I have several water heaters in the house. One for the master which is a 240v unit and a couple of smaller units in other places in the house. The master bedroom one is on the fritz, so we have been using the shower that is on one of the smaller tankless units. The nightly routine is usually to bathe the child, then the wife and i take showers one after the other. The water is scorching hot until about 10 mins before the last person showers. It seems to get cool, then luke warm....maybe it will heat up a little, but its not near as hot as it is when the routine starts. By that time the water has been running for maybe 30 mins or so.

Whats the deal? Why would a unit work great for so long, and then just peter out like that? Im on a well, but it doesnt have problems usually.

Any ideas? The unit is 5 years old or so...