Have a professional check the manifold pressure settings! This bit of adjustment is entirely unique to tankless water heaters and can make or break your experience. Results of incorrect manifold pressure settings can include water that is not hot enough, inconsistent, fluctuating temperature, or even error codes on the unit itself.
On my recent tankless water heater install, the water was only about 80-90 degrees. The installer checked absolutely everything: water pipes, gas pressure, etc. Finally, they called the manufacturer which tipped them off to this issue. They were almost to the point of replacing the unit itself. The manufacturer walked them through the procedure over the phone to properly adjusting the manifold pressure settings.
Update 5/29/15: I found out later that the reason this was necessary is that the gas piping diameter used in my house is less than what the tankless water heater is able to handle. So then the manifold pressure had to be adjusted to compensate. Theoretically this means I won't get the full heating performance of the tankless water heater but I have had no complaints at all with how well it has worked since.