Where I live, you are required to have a tempering valve on the output of the WH. Now, that doesn't say where you have to have it adjusted, just that you must have one. Unless changed, they normally come set at 119-120 degrees. I run my tank at 140, but have the boiler set to allow it to cool off more before it rewarms it. Often, with no use, it may not need to fire for a day. Adds to the efficency, especially in the summertime.
Now, I started a whole bunch of comments when I threw in an indirect. I know it isn't a tankless. But, I was responding to a comment that said you can't get a tank to provide 250 gallons the first hour. To that comment, BS! There are lots of tanks that can do that. Now, is it reasonable or economical, that's another story altogether. But, with a decent sized indirect tank (mine's 60-gallons) vs a typical tankless, you can get UNRESTRICTED FLOW at all locations at full output temperature that would take a VERY large tankless system, especially where I live in the winter. I can be filling the tub, washing clothes, running the dishwasher, and have someone washing hands, etc. all at the same time and have full use of the hot water. I've measured my inlet water at 33-degrees in the deep of winter. So, depending on your circumstances, there are good reasons to avoid a tankless. Now, again, lifestyle comes into play here, and I'm not going to comment on that and about how spoiled we are. Also note one of the comments on tank verses tankless are the standby losses. The tank I have is rated at less than 1/2-degree per hour of loss. So, if the power was off for a day, it still may not trigger the boiler to come on, if it could. If you were quick, you could probably get a few days worth or hot showers out of it without power. Try that with your tankless! A normal gas-fired tank would lose a lot more, but it's still not horrible. Fired by a mod-con, my system is rated at 94%. Since it modulates, and it is sized to be able to provide about 1/3 the normal design day needs at low end, it can run at barely 'idle', but ramp up to what's needed on a cold day, or to reheat the tank. Not all mod-cons have as large a modulation range. The tank is SS, and should outlive me. Expect the boiler will need replacing before the tank, but the guts are SS on it, too.
If you have a boiler and aren't using an indirect, I don't think you're being very efficient. If you live where the incoming water doesn't get really cold, a tankless has some benefit. If you don't want to put up with variable water temperatures, no hot on a low flow use, or need a high flow rate, then avoid a tankless. It can be set up to overcome that, but it gets more complicated, and a boiler may be a better choice. Ganging tankless units to get high flow is quite expensive in both the infrastructure (large gas lines) and available combustion air and flues. In some places, your no-use gas charge is based on possible demand, so you're paying more for gas, even when you aren't using any.
So, pick your poison, understand the benefits and limitations of the choices and make an informed decision. My energy use compared to my neighbors is about 1/3 less, and is without compromises they have. People can shower one-after-the-other all day.
A boiler is going to require less service than a tankless system since it is using less 'fresh', mineral laden water, so there'd be little scale. An indirect doesn't see high enough temperatures at the heat exchanger to precipitate out any amount of mineral deposits, so efficiency should stay reasonably constant. On a tankless, the goal is to heat the potable water very fast, and you'll get the mineral deposits (unless you also have a softener).
No one system is 'best' for everyone. Pick the best for your circumstances. To exclaim only one choice is viable is not taking into the whole account of use patterns, volumes required, existing equipment, tolerance for restrictions, and amount of money you wish to spend. Energy costs here may approach many other country's, but only if the tax rates go up. It costs the Europeans about the same for gas as here, but their taxes are MUCH more. There should be a move to make things more efficient. This is a good thing, but you have to weigh the costs/benefits. We could all drive mopeds and get 70mpg, but it just wouldn't be the same...not one answer is best for all.
If we continue to allow the powers-that-be to regulate US (and I mean all the USA) for carbon emissions, we grant them the power to legislate US all out of existence. We're all made of carbon and exhale it all day long. If it were not for us, the trees would all die and we wouldn't even have a "geen" planet to worry about.
