I have a Rheem power vent heater that acted much the same. I found the burner was not igniting when it was supposed to. The vent fan would kick on, but no flame. I explored the Rheem website and got some clues as to the problem. It was the ignition module. I replaced it myself for $65. End of story. You can get the module from Rheem, but it's around $100 plus shipping from them. They suggested I find a plumbing supply house locally, which I did. If you open the door over the controls, you will see a black box on the upper part of the enclosure. That's it. Easy to change. Replacing the hose bib on the drain is easy, but you do have to drain the tank. Draining the tank won't help or hurt you problem, but it is good maintainance to do that about once a year. Test the TP valve then, too. Water pressure has nothing to do with the problem either. When the tank is lighting off properly, you should be able to shower, wash clothes, and disher without running out of water. I don't think you could expect to run hot water full stream all day and not run out, but just regular usage should present no problems.