A tankless water heater is used differently from a regular water heater. With a hot water tank, we keep its temperature at maximum to get the longest possible shower. A tankless water heater never runs out of hot water. It should be run at the lowest possible temperature.
Shower require approximately 105°F water temperature. Add some to compensate for heat loss in the piping, and we arrive at 110°F or close setting for the water heater. The lower the better.
There are a few good reasons not to go higher than 110°F in a residential setting.
Convenience. If you set the water heater to your shower temperature (around 105°F), all you need to do to start shower is to open hot water. No adjustments to be made. Water temperature won’t change when other people open taps or flash toilets.
Safety. It takes only seconds to get scolded by 140°F water. It still impossible to get scolded by 105°F water, even for a child
Maintenance and the Life Span.
The more we heat the water, the more calcium and iron falls out of it