just my experience. ROI is something to consider before buying a tankless i agree but in the time i have owned one ,fuel keeps getting higher and it does save considerable fuel,do your own math. i agree "don't buy a budget unit. I agree, check out any peculiar install requirements like gas supply or even faucet conversions. as far as scale??? i have had my heat exchanger out and was very surprised to find no scale what so ever. i have a spare heat exchanger just cause that is how i am but a new exchanger for the bosch 125X is aprox 300 dollars and i haven't needed it. i also keep a rebuild kit for the valve and a spare ingniter. now in my opinion if you want to take a chance and save money!!!! install an electric tank and put a blanket on it and a switch or timer so you can turn it off during the day and only heat water when you need it. probably save a bundle.On a WA gun forum they are having a similar discussion.
"This is exactly what I tell my customers looking to convert to tankless. Don't buy it because it's more efficient. It is more efficient, but you won't get a ROI. Ever. I don't even think it affects the resale value of a house all that much compared to a nice, new tank unit.
The "cheap" tankless NG unit that's worth a ****, like a Rinnai RL94 or a Norritz NR98, is still 60-70% more than a standard NG tank heater (which have gone up >20% this year, btw), the installation may involve costly modifications to the install location, like pulling power or upsizing the gas line, and unlike tank heaters, tankless units are not necessarily maintenance-free. You know that scale that builds up in your shower? There's a delicate, expensive radiator-like piece of hardware in that tankless unit called a heat exchanger, and it builds up in there, too. And if you don't flush it out every so often, depending on water quality, it can make the heater malfunction, and it can break off and work their way through your water line into your faucets, shower valves, and toilets and make them malfunction, too.
Don't buy a tankless unit to save money. Buy a tankless unit if you want something that takes up less space, OR because you have a need for a large capacity. Like three daughters that take 30-minute showers each, and a 72"x40" soaker tub and a wife that demands to use it. It's a luxury item, not a way to save money.
If you want to save money heating water, you're best bang-for-buck is going to be converting form a standard electric to a heat pump unit. Assuming the thing stays functioning for the full 10-year warranty, it'll pay for itself if it's in the right location in the house."