I installed an on demand hot water heater 9 months ago Rheem brand. Over the past 2 weeks when I have tried to run bath water I could only get a few seconds of hot water. I unplugged it the first time and it seemed to heat the water again but last night it did the same thing. I wasn't able to get anymore than a few seconds of hot water. I have only had this issue with running a bath, not a shower. Does anyone have any ideas?