A typical tankless system is designed to raise the incoming water 70 degrees (farenheit) at it's design max flow rate. If you aren't using the max flow, there is typically a mixing valve on the output to temper that (now) hotter water. The tempering valves fail, and then you can have VERY hot water come out of the thing. Because they produce intense heat to be able to raise that temperature so fast, they need significant heat source. They come in both electrical and gas-fired versions. Either one may stress the typical supply source, if not already sized for them. On gas-fired ones, many people don't consider the make-up air requirements. One of these things is like adding another full-sized gas-fired furnace. Now, many of those are now using closed combustion with a piped outside air source. BUT, not all. Then, consider if you have a gas-fire dryer in the house. If all of them are running at the same time, you need huge amounts of fresh air to come into the house. So, that that into consideration. Say you have a system rated at 4 gallons/minute. That is 32 pounds of water. You want to raise that 70 degrees. That's 32*70*60 = 134,400BTU. That's a significant amount of burning. Also, now that that thing that can only flow 4 gallons/minute. You have a 40 gallon tub; it will take 10-minutes to fill the tub. Also, now run two showers at the same time - typically 2.5*2=5 gallons/minute. Now, your heater flows a max of 4, guess what - low flow, unsatisfying shower for both participants. Maybe it's not two showers, but one multihead job. It quickly gets pathetic. A tank type needs to be sized properly and have a quick enough recovery to satisfy your max needs, or again you will have compromises. But, you will get the max flow possible by your plumbing, with no restrictions by the heat source while you want it. A good tank, well insulated is very simple - the surface temperatures of the heat exchanger do not get as hot, so you don't get as much mineral deposits. Having soft water helps the life of either type, but is much more a factor with a tankless. I'm not particularly impressed with mine. I'm going to update not only my boiler but replace the hot water with an indirect heater. A small, 40-gallon tank on this system is rated at 150 gallons or so of hot water an hour. That's one shower running continuously; plus, when you want to fill the tub, you don't have to wait, either. Look carefully at the max input - not all of that input gets transferred to the water, so in my example above, at say 90% efficiency, the actual input would be 149,333 BTU or 43.725Kw (190Ahr) at 230vac. Like I said earlier, you need significant energy available to heat that water "instantaneously". Look carefully at the specs. Storing some water allows you to buffer that use, and makes that supply problem go away. A bigger tank, and or one with a faster recovery rate is still cheaper to install than a tankless, and over its life will cost less with fewer hassles. My unprofessional opinion.