I am sorry to hear you are having an issue. Starting ground water temperature and the amount of GPM being asked from the water heater makes a difference in how a smaller unit like this produces hot water. As you can see with the shower being a slower flow, the water is hotter. If you have any further sizing questions, please feel free to call our Technical Support Department at 1-800-582-8423 Ext 6 They may be able to give you some suggestions or make sure the unit is running correctly. Thank You