Yes preheat is a very good idea.
But the gas tankless heaters will turn off if the inlet temperature exceeds 90 to 100°F. I don't know if electric tankless heaters do that too.
I tried out the tankless heater in the shower and 110°F may be a little too cool for some people, I didn't get a nice real warm shower until I turned it up to 118 to 120°F.
Almost all of the hot water pipes are insulated and the run of hot water pipe is only about 20ft total.
So we may want to go with 120°F as the target temperature.
Do you know if the person has enough electrical capacity to run a big tankless heater?
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