1) What you describe does not sound like normal.
2) Since you do not have error code, issue could be plumbing problem.
http://waterheatertimer.org/pdf/No-Error-Code.pdf3) Open following link for useful troubleshooting information.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html4) Tankless installation and repair is a developing and complicated field as tankless numbers increase and companies hone their products, and customer relations, and installers get trained and certified.
5) I suggest you read your manual over and over to discover necessary maintenance and possible tricks and tips needed to keep these high-tech units working.
6) As a general rule, Rheem will refer you back to installer.
You might want a second professional technician to look over the problem to double-check first guy's work.
A lot of professionals will not get involved if they know they are assessing another guy's work.
Add a comment with your experience so other with same-similar problem can benefit.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.