Well, mark had the right nomenclature, the valves in the handle in the shower

not at the heater or boiler as there are none there.
Depending on the age of the building, if there were any updates to modern standards requiring pressure or temperature balance valves( in each shower stall

) if you even have one. I built mine in 1993 and the code required it. Or, I just put one in, memory lapse


remembering the older house we lived in where it got hot when someone flushed