This paper points out the fact that increasing the supply coolant temperature has a greater effect on the heater performance than revising the core efficiency. If two heater cores with different efficiencies are tested, the core with the higher efficiency results in a lower coolant temperature returning to the engine (it removes more heat out of that supplied). As a result, the supply coolant temperature for the next time point is lower than it would be for a core with lower efficiency. This is known as the feedback effect. It causes the difference in the discharge air and average interior temperatures in the higher and lower efficiency cores to be insignificant until the supply coolant temperature is increased to a higher level. Once at this stage, the lower efficiency core needs another heat exchanger (radiator) to help it in the heat rejection, while the higher effiency core continues to reject heat by itself.This phenomenon was revealed via the use of a transient heater warm-up computer program and further was affirmed by testing.