The difference between the incoming ground water temperature and the set temperature of the tankless unit. For example, if your tankless unit was set at 120°F and your incoming ground water temperature was 50°F your temperature rise would be 70°F (120-50=70). Temperature rise is used to determine BTU size of heater, when peak fixture demand is known. I.e. large temperature rise and high peak fixture demand requires a tankless heater with higher BTU input capacity.
BTU
British thermal unit, the amount of heat required to raise the temperature of 1 lb. of water 1°F
Fixture
A device that receives water from the water supply piping
Temperature Rise
The difference between the set temperature of the tankless heater and the ground water temperature (e.g. 120 Fahrenheit setting – 60 Fahrenheit ground water = 60 degree Fahrenheit rise.)