So this basic idea has been rattling around my head for a bit. I thought of it after reading various articles talking about how cheap and low effort PEX is to run for plumbing, and also about how electric tankless water heaters are now extremely cheap and pretty efficient. (they don’t beat gas tankless in cost to run but the difference isn’t large except in northern climates) That Ecosmart brand has them for just a few hundred bucks on Amazon.
So…if you were plumbing a new house, why not take the next logical step? Run just cold water lines in PEX to every grouping of fixtures. Then, for each fixture group (fixtures that are in close proximity, like dishwasher + kitchen sink or bathroom sink + bathroom shower), size an electric tankless heater for that location and install it either under the sink or on the other side of the wall, depending on the location.
Advantages : less piping to run to plumb a house or apartment, almost no waiting for hot water, the tankless heaters are far easier to work on and repair than tank units, no running out of hot water, and the efficiency is actually pretty good. If one unit fails, the rest of the residence has hot water still, and the residents can probably make due until they get the unit repaired.
Disadvantages : in cold climates, tank gas is cheaper. (because the heat that leaks out of the tank heater just warms the house and most of the year that is a good thing). More and thicker wires are needed to supply the tankless heater with power. If all tankless units are at full flow at the same time, the house main breaker will probably blow (without impractically large electrical service). More individual units to repair and maintain, even if each one can be taken apart right there on the wall and the replacement parts installed.