I have mega hard water and have had not one problem with the tankless heater where we were going thru a tank (even doing the proper maintenance on them as needed) every 4 to 5 years! They work different than the tanks and do not sit there and slowly fill up with sediment from the water. we did install a big sediment filter for the whole house at the same time tho. that may make a difference and if you are that worried putting a water softener only on the hot water before the tankless heater could make you feel better.k15n1 said:A new hot water heater should get over 90 % efficiency.
I looked into the instant heaters but ended up not getting one on account of the hard water in our region.