My Water Heater is old and about a year ago it stopped relighting automatically. I have been putting it on vacation mode, pushing down on the knob and clicking the restart pilot light button, turn it on & then turn the dial on thermostat mode. It stays on for about 15 minutes or more and then goes out. Sometimes it says on for a day or two but goes out. I have been doing this for a year. Is this okay to do or should I replace it? Thanks!