IDEAL usage of a tankless water heater is to set the unit's control AT the desired shower temperature and run ONLY the hot water tap ... in which case toilet flushing should have no effect. I've never been scalded in my shower in the two years I've had the tankless water heater .... but I am a single-person household so it's rare that anybody flushes a toilet while I'm showering. I also keep my tankless set at 102°F to 105°F, so scalding simply isn't possible. (I raise it temporarily for specific tasks, such as washing clothes.) However, being as very few people would use a tankless water heater that way, there may be a momentary increase in shower temperature if flushing a toilet decreases the COLD water flow at the shower, which would effectively increase the hot water flow ... but this wouldn't be much different than with a tank water heater. An unusual plumbing arrangement could possibly decrease the hot water flow as well, which may cause a momentary increase in hot water temperature due to it moving slower through the tankless unit ... depends on how quickly the unit can throttle its output to compensate. But in any case, seems like if any of that causes scalding, then the tankless setting is TOO HOT. If you are referring to thermostatic FAUCET controls IN the shower, doesn't seem to be any reason they won't work with a tankless water heater. The thermostatic control doesn't know if the hot water flowing through it is coming from a tankless or tank water heater.

thanks much for this information. yes, we do need to get a bigger tank for the well.... however, this problem has been solved with a pressure valve near the tank. wish i could tell you more specifics...but i can tell you that the installer tried putting in a valve (he heard that this would cure the problem) and it didn't help at all. when he returned after a few weeks, he decided to call rinnai again, and he talked to the right person. he needed a bigger valve of some sort.... installed it... and it fixed the problem! i'm so happy about this.... and we will work on getting the well in check as soon as po$$ible!

Set them at the same temperature. Nothing at all is gained with any other plan. If hot water uasge in a household is moderate, the top element could fail and no one would ever know. The bottom element will heat all the water in the tank. The top element heats only about the top 1/3 of the water in the tank when the top thermostat is chilled, such as from cold starts or when the stored hot water is being used rapidly.

Is this a gas or electric water heater?