To check if the air heater is operating normally, operate the blower for 5 minutes and then touch the air hose connected to the blower. This one should be quite warm.
If the hose is not warm, contact your distributor for instructions on how to change the blower.
If the air hose is warm, the blower is not defective and the feeling of cold may occur occasionally. This is due to the theory that air blown on wet skin will feel cold. For example, the air of the hand dryer in a public restroom will feel cold until the hands are dry.
Some people are more sensitive to this fact than others. The solution is to change position in the bathtub and the feeling of cold will decrease.