Like another product there is a problem for tankless water heater will occur during the usage time. They are really normal because some parts need to be replaced by new one to maintain their great performance. Most cases are have simple solution by replacing the broken parts. But sometime we push them too much and make their usage time reduce significantly because they are overwork. That’s why we need to read the guide to know how they are perform and their ability to supply hot water through our pipes.
My recommendation is if you doesn’t know what it is a better things to do is calling a professional help. You should have a local service and repairman phone number just in case you need it. And I think you will.
If the problem is small and can be solved by replacing easy to replaced parts, you can get it at local supplier or some big online store that have it.