC filter so I don't have to deal with salt. The only thing about that is that you have to make sure whoever is installing the Nav actually reads the TAC install instructions first. (My guy refused, insisted he knew everything, and his boss wound up paying for a replacement filter.) You should still do the maintenance once a year, but then you should be doing maintenance on all your major appliances. Descaling's not a big deal -- you just take off the cover and use a vinegar solution, use a couple of buckets. If you're rich, pay someone to come do it for you. But yeah, instant soft hot water that doesn't run out, I feel like we were living like animals before. I also have 1.8 gpm showerheads that are better than the fat lines I had before -- Sierra is the brand. Very strong flow. And low-flow aerators on the other faucets as well.
I replaced my furnace at the same time, and between the more efficient furnace and the Navien I've seen a substantial drop in gas use here even though we're home all the time during covid and we live in the deep-freeze of the upper midwest. I'd expected and am seeing about a 20% drop all told. Of course, gas is cheap. So if all you care about is dollars, and you're convinced that the only thing that shows you're smart is that you make big piles of them, you'll find plenty to grouch about in the very long payback time here. While it may boost the house price a bit on resale, I'm guessing it's not that big a deal unless you're selling to younger people who are coming to expect tankless in new builds. More like a penalty if they feel like they have to replace an obsolete tank.
The only thing I worry about with the Navien is that anything with a computer in it is vulnerable to power interruptions, software obsolescence, etc. The weather is not going to get less violent. I don't have a backup generator, but do have solar panels, and am considering getting battery storage. I'd rather wait, though, till the battery technology improves -- the work with the combination of charge density, rechargeability, size, and safety has a ways to go.
Between the solar panels, furnace, and Navien, and doing less driving, I reckon we avoid putting 250K lbs of CO2 into the air over the next 20 years, and overall we come out slightly ahead financially (including the financing costs), which I figure will actually mean it's about a wash dollarwise. Short of making the walls thicker it's probably not going to be possible to make this continental-climate suburban tract house much more efficient.
